The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. Organisms can be classified as unicellular (consisting of a single cell; including most bacteria) or multicellular (including plants and animals). Humans contain about 10 trillion (1013) cells. Most plant and animal cells are between 1 and 100 μm and therefore are visible only under the microscope.
The cell was discovered by Robert Hooke in 1665. In 1835, before the final cell theory was developed, Jan Evangelista Purkyje observed small "granules" while looking at the plant tissue through a microscope. The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.
The word cell comes from the Latin cella, meaning "small room". The descriptive term for the smallest living biological structure was coined by Robert Hooke in a book he published in 1665 when he compared the cork cells he saw through his microscope to the small rooms monks lived in.
The eukaryotes are distinguished from prokaryotes by the structural complexity of the cells characterized by having many functionall semi-autonomous elements, usually referred to as organelles. Many of these are membrane bounded and include the nucleus, the rough endoplasmic reticulum, dictyosomes (= golgi apparatus) lysosomes, and in protists contractile vacuoles and extrusomes. Other membrane bound organelles that are not always present include chloroplasts and mitochondria. Non-membrane-bound organelles include cytoskeletal elements (mostly tubulin = microtubule or actin = microfilament - based), contractile (actin-myosin assemblages, centrin) or other motility devices (mitotic spindle, myonemes, cilia, flagella).
The synapomorphic features of the eukaryotes is the nucleus with associated mitotic division system. The first eukaryotic cells probably also had a small array of cytoskeletal elements but these have yet to be identified.
Mitochondrial eukaryotes include all cells with nuclear genomes that at one time in their evolutionary history contained mitochondria. This broad class includes all plants, fungi, animals and most protists.
Crown eukaryotes are an artificial group of eukaryotic organisms found at the top of molecular phylogenetic trees including both eukaryotes and prokaryotes. They were originally thought to represent a late step of eukaryotic evolution (somewhat similar to a crown group) because they include multicellular and macroscopic lifeforms that represent the majority of the biomass of the planet while accounting for less than 1% of the genetic diversity. However, they are in fact the result of an artificial clustering of eukaryotic organisms with slowly evolving gene sequences. They are thus not a crown that excludes simpler eukaryotes, but correspond roughly to the initial radiation of eukaryotes. All eukaryotic lineages branching below the "crown" in phylogenetic trees are misplaced because of the long branch attraction phenomenon.
The group includes eukaryotic cells that, for the most part, have a single emergent flagellum, or are amoebae with no flagella. The unikonts include opisthokonts (animals, fungi, and related forms) and Amoebozoa.
The opisthokonts (Greek: opìsthios = "rear, posterior" + kontòs = "pole" i.e. "flagellum") or "Fungi/Metazoa group" are a broad group of eukaryotes, including both the animal and fungus kingdoms, together with the eukaryotic microorganisms that are sometimes grouped in the paraphyletic phylum Choanozoa (previously assigned to the protist "kingdom"). Both genetic and ultrastructural studies strongly support that opisthokonts form a monophyletic group.
One common characteristic of opisthokonts is that flagellate cells, such as most animal sperm and chytrid spores, propel themselves with a single posterior flagellum. This gives the group its name. In contrast, flagellate cells in other eukaryote groups propel themselves with one or more anterior flagella.
Holozoa is a group of organisms that includes animals and their closest single-celled relatives, but excludes fungi. Holozoa is also an old name for the tunicate genus Distaplia.
Because Holozoa is a clade including all organisms more closely related to animals than to fungi, some authors prefer it to recognizing paraphyletic groups such as Choanozoa, which mostly consists of Holozoa minus animals.
Perhaps the best-known holozoans, apart from animals, are the choanoflagellates, which strongly resemble the collar cells of sponges, and so were theorized to be related to sponges even in the 19th century. Proterospongia is an example of a colonial choanoflagellate that may shed light on the origin of sponges.
The affinities of the other single-celled holozoans only began to be recognized in the 1990s. A group of mostly parasitic species called Icthyosporea or Mesomycetozoea is sometimes grouped with other species in Mesomycetozoa (note the difference in the ending). The amoeboid genera Ministeria and Capsaspora may be united in a group called Filasterea by the structure of their thread-like pseudopods. Along with choanoflagellates, filastereans may be closely related to animals, and one analysis grouped them together as the clade Filozoa.
The Filozoa are a monophyletic grouping within the Opisthokonta. They include animals along with their nearest unicellular relatives (Filasterea and Choanoflagellata).
The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. As the name suggests, choanoflagellates (collared flagellates) have a distinctive cell morphology characterized by an ovoid or spherical cell body 3-10 µm in diameter with a single apical flagellum surrounded by a collar of 30-40 microvilli. Movement of the flagellum creates water currents that can propel free-swimming choanoflagellates through the water column and trap bacteria and detritus against the collar of microvilli where these foodstuffs are engulfed. This feeding provides a critical link within the global carbon cycle, linking trophic levels. In addition to their critical ecological roles, choanoflagellates are of particular interest to evolutionary biologists studying the origins of multicellularity in animals. As the closest living relatives of animals, choanoflagellates serve as a useful model for reconstructions of the last unicellular ancestor of animals.
Relationship of Choanoflagellates to Metazoans
The morphological similarities of choanoflagellates and sponge choanocytes and proposed the possibility of a close relationship as early as 1841. Over the past decade, this hypothesized relationship between choanoflagellates and animals has been upheld by independent analyses of multiple unlinked sequences. Importantly, comparisons of mitochondrial genome sequences from a choanoflagellate and three sponges confirm the placement of choanoflagellates as an outgroup to Metazoa and negate the possibility that choanoflagellates evolved from metazoans (Lavrov, et al., 2005). Finally, recent studies of genes expressed in choanoflagellates have revealed that choanoflagellates synthesize homologues of metazoan cell signaling and adhesion genes. Genome sequencing shows that among living organisms, the choanoflagellates are most closely related to animals.
Because choanoflagellates and metazoans are closely related, comparisons between the two groups promise to provide insights into the biology of their last common ancestor and the earliest events in metazoan evolution. The choanocytes (also known as "collared cells") of sponges (considered the most basal metazoa) have the same basic structure as choanoflagellates. Collared cells are found in other animal groups, such as ribbon worms, suggesting this was the morphology of their last common ancestor. The last common ancestor of animals and choanoflagellates was unicellular, perhaps forming simple colonies; in contrast, the last common ancestor of all eumetazoan animals was a multicellular organism, with differentiated tissues, a definite "body plan", and embryonic development (including gastrulation). The timing of the splitting of these lineages is difficult to constrain, but was probably in the late Precambrian, >600 million years ago.
Choanoflagellate colony formation
All animals are members of the Kingdom Animalia, also called Metazoa. This Kingdom does not contain the prokaryotes (Kingdom Mondera, includes bacteria, blue-green algae) or the protists (Kingdom Protista, includes unicellular eukaryotic organisms). All members of the Animalia are multicellular, and all are heterotrophs (that is, they rely directly or indirectly on other organisms for their nourishment). Most ingest food and digest it in an internal cavity.
Animal cells lack the rigid cell walls that characterize plant cells. The bodies of most animals (all except sponges) are made up of cells organized into tissues, each tissue specialized to some degree to perform specific functions. In most, tissues are organized into even more specialized organs. Most animals are capable of complex and relatively rapid movement compared to plants and other organisms. Most reproduce sexually, by means of differentiated eggs and sperm. Most animals are diploid, meaning that the cells of adults contain two copies of the genetic material. The development of most animals is characterized by distinctive stages, including a zygote, formed by the product of the first few division of cells following fertilization; a blastula, which is a hollow ball of cells formed by the developing zygote; and a gastrula, which is formed when the blastula folds in on itself to form a double-walled structure with an opening to the outside, the blastopore.
Somewhere around 9 or 10 million species of animals inhabit the earth; the exact number is not known and even our estimates are very rough. Animals range in size from no more than a few cells to organisms weighing many tons, such as blue whales and giant squid. Most animals inhabit the seas, with fewer in fresh water and even fewer on land.
A section of Metazoa that includes the phyla above the Porifera (Sponges). Phylogenetic analysis suggests that the Porifera and Ctenophora diverged before a clade that gave rise to the Bilateria. The sponges (Porifera) were long thought to have diverged from other animals early. They lack the complex organization found in most other phyla. Their cells are differentiated, but in most cases not organized into distinct tissues. Sponges typically feed by drawing in water through pores. Eumetazoans are divided into two basic groups: Radiata (organisms with radial symmetry, including jellyfish and their relatives) and Bilateria (organisms with twofold symmetry that gives them definite front and rear, and left and right, body surfaces). Bilaterians, the first of which likely evolved in the Precambrian, are a step up in multicellular complexity from Radiata. Whereas Radiata develop from two embryonic tissue layers, an inner endoderm and outer ectoderm (diploblastic), the Bilateria have a third tissue layer, the mesoderm, between the endo- and ectoderm (triploblastic). This layer forms the muscles and most organs located between the digestive tract and the outer covering of the animal. The vertebrate circulatory and skeletal systems also stem from the mesoderm. Most bilaterians also show cephalization: the evolutionary trend toward the concentration of sensory structures (such as the mouth, nerves, etc.) at the anterior end of the body -- the end of a moving animal that is usually first to encounter food, danger, or other stimuli.
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back. The bilateralia are a subregnum (a major group) of animals, including the majority of phyla; the most notable exceptions are the sponges, belonging to Parazoa, and cnidarians belonging to Radiata. For the most part, Bilateria have bodies that develop from three different germ layers, called the endoderm, mesoderm, and ectoderm. From this they are called triploblastic. Nearly all are bilaterally symmetrical, or approximately so. The most notable exception is the echinoderms, which achieve near-radial symmetry as adults, but are bilaterally symmetrical as larvae.
Except for a few highly reduced forms, the Bilateria have complete digestive tracts with separate mouth and anus. Most Bilateria also have a type of internal body cavity, called a coelom. It was previously thought that acoelomates gave rise to the other group, but there is some evidence now that in the main acoelomate phyla (flatworms and gastrotrichs) the absence could be secondary.
The type of body cavity places an organism into one of three groups:
Acoelomate animals, like flatworms, have no body cavity at all. Organs have direct contact with the epithelium. Semi-solid mesodermal tissues between the gut and body wall hold their organs in place.
Pseudocoelomate animals have a pseudocoel (literally "false cavity"), which is a fully functional body cavity. Tissue derived from mesoderm only partly lines the fluid filled body cavity of these animals. Thus, although organs are held in place loosely, they are not as well organized as in a coelomate. All pseudocoelomates are protostomes; however, not all protostomes are pseudocoelomates. An example of a Pseudocoelomate is the roundworm. Pseudocoelomate animals are also referred to as Hemocoel and Blastocoelomate.
Coelomates or Coelomata (also known as eucoelomates - "true coelom") have a fluid filled body cavity called a coelom with a complete lining called peritoneum derived from mesoderm (one of the three primary tissue layers). The complete mesoderm lining allows organs to be attached to each other so that they can be suspended in a particular order while still being able to move freely within the cavity. Most bilateral animals, including all the vertebrates, are coelomates.
There are two or more superphyla (main lineages) of Bilateria. The deuterostomes include the echinoderms, hemichordates, chordates, and possibly a few smaller phyla. The protostomes include most of the rest, such as arthropods, annelids, mollusks, flatworms, and so forth. There are a number of differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes the mouth in protostomes, and the anus in deuterostomes
In both deuterostomes and protostomes, a zygote first develops into a hollow ball of cells, called a blastula. In deuterostomes, the early divisions occur parallel or perpendicular to the polar axis. This is called radial cleavage, and also occurs in certain protostomes, such as the lophophorates. Most deuterostomes display indeterminate cleavage, in which the developmental fate of the cells in the developing embryo are not determined by the identity of the parent cell. Thus if the first four cells are separated, each cell is capable of forming a complete small larva, and if a cell is removed from the blastula the other cells will compensate.
In deuterostomes the mesoderm forms as evaginations of the developed gut that pinch off, forming the coelom. This is called enterocoely.
There are three extant phyla of deuterostomes:
Phylum Chordata (and their kin)
Phylum Hemichordata (acorn worms and possibly graptolites)
It seems very likely that 555 million years old Kimberella was a member of the protostomes. If so, this means that the protostome and deuterostome lineages must have split some time before Kimberella appeared at least 550 millions years ago, and hence well before the start of the Cambrian 542 million years ago. The Ediacaran fossil Ernietta, from about 549 to 543 million years ago, may represent a deuterostome animal.
Kimberella (9 cm. - 3.5 in.)
Ernietta (15 cm. - 6 in.)
Xenoturbellida is derived from a Greek root that means stranger (ξένος - zenos) and a Latin root (turba) that means turbulence (turbellida is the Latin diminutive of turba). The reference is to an odd animal that resembles a free-living flatworm (called Turbellaria because the ciliated epithelium causes turbulence around the body of the animal). These are the strange or odd Turbellarians because, although they superficially resemble free-living flatworms, the Xenoturbellarians have been demonstrated to be quite different.
Telford (2008) describes Xenoturbella as a "flatworm" with a ventral mouth and a blind gut. In fact, its digestive system almost has only a ventral mouth and an internal gastric cavity that has no pharynx. The nervous system is almost nothing more than a nerve net with four longitudinal nerves with no major nerve cords or ganglia. The anterior end of the animal, which is about 3 cm long and 0.5 cm wide, is denoted by a field of pigmented dots. The middle of the body has an obvious groove.
As of now, the group contains only two recognized species in the same genus. Xenoturbellida has taken a winding road through the animal kingdom. It has been treated as a basal metazoan, an acoel flatworm, a paedomorphic larva of an unknown deuterostome, a bivalve mollusk, and a basal deuterostome!
A longitudinal section of Xenoturbella showing the statocyst (st), mouth (m), gastric cavity (gc), and the ring furrow (rf). Image from: Telford (2008) - Source
Bourlat et al. (2006), in an analysis using 170 nuclear proteins and 13 mitochondrial proteins showed that Xenoturbellida emerged within the Deuterostomata and were basal to the Echinoderms and Hemichordates, a group called Ambulacria. Perseke et al. (2007) in analyzing mitochondrial comparisons conclude that Xenoturbellida is basal to the deuterostomes. Telford (2008) in attempting to reconcile the obvious similarities with the Acoels and the dawning understanding of the relationship of Xenoturbellida with the deuterostomes, suggested that Xenoturbellida and Acoelomata might be sister groups, which together are basal in the Ambulacria clade. The implications of these scenarios are not trivial. Consider the occurrence of gill slits, a character shared by members of both major clades of the deuterostomes. Because it is very unlikely that such structures could have evolved independently, the presumed deuterostome ancestor must have possessed that character. If so, Xenoturbellida is a secondarily simplified deuterostome.
The last common ancestor of the Chordata appeared after the xenoturbellids.
Xenoturbella has a very simple body plan: it has no brain, no through gut, no excretory system, no organized gonads (but does have gametes; eggs and embryos occur in follicles), or any other defined organs except for a statocyst containing flagellated cells; it has cilia and a diffuse nervous system. The animal is up to 4 centimetres (1.6 in) long, and has been found off the coasts of Sweden, Scotland and Iceland.
Image fron the on line version of Harold L. Levin's: The Earth Through Time (8th Ed.)
Chordates form a phylum of creatures that are based on a bilateral body plan, and is defined by having at some stage in their lives all of the following:
The notochord is an elongate, rod-like, skeletal structure dorsal to the gut tube and ventral to the nerve cord. The notochord should not be confused with the backbone or vertebral column of most adult vertebrates. The notochord appears early in embryogeny and plays an important role in promoting or organizing the embryonic development of nearby structures. In most adult chordates the notochord disappears or becomes highly modified. In some non-vertebrate chordates and fishes the notochord persists as a laterally flexible but incompressible skeletal rod that prevents telescopic collapse of the body during swimming.
A dorsal, hollow, ectodermal nerve cord (compare with Annelida and Arthropoda which have ventral, solid, mesodermal nerve cords) typically forms by an infolding of the ectoderm tissue, which then "pinches off" and becomes surrounded by mesoderm. In most species it differentiates in embryogeny into the brain anteriorly and spinal cord that runs through the trunk and tail. Together the brain and spinal cord are the central nervous system to which peripheral sensory and motor nerves connect.
The visceral (also called pharyngeal or gill) clefts and arches are located in the pharyngeal part of the digestive tract behind the oral cavity and anterior to the esophagus. The visceral clefts appear as several pairs of pouches that push outward from the lateral walls of the pharynx eventually to reach the surface to form the clefts. Thus the clefts are continuous, slit-like passages connecting the pharynx to the exterior. The soft and skeletal tissues between adjacent clefts are the visceral arches. The embryonic fate of the clefts and slits varies greatly depending on the taxonomic subgroup. In many of the non-vertebrate chordates, such as tunicates and cephalochordates, the clefts and arches are elaborated as straining devices concerned with capture of small food particles from water. In typical fish-like vertebrates and juvenile amphibians the walls of the pharyngeal clefts develop into gills that are organs of gas exchange between the water and blood. In adult amphibians and the amniote tetrapods (= reptiles, birds and mammals) the anterior most cleft transforms into the auditory (Eustachian) tube and middle ear chamber, whereas the other clefts disappear after making some important contributions to glands and lymphatic tissues in the throat region. The skeleton and muscles of the visceral arches are the source of a great diversity of adult structures in the vertebrates. For example, in humans (and other mammals) visceral arch derivatives include the jaw and facial muscles, the embryonic cartilaginous skeleton of the lower jaw, the alisphenoid bone in the side wall of the brain case, the three middle ear ossicles (malleus, incus and stapes), the skeleton and some musculature of the tongue, the skeleton and muscles of the larynx, and the cartilaginous tracheal rings.
The post-anal tail. A muscular tail that extends backwards behind the anus.
Pikaia(Drawing by Mary Parrish courtesy of the Smithsonian Institution)
Pikaia is a primitive creature without a well defined head It is less than 2 inches (5 centimetres) long. It swam in the mid-Cambrian seas, and is closely related to the ancestor of all backboned animals (vertebrates), from fish to amphibians, reptiles, and birds to mammals. When alive, Pikaia was a sideways-flattened, leaf-shaped animal. It swam by throwing its body into a series of S-shaped, zig-zag curves, similar to movement of snakes. Fish inherited the same swimming movement, but they generally have stiffer backbones. Pikaia had a pair of large head tentacles and a series of short appendages, which may be linked to gill slits, on either side of its head. In these ways, it differs from the living lancelet. Pikaia's anatomy is still not fully known and palaeontologists are still researching its details.
This primitive marine creature shows the essential prerequisites for backboned animals. The flattened body is divided into pairs of segmented muscle blocks, seen as faint vertical lines. The muscles lie on either side of a flexible rod which runs from the tip of the head to the tip of the tail.
At first glance, Pikaia does not seem like a vertebrate ancestor, and in fact there is a lot of debate regarding the topic in scientific circles. It looks like a worm that has been flattened sideways. But in detail, the fossils compressed within the Burgess Shale clearly show chordate features such as traces of an elongate notochord, dorsal nerve cord and blocks of muscles (myotomes) down either side of the body, all critical features for the evolution of the vertebrates.
The notochord is a flexible rod that runs along the back of the animal, lengthening and stiffening the body so that it can be flexed from side to side by the muscle blocks for swimming. In the fish and all subsequent vertebrates, the notochord forms the backbone (or vertebral column). The backbone strengthens the body, supports strut-like limbs, and protects the vital dorsal nerve cord, while at the same time allowing the body to bend.
Surprisingly, a Pikaia lookalike still exists today, the lancelet Branchiostoma (Anphioxus) (2-3 in). This little animal was familiar to biologists long before the Pikaia fossil was discovered. With notochord and paired muscle blocks, the lancelet and Pikaia belong to the chordate group of animals from which the vertebrates have descended. Molecular studies have refuted earlier beliefs that lancelets might be the closest living relative to the vertebrates, and instead favor Tunicates (but not all researchers agree) in this position. While the lancelet is a chordate, other living and fossil groups, such as acorn worms and graptolite, are more primitive. Called the hemichordates, they have only a notochord-like structure at an early stage of their lives.
The presence of a creature as complex as Pikaia some 530 million years ago reinforces the controversial view that the diversification of life must have extended back well beyond Cambrian times, deep into the Precambrian.
Appendicularia (formerly Larvacea)
Like other chordates, Tunicates have a notochord during their early development, but by the time they have completed their larval stages they have lost all myomeric segmentation throughout the body. As members of the Chordata they are true Coelomata with endoderm, ectoderm and mesoderm, but they do not develop very clear coelomic body-cavities if any at all. Whether they do or not, by the end of their larval development all that remain are the pericardial, renal, and gonadal cavities of the adults. Except for the heart, gonads, and pharynx (or branchial sac), the organs are enclosed in a membrane called an epicardium, which is surrounded by the jelly-like mesenchyme. Tunicates begin life in a mobile larval stage that resembles a tadpole.
The larval form ends when the tunicate finds a suitable rock to affix to and cements itself in place. The larval form is not capable of feeding, though it may have a digestive system, and is only a dispersal mechanism. Many physical changes occur to the tunicate's body, one of the most interesting being the digestion of the cerebral ganglion, which controls movement and is the equivalent of the human brain. From this comes the common saying that the sea squirt "eats its own brain". In some classes, the adults remain pelagic (swimming or drifting in the open sea), although their larvae undergo similar metamorphoses to a higher or lower degree.
In growing to adulthood, tunicates develop a thick protective covering, called a tunic, or test, around their barrel-shaped bodies.
Undisputed fossils of tunicates are rare. The best known (and earliest) is Shankouclava shankouense (pict left) from the Lower Cambrian Maotianshan Shale at Shankou village, Anning, near Kunming (South China). The tunicate identity of this organism is supported by the presence of a large and perforated branchial basket, a sac-like peri-pharyngeal atrium, an oral siphon with apparent oral tentacles at the basal end of the siphonal chamber, perhaps a dorsal atrial pore, and an elongated endostyle on the mid-ventral floor of the pharynx. As in most modern tunicates, the gut is simple and U-shaped, and is connected with posterior end of the pharynx at one end and with an atrial siphon at the other, anal end.
The Craniata are characterized by a skull (initially cartilaginous and fibrous), which includes three types of sensory organs derived in ontogeny from ectodermal placodes; that is, thickened patches of the embryonic skin that sink inward toward the brain where they develop into sensory chambers. Anteriormost of these is the olfactory organ, which is initially unpaired, and becomes paired in the Vertebrata. Behind it are the paired eyes, the photo receptors that develop as lateral outgrowths of the brain. The skin and connective tissues adjacent to the neural (photoreceptive) part of the eye add secondary structures in the Vertebrata (lens, intrinsic muscles, and eye lids). Posteriormost of these sensory organs in the head are the paired acoustic organs or inner ears. The inner ears are mechanoreceptors concerned with hearing, balance, and perception of position of movement. The sensory cells of the inner ear are enclosed in a cavity filled with a liquid, the endolymph, and which develops from one to three semicircular canals. The acoustic organs also comprise a special component, the lateral sensory system, which is lost in most terrestrial craniates (Amniota). It consists of lateralis nerve fibres derived from the acoustic nerve and superficial mechanoreceptors, the neuromasts, which are housed in grooves or canals on the surface of the head. These extend onto the body in the Vertebrata. True neuromasts, however, seem to be unique to the Vertebrata, and have never been observed in hagfishes.
The craniates are characterized by a skull; that is, a complex ensemble of skeletal elements which surrounds the brain and sensory capsules. The skull of hagfishes (top) consists of cartilaginous bars, but the brain is mostly surrounded by a fibrous sheath underlain by the notochord. The skull of lampreys has a more elaborate braincase and comprises a large "branchial basket" surrounding the gills. In the gnathostomes, the braincase is generally closed.
The skull also encloses the brain, always comprising five parts referred to as the rhombencephalon, metencephalon, mesencephalon, diencephalon, and telencephalon. The metencephalon is developed into a cerebellum in the Gnathostomata and some fossil jawless vertebrates. The nerve fibres are primitively non-myelinated and become myelinated only in the gnathostomes. The brain is continued posteriorly by the spinal cord, which is ribbon-shaped but becomes thicker in the gnathostomes. As in cephalochordates, the dorsal (sensory) and ventral (motor) spinal nerves are initially separate, but unite in the gnathostomes. In all craniates, the olfactory (I), optic (II), trigeminal (V), facial (VII), acoustic (VIII), glossopharyngeal (IX) and vagus (X) cranial nerves are present. Additional cranial nerves, the oculomotor (III), trochlear (IV) and abducent (VI) nerves occur only in the Vertebrata. Some consider that the latter have been secondarily lost in hagfishes.
The olfactory organ opens into a median duct, the nasopharyngeal duct, which also serves the intake of the respiratory water. In most vertebrates, however, this duct becomes a blind tube and the intake of respiratory water is made through the mouth or the gill slits. The nasopharyngeal duct lies ventrally against the diencephalon and there, in ontogeny, induces the formation of an important gland, the hypophysis, or pituitary organ, which comprises neural (neurohypophysis) and glandular (adenohypophysis) parts. The adenohypophysis is particularly complex in the Vertebrata, but very simple in hagfishes.
Craniates possess a unique embryonic tissue, the neural crest, that appears dorsal and lateral to the neural tube and which contributes to a great variety of adult tissues and structures including: sensory neurons (nerve cells), some skeletal and connective tissues in the skull, and some pigment containing cells and other integumentary tissues. In the skull, the neural crest cells give rise to the gill arches, jaws and parts of the braincase floor. In the gnathostomes and a number of fossil jawless vertebrates, the neural crest cells are also involved in the formation of the dermal skeleton (scales, teeth, and dermal bones).
The gills of craniates comprise gill filaments, made up by primary and secondary gill lamellae which insure gas exchanges. In hagfishes, the gills have no skeletal support, and are enclosed in pouches connected to the pharynx. Among vertebrates, a similar structure occurs in adult lampreys only, but here skeletal supports (gill arches) are present. The gills are derived from tissues of the embryonic gut (endoderm), but cells from the embryonic skin (ectoderm) are involved in their formation in the gnathostomes. The respiratory water flow is ensured by a special pumping and anti-reflux organ, the velum, situated at the limit between the mouth and the pharynx. There is a theory that the jaws of the gnathostomes are derived from the velum.
As chordates, all craniates develop a notochord, which is primitively large (hagfishes, lampreys), but becomes transitory in most vertebrates and is replaced by elements of the vertebral column, the centra and arcualia.
All craniates (except most tetrapods) possess a caudal fin strengthened by a number of cartilaginous radials. In vertebrates appear dorsal and anal fins, as well as radial muscles which ensure undulatory movements of the fin web. In the gnathostomes and some fossil jawless vertebrates, there are paired pectoral fins. Only the gnathostomes possess both pectoral and pelvic fins, which are modified into locomotory limbs in tetrapods.
All craniates possess an endoskeleton, which is primitively cartilaginous but becomes mineralized in various ways (bone, calcified cartilage) in the vertebrates. Only the gnathostomes and a number of fossil jawless vertebrates possess a mineralized exoskeleton which develops in the skin tissues. The exoskeleton is made up by a variety of tissues (bone, dentine, enamel).
Craniates have a circulatory system of arteries, capillaries and veins, and a chambered, muscular main heart located ventrally and anteriorly in the trunk. In the Vertebrata, the circulatory system is entirely closed. The two heart chambers, the atrium and ventricle are well apart. There are additional accessory venous hearts in the head and tail, which help in venous blood circulation, but these are lost in the Vertebrata. In gill-breathing craniates, the heart pumps venous blood anteriorly into arteries and capillaries in the gills for gas (oxygen and carbon dioxide) exchange with water. Oxygenated blood then collects dorsal to the gills and flows anteriorly to the head and posteriorly to the organs and muscles, and back to the heart. In some Vertebrata (Osteichthyes) diverticles of the digestive tract (lungs or air bladder) supplements or replaces gills as the repiratory organ.
The digestive tract of craniates is longitudinally differentiated into mouth and oral cavity, pharynx, esophagus, intestine, rectum and anus. A stomach is developed in the Gnathostomata and some fossil jawless Vertebrates. All craniates have a pancreas that produces digestive enzymes and hormones (insulin and glucagon) that regulate blood sugar level. The pancreas was ancestrally disseminated along the anterior part of the gut, but becomes condensed into a well-defined organ in the Vertebrata.
All craniates and the related cephalochordates have a liver or hepatic organ that serves many functions including food storage and production of fat emulsifiers (bile).
The kidneys are the chief excretory organs of vertebrates and these organs play an important role in water and salt balance. Although kidneys vary greatly in size, shape and position among species, all contain nephrons as the basic functional units. Each nephron is a nearly microscopic tubule that receives a filtrate of blood (lacking blood cells and very large molecules). The filtrate is processed by selective secretion and reabsorption of materials to produce an excretory product (generally called urine) that contains nitrogenous waste and other materials. Long and complex kidney tubules occur only in the vertebrates.
The Vertebrata have all the characteristics of the Craniata but share, in addition, a number of unique characteristics which do not occur in hagfishes (Hyperotreti). These characteristics are:
Metamerically arranged endoskeletal elements flanking the spinal cord. There are primitively two pairs of such elements in each metamere and on each side: the interdorsals and basidorsals. In the gnathostomes, there are two additional pairs ventrally to the notochord: the interventrals and basiventrals. These elements are called arcualia and can fuse to a notochordal calcification, the centrum. The ensemble of the arcualia + centrum is the vertebra, and the ensemble of the vertebrae is the vertebral column. The vertebrates are characterized by a vertebral column; that is, a variable number of endoskeletal elements aligned along the notochord and flanking the spinal cord. In lampreys, the vertebral elements are only the basidorsal and the interdorsals . In the gnathostomes, there are in addition ventral elements, the basiventrals and interventrals , and the notochord may calcify into centra.
Extrinsic eye muscles. These muscles are attached to the eyeball and orbital wall, and ensure eye movements
Radial muscles in fins. These are small muscles associated with each of the cartilaginous radials of the unpaired and paired fins. They ensure the undulatory movements of the fin web.
Atrium and ventricle of heart closely-set.
Nervous regulation of heart. The heart in the embryo of the vertebrates is aneural, like the heart of adult hagfishes. In adult vertebrates, however, the heart is innervated by a branch of the vagus nerve.
Typhlosole in the intestine. This is a spirally coiled fold of the intestinal wall. In the Gnathostomes, it can be developed into a complex spiral valve.
At least two vertical semicircular canals in the labyrinth
True neuromasts in the sensory-line system
Myllokunmingia is a chordate from the Lower Cambrian Maotianshan shales of China, thought to be a vertebrate, although this is not conclusively proven. It is 28 mm long and 6 mm high.
It is among the oldest possible craniates, found in the lower Cambrian Chengjiang (524 million years ago). It appears to have a skull and skeletal structures made of cartilage. There is no sign of mineralization of the skeletal elements (biomineralization).
The holotype was found in the Yuanshan member of the Qiongzhusi Formation in the Eoredlichia Zone near Haikou at Ercaicun, Kunming City, Yunnan, China. The animal has a distinct head and trunk with a forward sail-like (1.5 mm) dorsal fin and a ventral fin fold (probably paired) further back. The head has five or six gill pouches with hemibranchs. There are 25 segments (myomeres) with rearward chevrons in the trunk. There is a notochord, a pharynx and digestive tract that may run all the way to the rear tip of the animal. The mouth cannot be clearly identified. There may be a pericardic cavity. There are no fin radials. There is only one specimen, which has the tip of the tail buried in sediment.
Gnathostomata is the group of vertebrates with jaws. The term derives from Greek γναθος (gnathos) "jaw" + στομα (stoma) "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates.
Gnathostomes are characterized by:
A vertically biting device called jaws, and which is primitively made up by two endoskeletal elements, the palatoquadrate and Meckelian cartilage, and a number of dermal elements called teeth, sometimes attached to large dermal bones. The skull of a gnathostome, or jawed vertebrates, are characterized by vertically biting jaws consisting of the palatoquadrate dorsally and the Meckelian cartilage ventrally. The gill arches are situated internally to the gill filaments, and the nasal capsules open to the exterior by means of paired nostrils.
The Meckelian Cartilage, also known as "Meckel's Cartilage", is a piece of cartilage from which the mandibles (lower jaws) of vertebrates evolved. Originally it was the lower of two cartilages which supported the first gill arch (nearest the front) in early fish. Then it grew longer and stronger, and acquired muscles capable of closing the developing jaw.
In early fish and in chondrichthyans (cartilaginous fish such as sharks, which are not primitive in any sense of the word), the Meckelian Cartilage continued to be the main component of the lower jaw. But in the adult forms of osteichthyans (bony fish) and their descendants (amphibians, reptiles, birds, mammals), the cartilage was covered in bone - although in their embryos the jaw initially develops as the Meckelian Cartilage. In all tetrapods the cartilage partially ossifies (changes to bone) at the rear end of the jaw and becomes the articular bone, which forms part of the jaw joint in all tetrapods except mammals
Pelvic fins. These are the paired fins or limbs situated just in front of the anus.
Interventrals and basiventrals in the backbone. These are the elements of the backbone which lie under the notochord, and match the basidorsals and interdorsals respectively.
Gill arches which lie internally to the gills and branchial blood vessels, contrary to the gill arches of all jawless craniates, which are external to the gills and blood vessels.
A horizontal semicircular canal in the inner ear
Paired nasal sacs which are independent from the hypophysial tube. In all extant and fossil jawless craniates, the nasal sacs, which contain the olfactory organs, open into a median duct, the nasohypophysial duct, which takes part to the formation of the pituitary gland and either leads postriorly to the pharynx (e.g. in hagfish and galeaspids) or ends as a blind pouch (e.g. in lampreys and osteostracans). In the gnathostomes, this pouch remains as a thin canal in the palate, the buccohypophysial canal, whereas the nasal sacs open separately to the exterior by external nostrils.
There are numerous other characteristics of the soft anatomy and physiology (e.g. myelinated nerve fibres, sperms passing through urinary ducts, etc.), which are unique to the gnathostomes among extant craniates, but cannot by observed in fossils.
Teleostomi is a clade of jawed vertebrates that includes the tetrapods, bony fish, and the wholly extinct acanthodian fish. Key characters of this group include an operculum and a single pair of respiratory openings, features which were lost or modified in some later representatives. The teleostomes include all jawed vertebrates except the chondrichthyans and the placodermi.
The clade Teleostomi should not be confused with the similar-sounding fish clade Teleostei.
Teleostomes have two major adaptations that relate to aquatic respiration. First, the early teleostomes probably had some type of operculum, however, it was not the one-piece affair of living fish. The development of a single respiratory opening seems to have been an important step. The second adaptation, the teleostomes also developed a primitive lung with the ability to use some atmospheric oxygen. This developed, in later species, into the lung and (later) the swim bladder, used to keep the fish at neutral buoyancy.
Climatius (meaning inclined fish or tilted fish) is an extinct genus of spiny shark. Fossils have been found in both Europe and North America.
Climatius was an active swimmer judging from its powerful caudal fin and abundant stabilizing fins, and probably preyed on other fish and crustaceans. Its lower jaw was lined with sharp teeth which were replaced when worn, but the upper jaw had no teeth. It also had large eyes, suggesting that it hunted by sight.
Although it was a small fish, at 7.5 centimetres (3 in), to discourage predators, Climatius sported a total of fifteen sharp spines. There was one spine each on the paired pelvic and pectoral fins, and on the aingle anal and two dorsal fins, and a further four pairs without fins on the fish's underside.
Euteleostomi is a successful clade that includes more than 90% of the living species of vertebrates. Euteleostomes are also known as "bony vertebrates". Both major subgroups are successful today: Actinopterygii includes the majority of extant fish species, and Sarcopterygii includes the tetrapods. This clade is sometimes called "Osteichthyes", but since that name literally means "bony fish" and traditionally is a paraphyletic group that excludes tetrapods, the name Euteleostomi was coined as a substitute.
Euteleostomes originally all had endochondral bone, fins with lepidotrichs, and jaws lined by maxillary, premaxillary, and dentary bones. Many of these characters have since been lost by descendant groups, however, such as lepidotrichs lost in tetrapods, and bone lost among the chondrostean fishes. The image shows on the left a sarcopterigyan fin and on the right an actinopterygian fin.
The Sarcopterygii or lobe-finned fishes (from Greek sarx=flesh, and pteryx=fin) sometimes considered synonymous with Crossopterygii ("fringe-finned fish", from Greek krossos=fringe) constitute a clade (traditionally a class or subclass) of the bony fishes, though a strict classification would include the terrestrial vertebrates. The living sarcopterygians are the coelacanths, lungfishes, and the tetrapods.
Early sarcopterygians or crossopterygians are bony fish with fleshy, lobed, paired fins, which are joined to the body by a single bone. The fins of crossopterygians differ from those of all other fish in that each is borne on a fleshy, lobelike, scaly stalk extending from the body. Pectoral and pelvic fins have articulations resembling those of tetrapod limbs. These fins evolved into legs of the first tetrapod land vertebrates, amphibians. They also possess two dorsal fins with separate bases, as opposed to the single dorsal fin of actinopterygians (ray-finned fish).
The braincase of sarcoptergygians primitively has a hinge line, but this is lost in tetrapods and lungfish. Many early sarcopterygians have a symmetrical tail. All sarcopterygians possess teeth covered with true enamel.
Taxonomists who subscribe to the cladistic approach include the grouping Tetrapoda within this group, which in turn consists of all species of four-limbed vertebrates. The fin-limbs of sarcopterygians such as the coelacanths show a strong similarity to the expected ancestral form of tetrapod limbs. Sarcopterygii e Tetrapodi basali (in italian).
? Onychodontiformes †
The only animals with choana are the tetrapoda, and they could as well be called Choanata (they are also the only ones with a vomeronasal organ, which has an embryonic origin from the olfactory structure).
These internal nasal passages evolved while the vertebrates still lived in water. At this point they already needed to gulp air to get enough oxygen, and rather than open their jaws each time to do this, those mutants who acquired small openings to breathe through were more successful at living in the new environment. Fish
Fish do not have choanae, instead they have two pairs of external nostrils: each with two tubes whose frontal openings lie close to the upper jaw, and the posterior openings further behind near the eyes. Whether choanae of tetrapods are homologous to the posterior nostrils or not has been debated. Reasons for dispute have been that the posterior nostril in its evolution into choanae would have to switch position relative to other anatomical features, i.e. a nerve. Recent paleontologiclal findings support homology: a 400-million-year-old fossil lobe-finned fish called Kenichthys campbelli has something between a choana and the external nostrils seen on other fish, which makes it look like it has a cleft palate or cleft lip. The reason seems to be that the posterior opening of the external nostrils has migrated into the mouth for some reason.
A similar evolution has taken place in lungfish. Here the inner nostrils have generally been accepted as homologous to the posterior nostrils, but the homology to true choanae as internal nostrils has been a matter of controversy. The fossil lungfish Diabolepis shows an intermediate stage between posterior and interior nostril and supports the independent origin of internal nostrils in the lungfish. Tetrapods
Similar migration is still seen in the tetrapod embryo, and can cause a baby to be born with a cleft palate. Why it should migrate is a mystery, since the nostrils would be useless as a breathing device before their final position inside the mouth. They could also already breathe air through their spiracles.
Tetrapods are also equipped with a lacrimal duct, or tear duct. How it evolved is not known, but it has an internal connection with the choana. It is possible that the choana started as a natural crack between maxilla and premaxilla because of an incomplete fusion in air-breathing animals. If this gap got wider and deeper with time, the frontal part of it would have to fuse together to avoid weakening the upper jaw, creating a small opening on the upper lip. Some more migrating, and this gap would meet the anterior pair of the external nasal openings. The posterior pair of the openings was then free to form the lacrimal duct if a migration caused them to come in contact with the eyes. Choanae analogues in other animals and fossils
This would not have been the first time the jaws evolved some sort of opening. For instance, snakes have evolved a cleft in the lower jaw, allowing them to stick out their tongues without having to open the jaw. For an animal living in water, the formation of a paired cleft on the upper jaw would be quite logical. Terrestrial vertebrates would in any case need a way to breathe without needing to open their jaws each time.
Some fossil species are said to have both conventional external nostrils and a choana, but only more fossils will give a real answer to how the choanas evolved. Lungfish and hagfishes
In addition to tetrapods, the lungfish has internal nostrils too. These seem to have a different origin than those of the tetrapods, and lungfish have no tear duct either.
Hagfishes have a single internal nostril that opens inside the mouth cavity, while Chimaerae have open canals that leads water from their external nostrils into their mouth and through their gills.
Tetrapodomorpha is a clade of vertebrates, consisting of tetrapods (four-limbed vertebrates) and their closest sarcopterygian relatives.
Advanced transitional fossils between fish and the early labyrinthodonts, like Tiktaalik, are called 'fishapods' by their discoverers. They are half-fish half-tetrapods, in appearance and limb morphology.
1. The stem group tetrapoda, extinct fossil relatives of the crown group. This is a paraphyletic unit covering the fish to tetrapod transition. Tetrapodomorpha contains several groups of related lobe-finned fishes, collectively known as the osteolepiforms.
2. The crown group tetrapods, the last common ancestor of living tetrapods and all of its descendants.
Among the characters defining tetrapodomorphs are modifications to the fins, notably a humerus with convex head articulating with the glenoid fossa (the socket of the shoulder joint).
Tetrapodomorph fossils are known from the early Devonian onwards, and include Tungsenia paradoxa, Kenichthys, Osteolepis, Eusthenopteron, Panderichthys and Tiktaalik.
Kenichthys was considered the most primitive member of Tetrapodomorpha. Kenichthys has a primitive choana, an opening on the palate that connects the nasal cavity and mouth, which would become an important adaptation for terrestrial life in later tetrapodomorphs. The migration of this opening would have blocked arteries in the palate, implying that blood vessels shifted position during evolution.
Some stem-tetrapod species are believed to have moved onto land about 370 million years ago and gradually evolved into the earliest terrestrial vertebrates and eventually humans.
Previously, there was a gap of at least 16 million years from the oldest fossil record of the lungfish lineage to the earliest known stem-tetrapod Kenichthys, a finned stem-tetrapod.
A new discovery pushes the fossil record of tetrapods back by some 10 million years and, as a result, the first appearance of the tetrapod superclass has been drawn far closer to the estimated time of the lungfish-tetrapod split.
The study further fills in the morphological gap between tetrapods and lungfish and unveils the evolutionary pattern of character changes during the initial diversification of stem-tetrapods, the report said.
In addition, an X-ray tomography study of the skull has provided new fossil evidence on the origin of the tetrapod brain.
A simulation of its brain conditions indicated that some important brain modifications related to terrestrial life occurred at the beginning of tetrapod evolution, much earlier than previously thought.
The newly discovered stem-tetrapod was named "Tungsenia paradoxa", a tribute to renowned Chinese geologist Liu Dongsheng (1917-2008).
Below is a cladogram modified from Ahlberg and Johanson (1998) "Osteolepiforms and the ancestry of tetrapods". Nature 395 (6704): 792-794
Osteolepiformes (or Osteolepidida) are a group of prehistoric lobe-finned fishes which appears first time during the Devonian period. The order contains five families: Canowindridae, Elpistostegidae, Megalichthyidae, Osteolepidae and Tristichopteridae. The superorder is generally considered to be paraphyletic because the characters that define it are mainly attributes of stem tetrapodomorphs.
Osteolepiform fish are thought to the ancestors of the tetrapods because of the structure of their paired fins and also because they may have had choanae (an intrabuccal opening possible posterior nostril (excurrent) possibly shared with the tetrapods. Despite there being numerous species, structurally they where quite homogeneous.
Gogonasus lived in the late Devonian period, on what was once a 1400 kilometre coral reef off the Kimberley coast surrounding the north-west of Australia. Gogonasus was a small fish reaching 30-40 cm (1 ft) in length.
Its skeleton shows several features that were like those of a four-legged land animal (tetrapod). They included the structure of its middle ear, and its fins show the precursors of the forearm bones, the radius and ulna. Researchers believe it used its forearm-like fins to dart out of the reef to catch prey.
The specimen (NMV P221807) shows some surprising new data not seen in any of the other specimens. Firstly, there were large spiracular openings on top of the skull, with a distinct down folded cosmine-covered lamina of bone present on the tabular bone. This indicated its spiracles were almost as large as in the elpistostegalian fishes (like Tiktaalik) and early tetrapods (e.g. Acanthostega). Secondly, after preparation of its pectoral fins, the internal limb skeleton showed closer resemblances to that of the elpistostegalians than to other more generalised tetrapodomorph fishes like Eusthenopteron. For almost 100 years Eusthenopteron had been the well-used role model for demonstrating stages in the evolution of lobe-finned fishes to tetrapods. Gogonasus now replaces Eusthenopteron in being a better preserved representative without any ambiguity in interpreting its anatomy.
Osteolepis ('bone scale') is an extinct genus of lobe-finned fish from the Devonian period. It lived in the Lake Orcadie of northern Scotland.
Osteolepis was about 20 centimetres (7.9 in) long, and covered with large, square scales. The scales and plates on its head were covered in a thin layer of spongy, bony material called cosmine. This layer contained canals which were connected to sensory cells deeper in the skin. These canals ended in pores on the surface, and were probably for sensing vibrations in the water.
Osteolepis was a rhipidistian, having a number of features in common with the tetrapods (land-dwelling vertebrates and their descendants), and was probably close to the base of the tetrapod family tree.
TheTtristichopterids were the most diverse and successful of the tetrapodomorph fishes throughout the Late Devonian, but were extinct by the end of the Famennian.
What is exceedingly odd about the disappearance of tristichopterids is that they are very similar to elpistostegalians, the immediate ancestors of Tetrapods. Almost all of the characters of Tristichopterids are shared by Elpistostegalians, whether by inheritance or convergence. One distinction: the tendency to reinforce the cranial arch anteriorly, rather than lighten the rostrum as in Panderichthys. This may be the only significant qualitative distinction.
The Tristichopteridae include the much celebrated Eusthenopteron foordi from Miguasha in Canada made famous by Jarvik, who spent almost quarter of a century studying it. Eusthenopteron has lost the cosmine of the osteolepids, has a somewhat diphyceral tail compared to epiceral tail of Osteolepis has more angular posteriorly placed dorsal fins and an elongated snout (although juveniles have a shorter snout).
The most notable features of Eusthenopteron are the powerfully built pectoral and pelvic fins. This led to early speculation that Eusthenopteron could use these fins to crawl out of the water and onto land. This in turn led to Eusthenopteron being classed by some as a link to the early tetrapods.
Today however, Eusthenopteron is more widely accepted to have stayed in the water, but developed the parts that would allow for the evolution of legs. Reinforcement for this view comes from the study of transitional fossils such as Tiktaalik, which seems to suggest that primitive legs would have evolved in creatures that were still primarily aquatic. These continuing adaptations including primitive fingers and leg joints for navigating dense weeds and shallow waters, also proved useful for terrestrial locomotion as well.
Eusthenopteron did still have some of the features that would become present in later terrestrial amphibians. The teeth displayed folded enamel like the labyrinthodonts, and it also had internal nostrils. The bones of the pectoral fins also display clear upper and lower portions with bones that are analogous to a humerus, ulna and radius. The pelvic fins also have a similar arrangement but the bones here would be the equivalent of a femur, tibia and fibula.
Elpistostegalia or Panderichthyida is an order of prehistoric lobe-finned fishes which lived during the Late Devonian period (about 385 to 374 million years ago). They represent the advanced tetrapodomorph stock, the fishes more closely related to tetrapods than the osteolepiform fishes. The elpistostegalians, combining fishlike and tetrapod-like characters, are sometimes called fishapods, a phrase coined for the advanced elpistostegalian Tiktaalik.
A rise in global oxygen content allowed for the evolution of large, predatory fish that were able to exploit the shallow tidal areas and swamplands as top predators. Several groups evolved to fill these niches, the most successful were the elpistiostegalians. In such environments, they would have been challenged by periodic oxygen deficiency. In comparable modern aquatic environments like shallow eutrophic lakes and swampland, modern lungfish and some genera of catfish also rely on the more stable, atmospheric source of oxygen.
Being shallow-water fishes, the elpistostegalians evolved many of the basic adaptions that later allowed the tetrapods to become terrestrial animals. The most important ones were the shift of main propulsion apparatus from the tailfin to the pectoral and pelvic fins, and a shift to reliance on lungs rather than gills as the main means of obtaining oxygen. Both of these appear to be a direct result of moving to an inland freshwater mode of living.
Professor Per Ahlberg has identified the following traits as synapomorphic for Elpistostegalia (and thus Tetrapoda):
The endocranium is hinged, the hinge forming the profundus nerve foramen. The cranial kinesis is also visible in the skull roof, between the parietal bones and the postparietal bones.
A rather small shoulder girdle.
The anal and posterior dorsal fin supported by a basal plate and three unjointed radials.
The pectoral fin skeleton composed of bones homologous to the tetrapod humerus, ulna and radius, followed by a host of smaller bones anchoring the fin rays, the pelvic fin skeleton similarly has femur, tibia and fibula.
Tiktaalik is a monospecific genus of extinct sarcopterygian (lobe-finned "fish") from the late Devonian period, with many features akin to those of tetrapods (four-legged animals). It is an example from several lines of ancient sarcopterygian "fish" developing adaptations to the oxygen-poor shallow-water habitats of its time, which led to the evolution of tetrapods. Well-preserved fossils were found in 2004 on Ellesmere Island in Nunavut, Canada.
Tiktaalik lived approximately 375 million years ago. Paleontologists suggest that it is representative of the transition between non-tetrapod vertebrates ("fish") such as Panderichthys, known from fossils 380 million years old, and early tetrapods such as Acanthostega and Ichthyostega, known from fossils about 365 million years old. Its mixture of primitive "fish" and derived tetrapod characteristics led one of its discoverers, Neil Shubin, to characterize Tiktaalik as a "fishapod".
Tiktaalik roseae is the only species classified under the genus. The name Tiktaalik is an Inuktitut word meaning "burbot", a freshwater fish related to true cod. The "fishapod" genus received this name after a suggestion by Inuit elders of Canada's Nunavut Territory, where the fossil was discovered.
Tiktaalik provide insights on the features of the extinct closest relatives of the tetrapods. Unlike many previous, more fishlike transitional fossils, the "fins" of Tiktaalik have basic wrist bones and simple rays reminiscent of fingers. The homology of distal elements is uncertain, but the proximal series can be directly compared to the ulnare and intermedium of tetrapods. The fin was clearly weight bearing, being attached to a massive shoulder with expanded scapular and coracoid elements and attached to the body armor, large muscular scars on the ventral surface of the humerus, and highly mobile distal joints. The bones of the fore fins show large muscle facets, suggesting that the fin was both muscular and had the ability to flex like a wrist joint. These wrist-like features would have helped anchor the creature to the bottom in fast moving current.
Also notable are the spiracles on the top of the head, which suggest the creature had primitive lungs as well as gills. This would have been useful in shallow water, where higher water temperature would lower oxygen content. This development may have led to the evolution of a more robust ribcage, a key evolutionary trait of land living creatures. The more robust ribcage of Tiktaalik would have helped support the animal's body any time it ventured outside a fully aquatic habitat. Tiktaalik also lacked a characteristic that most fishes have‹bony plates in the gill area that restrict lateral head movement. This makes Tiktaalik the earliest known fish to have a neck, with the pectoral girdle separate from the skull. This would give the creature more freedom in hunting prey either on land or in the shallows.
Tiktaalik is a transitional fossil; it is to tetrapods what Anchiornis is to birds, troodonts and dromaeosaurids. While it may be that neither is ancestor to any living animal, they serve as evidence that intermediates between very different types of vertebrates did once exist.
The mixture of both "fish" and tetrapod characteristics found in Tiktaalik include these traits:
half-fish, half-tetrapod limb bones and joints, including a functional wrist joint and radiating, fish-like fins instead of toes
half-fish, half-tetrapod ear region
tetrapod rib bones
tetrapod mobile neck with separate pectoral girdle
The ancestry of mammals
Sauropsids (dinosaurs - reptiles - birds)
Tetrapoda is a crown group including modern amphibians and amniotes, their most recent common ancestor and all of its descendants. The animals have obvious, separate digits (fingers and toes) and well-defined joints in their limbs. Such limbs are called chiridii (chiridium, singular) (Laurin, 1998). Non-tetrapod tetrapodomorphs also possess chiridii, but true tetrapods always possess five or fewer functional digits per limb. Interestingly, the name Tetrapoda means "four feet," yet snakes, whales, and limbless amphibians are tetrapods. These creatures have simply lost some or all of their limbs. Even when lost, the limbs are not lost uniformly. Cephalization is the tendency for the head and anatomical units close to the head to form early in ontogeny (life development) and to be well-developed. Cephalization is characteristic of vertebrates and would, therefore, create expectations for patterns in limb development of animals. In most animal embryos, the head and front appendages form first and the lower extremities appear later. This might cause one to assume that if a limb were going to be lost, it would be the hind limbs to be lost because the front limbs appear earlier in ontogeny. Indeed, this is the case in whales where the forelimbs are preserved and the hindlimbs represented by only a vestigal splint for the femur and (in some species) a splint for the femur. This tendency does not hold true, however, in snakes where the hindlimb is represented by a vestigal ilium in primitive forms, but the forelimb is completely absent.
Acanthostega (meaning spiny roof) is an extinct labyrinthodont genus, among the first vertebrate animals to have recognizable limbs. It appeared in the Upper Devonian (Famennian) about 365 million years ago, and was anatomically intermediate between lobe-finned fishes and the first tetrapods fully capable of coming onto land.
It had eight digits on each hand (the number of digits on the feet is unclear) linked by webbing, it lacked wrists, and was generally poorly adapted to come onto land. Acanthostega also had a remarkably fish-like shoulder and forelimb. The front foot of Acanthostega could not bend forward at the elbow, and thus could not be brought into a weight bearing position, appearing to be more suitable for paddling or for holding on to aquatic plants. It had lungs, but its ribs were too short to give support to its chest cavity out of water, and it also had gills which were internal and covered like those of fish, not external and naked like those of some modern amphibians which are almost wholly aquatic. Acanthostega is the first tetrapod to show the shift in locomotory dominance from the pectoral to pelvic girdle. There are many morphological changes that allowed the pelvic girdle of Acanthostega to become a weight-bearing structure. In more ancestral states the two sides of the girdle were not attached. In Acanthostega there in contact between the two sides and fusion of the girdle with the sacral rib of the vertebral column. These fusions would have made the pelvic region more powerful and equipped to counter the force of gravity when not supported by the buoyancy of an aquatic environment.
Ichthyostega (Greek: "fish roof") is an early tetrapod genus that lived at the end of the Upper Devonian period (Famennian age, 374 359 million years ago). It was a labyrinthodont, one of the first tetrapods in the fossil record. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps. Though undoubtedly of amphibian build and habit, it is not considered a true member of the group in the narrow sense, as the first true amphibians appeared in the Carboniferous period. Until finds of other early tetrapods and closely related fishes in the late 20th century, Ichthyostega stood alone as the transitional fossil between fish and tetrapods, combining a fishlike tail and gills with an amphibian skull and limbs.
Tulerpeton is a fossil of an extinct genus of Devonian labyrinthodont. This genus and the closely related Acanthostega and Ichthyostega genara represent the earliest tetrapods, though Tulerpeton has been suggested as the first reptiliomorph.
Tulerpeton is considered one of the first true tetrapods to have arisen. It is known from a fragmented skull, the left side of the pectoral girdle, and the entire right forelimb and right hindlimb along with a few belly scales. This species is differentiated from the less derived "aquatic tetrapods" (such as Acanthostega and Ichthyostega) by a strengthened limb structure. These limbs consist of 6 toes and fingers. Additionally, its limbs appear to have evolved for powerful paddling rather than walking.
The fossil fragments also indicate that its head was disconnected from the pectoral girdle. From the absence of the rough postbranchial lamina of the pectoral girdle, it has been determined that Tulerpeton had no gills and was therefore entirely dependent on breathing air.
Tulerpeton lived approximately 365 million years ago, in the Late Devonian period when the climate was fairly warm and there were no glaciers. Land had already been colonized by plants. During the Devonian period, the first truly terrestrial tetrapods the ancestors of present day reptiles, birds, amphibians and mammals - first began to appear.
Even though Tulerpeton breathed air, it lived mainly in shallow marine water. The Andreyevka fossil bed where it was discovered was at least 200 km from the nearest landmass during this era. The fossils of plants in the area tell us that the salinity of the waters where it lived fluctuated wildly, indicating that the waters were quite shallow. Because the bones of the neck and the pectoral girdle were disconnected, Tulerpeton could lift its head. Therefore, in shallow water, it had a considerable advantage over the other animals whose heads only moved side to side. The later land animals that descended from Tulerpeton's relatives needed this head flexion on land, but the condition probably evolved because of the advantage that this gave it in shallow marine waters, not for land. In the book "Vertebrate Life", authors Pough, Janis, and Heiser say that," The development of a distinct neck, with the loss of the opercular bones and the later gain of a specialized articulation between the skull and the vertebral column (not yet present in the earliest tetrapods), may be related to lifting the snout out of the water to breath [sic?] air or to snap at prey items." The six fingered hands and toes were stronger than the fins from which they developed, therefore "tulerpeton" had an advantage in propelling itself through shallow and brackish water, but the limbs do not yet seem strong enough for extensive use on land.
Tulerpeton is one of the early transition tetrapods a marine animal capable of living on land. The separation of the pectoral-shoulder girdle from the head allowed the head to move up and down, and the strengthening of the legs and arms allowed the early tetrapods to propel themselves on land.
Crassigyrinus (meaning "thick tadpole") is an extinct genus of carnivorous stem tetrapod from the Early Carboniferous
Crassigyrinus is taxonomically enigmatic, having confused paleontologists for decades with its apparent fish-like and tetrapod features. It was traditionally placed within the group Labyrinthodontia along with many other early tetrapods. Some paleontologists have even considered it as the most basal Crown group tetrapod, while others hesitate to even place it within the Tetrapoda superclass. Crassigyrinus had unusually large jaws, enabling it to eat other animals it could catch and swallow. It had two rows of sharp teeth in its jaws, the second row having a pair of fangs. Crassigyrinus had large eyes, suggesting that it was either nocturnal, or lived in very murky water.
Its peculiar stunted forelimbs were tiny and the humerus was only 35 mm long (the whole animal was about 1.5 m long). Various foramina on the humeral surfaces are very similar to those seen in Ichthyostega, Acanthostega, and lobe-finned fishes like Eusthenopteron. The hindlimbs were much larger than the forelimbs, and in the pelvis the ilium lacked a bony connection to the vertebral column (a classic feature of aquatic tetrapods). The tail only known from a few vertebrae fragments, is assumed to have been long and laterally compressed.
Pederpes ('Peter's Foot') is an extinct genus of early Carboniferous tetrapod, dating from the Tournaisian age (lower Mississippian, 359 - 345 Ma). Pederpes contains one species, P. finneyae.
This most basal Carboniferous tetrapod had a large, somewhat triangular head, similar to that of later American sister-genus Whatcheeria, from which it is distinguished by various skeletal features, such as a spike-like latissimus dorsi (an arm muscle) attachment on the humerus and several minor skull features. The feet had characteristics that distinguished it from the paddle-like feet of the Devonian Ichthyostegalia and resembled the feet of later, more terrestrially adapted Carboniferous forms. Pederpes is the earliest-known tetrapod to show the beginnings of terrestrial locomotion and despite the probable presence of a sixth digit on the forelimbs it was at least functionally pentadactyl.
Pederpes was discovered in 1971 in central Scotland and classified as a lobe-finned fish and It was not until 2002 that Jennifer Clack named and reclassified the fossil as a primitive tetrapod.
Pederpes is an important fossil because it comes from the period of time known as Romer's Gap and provides biologists with rare information about the development of tetrapods in a time where terrestrial life was rare. Pederpes was 1 m long, making it average-sized for an early tetrapod.
The shape of the skull and the fact that the feet face forward rather than outward indicate that Pederpes was well adapted to land life. It is currently the earliest known fully terrestrial animal, although the structure of the ear shows that its hearing was still much more functional underwater than on land, and may have spent much of its time in the water and could have hunted there.
The narrow skull suggests that Pederpes breathed by inhaling with a muscular action like most modern tetrapods, rather than by pumping air into the lungs with a throat pouch the way many modern amphibians do.
From Eusthenopteron to Pederpes
Reptiliomorpha refers to an order or subclass of reptile-like amphibians, which gave rise to the amniotes in the Carboniferous. Under phylogenetic nomenclature, the Reptiliomorpha includes their amniote descendants though, even in phylogenetic nomenclature, the name is mostly used when referring to the non-amniote reptile-like labyrinthodont grade. An alternative name, Anthracosauria is commonly used for the group, but is confusingly also used for the "lower" grade of reptiliomorphs.
During the Carboniferous and Permian periods, tetrapods evolved along a number of parallel lines towards a reptilian condition. Some of these tetrapods (e.g. Archeria, Eogyrinus) were elongate, eel-like aquatic forms with diminutive limbs, while others (e.g. Seymouria, Solenodonsaurus, Diadectes, Limnoscelis) were so reptile-like that until quite recently they actually had been considered true reptiles, and it is likely that to a modern observer they would have appeared as large to medium-sized, heavy-set lizards. Several groups however remained aquatic or semiaquatic. The two most terrestrially adapted groups were the medium sized insectivorous or carnivorous Seymouriamorpha and the mainly herbivorous Diadectomorpha, with many large forms. The latter group is in most analysis the closest relatives of the Amniotes.
The following cladogram simplified from:
Marcello Ruta, Michael I. Coates and Donald L. J. Quicke (2003). "Early tetrapod relationships revisited". Biological Reviews 78 (2): 251-345.
Crown group Tetrapoda
Temnospondyli + Lissamphibia
Diadectes (now Silvadectes)
Caerorhachis (meaning "suitable spine" in Greek) is an extinct genus of early tetrapod from the Early Carboniferous of Scotland. Its placement within Tetrapoda is uncertain, but it is generally regarded as a primitive member of the group. Caerorhachis has usually been placed as a basal anthracosaur or a close relative of anthracosaurs. In this classification, Caerorhachis is a close ancestor of amniotes, or tetrapods that lay eggs on land. Caerorhachis has also been classified as the sister taxon of temnospondyls, a large group of extinct amphibians, based on the presence of several primitive traits. The vertebrae of Caerorhachis are more similar to anthracosaurs, however. Caerorhachis is thought to have had a primarily terrestrial lifestyle. It lacks the lateral lines across the skull that served as an adaptation for earlier aquatic tetrapods and their ancestors. The large, well developed limbs suggest it was able to move on land better than other early tetrapods like colosteids and baphetids. Robert Holmes and Robert L. Carroll, the first to describe Caerorhachis, interpreted it as "[an] animal spending much of its life in the damp mud on the margins of ponds or streams, feeding on stranded fish, or occasionally venturing into the water to catch aquatic larvae of other amphibians.
Seymouria was a reptile-like labyrinthodont from the early Permian of North America and Europe (approximately 280 to 270 million years ago). It was small, only 2 ft (60 cm) long. The dry climate of the Permian suited reptiles better than amphibians, but Seymouria had many reptilian features that helped it in this harsh environment. It had long and muscular legs, and may have had dry skin and the ability to conserve water. It may have been able to excrete excess salt from its blood through a gland in its nose, like modern reptiles. All of this meant that Seymouria, unlike amphibians and other early tetrapods, might have lived for extended periods of time away from water. If so, this would have allowed it to move about the landscape in search of insects, small amphibians, and other possible preys, such as the eggs of reptiles. Male Seymouria had thick skulls that may have been used to batter rivals in mating contests. After mating, the females would have had to return to water to lay their eggs. As in amphibians, the larvae would develop in water, hunting for worms and insects until they were strong enough to live on land.
From aquatic to terrestrial eggs
Their terrestrial life style combined with the need to return to the water to lay eggs hatching to larvae (tadpoles) lead to a drive to abandon the larval stage and aquatic eggs. A possible reason may have been competition for breeding ponds, to exploit drier environments with less access to open water, or to avoid predation on tadpoles by fish, a problem still plaguing modern amphibians. Whatever the reason, the drive led to internal fertilization and direct development (completing the tadpole stage within the egg). A striking parallel can be seen in the frog family Leptodactylidae, which has a very diverse reproductive system, including foam nests, non-feeding terrestrial tadpoles and direct development. The Diadectomorphans generally being large animals would have had correspondingly large eggs, unable to survive on land.
Fully terrestrial life was achieved with the development of the amniote egg, where a number of membranous sacks protect the embryo and facilitating gas exchange between the egg and the atmosphere. The first to evolve was probably the allantois, a sack that develops from the gut/yolk-sack. This sack contains the embryo's nitrogenous waste (urea) during development, stopping it from poisoning the embryo. A very small allantois is found in modern amphibians. Later came the amnion surrounding the fetus proper, and the chorion, encompassing the amnion, allantois, and yolk-sack.
Solenodonsaurus is an extinct genus of Reptiliomorpha, which lived about 320-305 million years ago. Classification is uncertain, but it was possibly an early reptile or an amphibian close to the diadectomorphs.
Solenodonsaurus was likely best adapted to life on land, as opposed to living in an aquatic environment like many other early tetrapods. The limbs and pelvis are incomplete in all known specimens of Solenodonsaurus, making it difficult to infer how the animal may have moved. One feature that suggests a terrestrial lifestyle is the 90° rotation of the ends of the humerus, which orients the forelimb forward rather than out to the side. Several presumably terrestrial groups of Paleozoic tetrapods, including amphibamid temnospondyls, microsaurs, and the first amniotes, have a similar degree of rotation in their humeri. The short, triangular shape of the skull of Solenodonsaurus distinguishes it from most aquatic forms, which have either long and narrow or broad and parabolic heads.
Diadectomorpha are a clade of large reptile-like amphibians that lived in Euramerica during the Carboniferous and Early Permian periods, and are very close to the ancestry of the Amniota. They include both large (up to 2 meters long) carnivorous and even larger (to 3 meters) herbivorous forms, some semi-aquatic and others fully terrestrial.
Diadectomorphs possess both amphibian and reptilian characteristics. Originally these animals were included under the order Cotylosauria, and were considered the most primitive and ancestral lineage of reptiles. More recently they have been reclassified as amphibian-grade tetrapods, closely related to the first true amniotes. Contrary to other Reptiliomorph amphibians, the teeth of the Diadectomorpha lacked the infolding of the dentine and enamel that account for the name Labyrinthodontia for the non-amniote tetrapodes.
Limnoscelis is a genus of large (1.5 m in total length), very reptile-like diadectomorph (a type of reptile-like amphibian) from the Early Permian of North America. Contrary to other diadectomorphans, Limnoscelis appear to have been a carnivore. Though the post cranial skeleton is very similar to the early large bodied reptiles like pelycosaurs and pareiasaurs, the digits lacked claws, and the bones of the ankle bones were fused like in other reptile-like amphibians. This would not allow them to use their feet actively in traction, but rather as holdfasts, indicating Limnoscelis primarily hunted slow moving prey.
The reproduction of the Diadectomorphs has been the matter of some debate. If their group lay within the Amniota as has at times been assumed, they would have laid an early version of the amniote egg. Current thinking favours the amniote egg being evolved in very small animals, like Westlothiana or Casineria, leaving the bulky Diadectomorphs just on the amphibian side of the divide.
This would indicate the large and bulky Diadectomorphs had non-terrestrial anamniote eggs. However, no clearly diadectomorph tadpole is known. Whether this is due to an actual lack of tadpole stage or taphonomy (many diadectomorphs were upland creatures where tadpoles would have a poor probability of being fossilized) is uncertain. Alfred Romer indicated that the anamniote/amniote divide may not has been very sharp, leaving the question of the actual mode of reproduction of these large animals unanswered. Possible reproductive modes include full amphibian spawning with aquatic tadpoles, internal fertilization with or without ovoviviparity, aquatic eggs with direct development or some combination of these. The reproductive mode may also have varied within the group.
Diadectes (meaning crosswise-biter) is an extinct genus of large, very reptile-like amphibians that lived during the early Permian period (Cisuralian - Guadalupian epochs, between 299 and 271 million years ago). Diadectes was one of the very first herbivorous tetrapods, and also one of the first fully terrestrial animals to attain large size. Diadectes was a heavily built animal, 1.5 to 3 meters long, with a thick-boned skull, heavy vertebrae and ribs, massive limb girdles and short, robust limbs. The nature of the limbs and vertebrae clearly indicate a terrestrial animal. It possesses some characteristics of reptilians and amphibians, combining a reptile-like skeleton with a more primitive, seymouriamorph-like skull. Diadectes has been classified as belonging to the sister group of the amniotes. Among its primitive features, Diadectes has a large otic notch (a feature found in all labyrinthodonts, but not in reptiles) with an ossified tympanum. At the same time its teeth show advanced specialisations for an herbivorous diet that are not found in any other type of early Permian animal. The eight front teeth are spatulate and peg-like, and served as incisors that were used to nip off mouthfuls of vegetation. The broad, blunt cheek teeth show extensive wear associated with occlusion, and would have functioned as molars, grinding up the food. It also had a partial secondary palate, which meant it could chew its food and breathe at the same time, something many even more advanced reptiles were unable to do.
These traits are likely adaptations related to the animals' high-fiber herbivorous diet, and evolved independently of similar traits seen in some reptilian groups.
Westlothiana is a genus of reptile-like amphibian or possibly early reptile that bore a superficial resemblance to modern-day lizards. It lived during the Carboniferous period, about 350 million years ago. It is known from a single species, Westlothiana lizziae. Westlothiana's anatomy contained a mixtures of both labyrinthodont and reptilian features, and was originally regarded as the first reptile. Most scientists place them among the Reptilomorpha, as a sister group to the first amniotes. This species probably lived near a freshwater lake, probably hunting for other small creatures that lived in the same habitat. It was a slender animal, with rather small legs and a long tail. Together with Casineria, another transitional fossil found in Scotland, it is one of the smallest reptile-like amphibians known, being a mere 20 cm in adult length. The small size has made it a key fossil in the search for the earliest amniote, as amniote eggs are thought to have evolved in very small animals. Advanced features that ties it in with the reptilian rather than amphibian group is unfused ankle bones, lack of labyrinthodont infolding of the dentin, a lack of an otic notch and a generally small skull.
The phylogenetic placement of Westlothiana has varied from basal amniote (i.e. a primitive reptile) to a basal Lepospondyl, in an analysis with the lepospondyls branching of from within Reptiliomorpha. The actual phyllogenetic position of Westlothiania is uncertain, reflecting both the fragmentary nature of the find and the uncertainty of labyrinthodont phylogeny in general.
Origin of amniotes
Exactly where the border between reptile-like amphibians (non-amniote reptiliomorphs) and amniotes lies will probably never be known, as the reproductive structures involved fossilize poorly, but various small, advanced reptiliomorphs have been suggested as the first true amniotes, including Solenodonsaurus, Casineria and Westlothiana. Such small animals lay small eggs, 1 cm in diameter or less. Such eggs will have a small enough volume to surface ratio to be able to develop on land without the amnion and chorin actively effecting gas exchange, setting the stage for the evolution of true amniotic eggs.
Although the first amniote probably appeared as early as the latest Mississippian period (Middle Carboniferous), non-amniote (or amphibian) reptiliomorphs continued to flourish alongside their amniote descendants for many millions of years. By the middle Permian the non-amniote terrestrial forms had died out, but several aquatic non-amniote groups continued to the end of the Permain, and in the case of the Chroniosuchids survived the end Permian mass extinction, only to die out at the end of the Early Triassic. Meanwhile, the single most successful daughter-clade of the reptiliomorphs, the amniotes, continued to flourish and to inherit the Earth.
The amniotes are a group of tetrapods (four-limbed animals with backbones or spinal columns) that have a terrestrially adapted egg. They include synapsids (mammals along with their extinct kin) and sauropsids (reptiles and birds), as well as their fossil ancestors.
Many amniote synapomorphies are widely interpreted as adaptations to the rigors of life on land. Indeed Amniota owes its name to what may be its most distinctive attribute, alarge and hard-shelled "amniotic" egg which possesses of a unique set of membranes: amnion, chorion, and allantois. The amnion surrounds the embryo and creates a fluid-filled cavity in which the embryo develops. The chorion forms a protective membrane around the egg. The allantois is closely applied against the chorion, where it performs gas exchange and stores metabolic wastes (and becomes the urinary bladder in the adult).
As in other vertebrates, nutrients for the developing embryo are stored in theyolk sac, which is much larger in amniotes than in vertebrates generally. Hatchling amniotes also possess an egg-tooth and horny caruncle on the snout tip to facilitate exit from their hard-shelled eggs. The amniotic egg, together with a penis for internal fertilization, loss of a free-living larval stage in the life cycle, and the ability to bury their eggs, enabled amniotes to escape the bonds that confined their ancestors' reproductive activities to aquatic environments.
Some components of the amniotic egg have been variously modified within Amniota. Placental mammals, for example, have suppressed the egg shell and yolk sac, and elaborated the amniotic membranes to enable nutrients and wastes to pass directly between mother and embryo.
Development of extra embryonic membranes in an amniote egg (chick). In this early developmental stage, the yolk sac is expanding over the yolk. The amnion and chorion are expanding over the embryo and will eventually form the amniotic chamber. The allantois is expanding toward the chorion, with which it will form a respiratory membrane, in addition to storing metabolic wastes of the embryo.
The comparative aridity of the terrestrial environment affects all aspects of amniote biology, and not just their reproductive systems. Thus, amniotes have highly keratinized skins that are relatively impervious and reduce water loss. They also possess horny nails that, among other things, enable them to use their forelimbs to dig burrows into which they can retreat during the heat of the day.
The imperative to reduce water loss is equally evident in the density of renal tubules in the metanephric kidney of amniotes, in the larger size of their water-resorbing large intestines, and in the full differentiation of the Harderian and lacrimal glands in the eye socket whose antibacterial secretions help to moisten and, along with a third eyelid (the nictitans), to further protect the eye from desiccation.
The commitment of amniotes to a life on land is also revealed by anextensive system of muscle stretch receptors that enables finer coordination and greater agility during locomotion, their enlarged lungs (which are the only remaining organs of gas exchange owing to the loss of gills), and the complete loss of the lateral line system other vertebrates use to detect motion in water.
Many of these features are rarely preserved in fossils, but there are some novelties in the skeleton that are no less diagnostic of amniotes. For example, amniotes have at least two pairs of sacral ribs, instead of just one pair. They also have an astragalus bone in the ankle, instead of separate tibiale, intermedium, and proximal centrale bones. Finally, they have paired spinal accessory (11th) and hypoglossal (12th) cranial nerves incorporated into the skull, in addition to the ten pairs of cranial nerves present in amphibians.
Synapsida is one of two great branches on the amniote family tree. This is the branch that includes us. Synapsids (Greek, 'fused arch'), synonymous with theropsids (Greek, 'beast-face'), are a group of animals that includes mammals and every animal more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having a temporal fenestra, an opening low in the skull roof behind each eye, leaving a bony arch beneath each; this accounts for their name. Primitive synapsids are usually called pelycosaurs; more advanced mammal-like ones, therapsids. The non-mammalian members are described as mammal-like reptiles in classical systematics, but are referred to as "stem mammals" (or sometimes "protomammals") under cladistic terminology. Synapsids evolved from basal amniotes and are one of the two major groups of the later amniotes; the other is the sauropsids, a group that includes modern reptiles and birds. Their distinctive temporal fenestra developed in the ancestral synapsid about 324 million years ago (mya) during the late Carboniferous period.
Synapsids were the largest terrestrial vertebrates in the Permian period, 299 to 251 million years ago. As with almost all groups then extant, their numbers and variety were severely reduced by the Permian-Triassic extinction. Though some species survived into the Triassic period, archosaurs became the largest and most numerous land vertebrates in the course of this period. Few of the nonmammalian synapsids outlasted the Triassic, although survivors persisted into the Cretaceous. However, as a phylogenetic unit, they included the mammals as descendants, and in this sense synapsids are still very much a living group of vertebrates. After the Cretaceous-Paleogene extinction event, the synapsids (in the form of mammals) again became the largest land animals.
Characteristics Temporal openings
Synapsids evolved a temporal fenestra behind each eye orbit on the lateral surface of the skull. It may have evolved to provide new attachment sites for jaw muscles. A similar development took place in the Diapsids, which evolved two rather than one opening behind each eye. Originally, the opening in the skull left the inner cranium only covered by the jaw muscles, but in higher therapsids and mammals, the sphenoid bone has expanded to close the opening. This has left the lower margin of the opening as an arch extending from the lower edges of the braincase.
Synapsids are characterized by having differentiated teeth. These include the canines, molars, and incisors. The trend towards differentiation is found in some labyrinthodonts and early anapsid reptilians in the form of enlargement of the first teeth on the maxilla, forming a form of protocanines. This trait was subsequently lost in the Sauropsid line, but developed further in the synapsids. Early synapsids could have two or even three enlarged "canines", but in the therapsids, the pattern had settled to one canine in each upper jaw half. The lower canines developed later. Jaw
Most paleontologists hold fossilized jaw remains to be the distinguishing feature used to classify synapsids and reptiles. The jaw transition is a good classification tool, as most other fossilized features that make a chronological progression from a reptile-like to a mammalian condition follow the progression of the jaw transition. The mandible, or lower jaw, consists of a single, tooth-bearing bone in mammals (the dentary), whereas the lower jaw of modern and prehistoric reptiles consists of a conglomeration of smaller bones (including the dentary, articular, and others). As they evolved in synapsids, these jaw bones were reduced in size and either lost or, in the case of the articular, gradually moved into the ear, forming one of the middle ear bones: while mammals possess the malleus, incus and stapes, mammal-like reptiles (like all other tetrapods) possess only a stapes. The malleus is derived from the articular (a lower jaw bone) while the incus is derived from the quadrate (a skull bone).
Mammalian jaw structures are also set apart by the dentary-squamosal jaw joint. In this form of jaw joint, the dentary forms a connection with a depression in the squamosal known as the glenoid cavity. In contrast, all other jawed vertebrates, including reptiles and nonmammalian synapsids, possess a jaw joint in which one of the smaller bones of the lower jaw, the articular, makes a connection with a bone of the skull called the quadrate bone to form the articular-quadrate jaw joint. In forms transitional to mammals, the jaw joint is composed of a large, lower jaw bone (similar to the dentary found in mammals) that does not connect to the squamosal, but connects to the quadrate with a receding articular bone. Palate
Over time, as synapsids became more mammalian and less 'reptilian', they began to develop a secondary palate, separating the mouth and nasal cavity. In early synapsids, a secondary palate began to form on the sides of the maxilla, still leaving the mouth and nostril connected.
Eventually, the two sides of the palate began to curve together, forming a U-shape instead of a C-shape. The palate also began to extend back toward the throat, securing the entire mouth and creating a full palatine bone. The maxilla is also closed completely. In fossils of one of the first eutheriodonts, the beginnings of a palate are clearly visible. The later Thrinaxodon has a full and completely closed palate, forming a clear progression. Skin
The actual skin of the synapsids has been subject to some discussion. Basal reptilian skin is rather thin, and lacks the thick dermal layer that produces leather in mammals. Exposed parts of reptiles are protected by horny scales or scutes. Mammal hide has a thick, fibrous dermis and rarely exhibits scutes. A hallmark of mammals is the presence of copious glands and hair follicles.
Among the pelycosaurs, only two species of small varanopids have been found to possess scutes; fossilized rows of osteoderms indicate horny armour on the neck and back, and skin impressions indicate some retained rectangular scutes on their undersides. The pelycosaur scutes probably were nonoverlapping dermal structures with a horny overlay, like those found in modern crocodiles and turtles. These differed in structure from the scales of lizards and snakes, which are an epidermal feature (like mammalian hair or avian feathers). The remaining upper surface of the pelycosaurs may have borne scutes, too, or may have been glandular and leathery like that of a mammal.
It is currently unknown at what stage the synapsids acquired mammalian characteristics such as body hair and mammary glands, as the fossils only rarely provide direct evidence for soft tissues. An exceptionally well-preserved skull of Estemmenosuchus, a therapsid from the Upper Permian shows smooth, hairless skin with what appears to be glandular depressions. The oldest known fossil showing unambiguous imprints of hair is the Callovian (late middle Jurassic) Castorocauda, an early mammal. The more advanced therapsids could have had a combination of naked skin, whiskers and scutes. A full pelage likely did not evolve until the therapsid-mammal transition. The more advanced, smaller therapsids could have had a combination of hair and scutes, a combination still found in some modern mammals, such as rodents and the opossum. Metabolism
Sail-back pelycosaurs like Edaphosaurus indicate an early trend toward temperature regulation in synapsids.
The first pelycosaurs had the usual reptilian cold-blooded metabolism by all indications, including a sprawling gait and a low slung body. However, there appears to have been an early trend towards a form of temperature regulation in several pelycosaur lines, as indicated by the large "sails" in both edaphosaurids and sphenacodontids (e.g. Dimetrodon).
The sphenacodontids gave rise to the therapsids, which may have inherited the temperature regulation. The legs and feet of the early therapsid groups point to a more erect posture, traditionally interpreted as a sign of more efficient metabolism. The presence of large turbinatae acting as moisture traps in the nasal passage found in therapsids, but not in pelycosaurs, confirms the shift in metabolism. None of them shows any sign of a sail, indicating any temperature regulation would have relied on the creatures' own metabolism rather than external heat. In the later cynodonts, the presence of a secondary palate, erect posture and other indicators of high metabolic rate suggests many mammalian features had evolved by this stage. Being rather sizeable creatures, the balmy temperatures of the Triassic likely did allow them to rely on inertial homothermy to keep temperatures steady. The high metabolism of the advanced forms only forced the evolution of hair when mouse-sized animals evolved in the synapsid-mammal transition.
Cotylorhynchus was a very large synapsid that lived in the southern part of what is now North America during the Early Permian period and persisted until the late-Mid Permian (about 265 mya). Cotylorhynchus are the most well known of the synapsid clade Caseidae and are in the order Pelycosauria.
The Cotylorhynchus were among the largest pelycosaurs, the largest terrestrial vertebrates of their time, as well as the terrestrial vertebrates up to that time and the largest non-mammalian synapsid that ever lived. They were herbivores, and because of their enormous size they probably did not fear any of the carnivores.
Cotylorhynchus was a massively built animal with a disproportionately small head and a huge barrel-shaped body. They could grow to lengths of up to 20 feet and could achieve weights of up to 2 tons. Their skeletal features included a massive scapulocoracoid, humeri with large flared ends, stout forearm bones and broad, robust hands that had large claws. Certain features of their hands indicate that they had to dig considerably to obtain their food supply and also they may have used these features to dig burrows for shelter or safety. Their digits were believed to have a considerable range of motion and large retractor processes on the ventral surfaces of the unguals allowed them to flex their claws with powerful motions. Also, the articulatory surfaces of their phalanges were oblique to the bone's long axis rather than perpendicular to it. This allowed for much more surface area for the flexor muscles.
Their skulls are distinctive in the presence of large temporal openings and very large nostril openings, which could have been utilized for better breathing or may have housed some sort of sensory or moisture conserving organ. Also they featured large pineal openings and a snout or upper jaw that overhangs the row of teeth to form a projecting rostrum. Rounded deep pits and possibly large depressions were present on the outer surface of the skull. Their teeth were very similar to those of iguanas with posterior marginal teeth that bore a longitudinal row of cusps.
The Eupelycosauria originally referred to a suborder of 'pelycosaurs' (Reisz 1987), but has been redefined (Laurin and Reisz 1997) to designate a clade of synapsids that includes most pelycosaurs, as well as all therapsids and mammals. They first appear during the Early Pennsylvanian epoch (i.e: Archaeothyris, and perhaps an even earlier genus, Protoclepsydrops), and represent just one of the many stages in the acquiring of mammal-like characteristics (Kemp 1982), in contrast to their earlier amniote ancestors.
Many non-therapsid Eupelycosaurs were the dominant land animals from the latest Carboniferous to the end of the early Permian epoch. Ophiacodontids were common since their appearance, from late Carboniferous (Pennsylvanian) to early Permian, but they became progressively smaller as early Permian went by. The Edaphosaurids, along with the Caseids, were the dominant herbivores in the early part of Permian, ranging from the size of a pig to the size of rhinoceroses. The most renowned Edaphosaurid is Edaphosaurus, a large [1012-foot-long (3.03.7 m)] herbivore which had a sail on its back, probably used for regulating heat and mating. Sphenacodontids, a family of carnivorous eupelycosaurs, included the famous Dimetrodon, which is sometimes mistaken for a dinosaur, and was the largest predator of the period. Like Edaphosaurus, Dimetrodon also had a distinctive sail on its back, and it probably served the same purpose - regulating heat. The Varanopseid family somewhat resembled today's monitor lizards and may have had the same lifestyle.
Therapsids descended from a clade closely related to the Sphenacodontids. They became the succeeding dominant land animals for the rest of the Permian and in the later part of the Triassic, therapsids gave rise to the first mammals. All non-therapsid pelycosaurs, as well as many other life forms, became extinct at the end of Permian period.
Archaeothyris is an extinct genus of ophiacodontid synapsid that lived during the Late Carboniferous and is known from Nova Scotia. Dated to 306 million years ago, Archaeothyris is the oldest undisputed synapsid known. Archaeothyris was relatively large, measuring 50 centimetres (20 in) head to tail. It was also more advanced than the early sauropsids, having strong jaws that could open wider than those of the early reptiles. While its sharp teeth were all of the same shape, it did possess a pair of enlarged canines, suggesting that it was a carnivore.
Edaphosaurus (meaning "pavement lizard" for dense clusters of teeth) is a genus of extinct edaphosaurid synapsid that lived around 303 to 275 million years ago, during the Late Carboniferous to Early Permian periods. The American paleontologist Edward Drinker Cope first described Edaphosaurus in 1882, naming it for the "dental pavement" on both the upper and lower jaws, from the Greek edaphos/εδαφος ("ground"; also "pavement") and σαυρσς/sauros ("lizard").
Edaphosaurus is important as one of the earliest known large plant-eating (herbivorous) amniote tetrapods (four-legged land-living vertebrates). In addition to the large tooth plates in its jaws, the most characteristic feature of Edaphosaurus is a sail on its back. A number of other synapsids from the same time period also have tall dorsal sails, most famously the large apex predator Dimetrodon. However, the sail on Edaphosaurus is different in shape and morphology.
The name Edaphosaurus, meant as "pavement lizard", is often translated inaccurately as "earth lizard," "ground lizard," or "foundation lizard" based on other meanings for Greek edaphos such as "soil, earth, ground, land, base" used in Neo-Latin scientific nomenclature (Edaphology). However, older names in paleontology such as Edaphodon Buckland, 1838 "pavement tooth" (a fossil fish) match Cope's clearly intended meaning "pavement" for Greek edaphos in reference to the animal's teeth.
The head of Edaphosaurus was short, relatively broad, triangular in outline, and remarkably small compared to its body size. The deep lower jaw likely had powerful muscles and the marginal teeth along the front and sides of its jaws had serrated tips, helping Edaphosaurus to crop bite-sized pieces from tough terrestrial plants. Back parts of the roof of the mouth and the inside of the lower jaw held dense batteries of peglike teeth, forming a broad crushing and grinding surface on each side above and below. Its jaw movements were propalinal (front to back).
Early descriptions suggested that Edaphosaurus fed on invertebrates such as mollusks, which it would have crushed with its tooth plates. However, paleontologists now think that Edaphosaurus ate plants, although tooth-on-tooth wear between its upper and lower tooth plates, indicates only "limited processing of food" compared to other early plant-eaters such as Diadectes, a large non-amniote reptiliomorph (Diadectidae) that lived at the same time. Early members of the Edaphosauridae such as Ianthasaurus lacked tooth plates and ate insects.
The sail along the back of Edaphosaurus was supported by hugely elongated neural spines from neck to lumbar region, connected by tissue in life. When compared with the sail of Dimetrodon, the vertebral spines are shorter and heavier, and bear numerous small crossbars. Edaphosaurus and other members of the Edaphosauridae evolved tall dorsal sails independently of sail-back members of the Sphenacodontidae such as Dimetrodon and Secodontosaurus that lived at the same time, an unusual example of parallel evolution. The function(s) of the sail in both groups is still debated. Researchers have suggested that such sails could have provided camouflage, wind-powered sailing over water, anchoring for extra muscle support and rigidity for the backbone, protection against predator attacks, fat storage areas, body temperature control surfaces, or sexual display and species recognition. The height of the sail, curvature of the spines, and shape of the crossbars are distinct in each of the described species of Edaphosaurus and show a trend for larger and more elaborate (but fewer) projecting processes over time. Romer and Price suggested that the projections on the spines of Edaphosaurus might have been embedded in tissue under the skin and might have supported food-storage or fat similar to the hump of a camel.
Bennett argued that the bony projections on Edaphosaurus spines were exposed and could create air turbulence for more efficient cooling over the surface of the sail to regulate body temperature. Recent research that examined the microscopic bone structure of the tall neural spines in edaphosaurids has raised doubts about a thermoregulatory role for the sail and suggests a display function is more plausible.
Edaphosaurus species measured from 1 m (3 ft) to almost 3.5 m (11 ft) in length and weighed over 300 kilograms (660 lb). This large herbivore had a massive wide body, thick tail, and short limbs which show that it was a slow moving animal. Its broad gut would have allowed it to digest tough, fibrous plant material.
Sphenacodontia is a stem-based clade of derived synapsids. It was defined by Amson and Laurin (2011) as "the largest clade that includes Haptodus baylei, Haptodus garnettensis and Sphenacodon ferox". They first appear during the Late Pennsylvanian epoch. The defining characteristics include a thickening of the maxilla visible on its internal surface, above the large front (caniniform) teeth; and the premaxillary teeth being set in deep sockets. All other (sister group and more primitive) synapsid clades have teeth that are set in shallow sockets.
Basal Sphenacodontia constitute a transitional evolutionary series from early pelycosaurs to ancestral therapsids (which in turn were the ancestors of more advanced forms and finally the mammals). One might say that the Sphenacodontians are proto-therapsids.
Haptodus was at least 1.5 metres (5 ft) in length. It lived from Latest Carboniferous to Early Permian. It was a medium-sized predator, feeding on insects and small vertebrates. H. garnettensis is currently the basalmost sphenacodontian.
The main family of Sphenacodontia are Sphenacodontidae, a family of small to large, advanced, carnivorous. During the later part of the early Permian these animals grew progressively larger (up to 3 meters or more), to become the top predators of their environments.
The skull is long, deep and narrow, an adaptation for strong jaw muscles. The front teeth are large and dagger-like, whereas the teeth in the sides and rear of the jaw are much smaller (hence the name of the well-known genus Dimetrodon - "two-measure tooth", although all members of the family have this attribute).
Several large and advanced members of this group (Ctenospondylus, Sphenacodon, Secodontosaurus and Dimetrodon) are distinguished by a tall sail along the back, made up of elongated vertebral neural spines, which in life must have been covered with skin and blood vessels, and presumably functioned as a thermoregulatory device. However, possession of a sail does not appear to have been essential for these animals. For example there is the case in which one genus (Sphenacodon - fossils known from New Mexico) lacks a sail, while a very similar and closely related genus (Dimetrodon - fossils known from Texas) has one. During the Permian, these two regions were separated by a narrow sea-way, but it is not clear why one geographically isolated group should evolve a sail, but the other group not.
The family Sphenacodontidae is actually paraphyletic as originally described, defined by shared primitive synapsid characters; these animals constitute an evolutionary gradation from primitive synapsid to early therapsid.
Sphenacodon (meaning "wedge point tooth") is an extinct genus of synapsid that lived from about 300 to about 280 million years ago during the Late Carboniferous and Early Permian periods. Like the closely related Dimetrodon, Sphenacodon was a carnivorous member of the Eupelycosauria family Sphenacodontidae. However, Sphenacodon had a low crest along its back, formed from blade-like bones on its vertebrae (neural spines) instead of the tall dorsal sail found in Dimetrodon
The skull of Sphenacodon is very similar to that of Dimetrodon. It is narrow from side to side and vertically deep, with an indented notch at the front of the maxillary bone in the upper jaw. The upper and lower jaws are equipped with an array of powerful teeth, divided into sharp pointed "incisors" [precaniniforms], large stabbing "canines" [caniniforms], and smaller slicing back teeth [postcaniniforms]. The orbit is set high and far back with a single opening (temporal fenestra) behind and partly below the eye, a characteristic of synapsids.
Body proportions are also similar to Dimetrodon, with a very large head, short neck, robust trunk, relatively short front and hind limbs, and a tapering tail that makes up about half the animal's entire length. However, the tops of the neural spines along the back bone are strikingly different in each genus. In Dimetrodon, the neural spines develop into long, narrow, cylindrical projections that support a tall vertical dorsal sail that ends near the base of the tail. In Sphenacodon, the neural spines are enlarged but retain a flat-tipped, blade-like shape along the back and tail, and form a crest rather than a tall sail. (The sphenacodontid genus Ctenospondylus also has blade-like neural spines, but its dorsal crest is taller than in Sphenacodon, although not as tall as the sail in Dimetrodon.)
There is evidence for strong epaxial muscles along the base of the raised neural spines in both Sphenacodon and Dimetrodon, likely helping to stiffen and strengthen the backbone for walking and for lunging at prey by restricting side-to-side flexing motion. A recent study of the structure of the neural spines on Sphenacodon confirms that the upper parts were not encased in a thick muscular hump and instead protruded above a layer of muscle to form a low dorsal crest. Finds of sphenacodontid specimens in which postmortem distortion of the body caused the dorsal spines to overlap suggests that the spines were not connected by hard or particularly tough tissue. The possible function of a low, skin-covered crest in Sphenacodon is debated. A thermoregulatory role seems unlikely, although the taller crest in Sphenacodon ferocior is allometrically larger than in S. ferox. Recent research has favored a display role for the tall sails in Dimetrodon and Edaphosaurus.
Both Sphenacodon and Dimetrodon have been depicted with their short limbs splayed outward at 90 degrees from the body in a wide pushup position and with the tail (and even belly) dragging on the ground, similar to modern lizards and crocodiles. A sprawling stance is also typical for Sphenacodon and Dimetrodon skeletons as currently mounted in museums
Therapsid family tree
Therapsida is a group of the most advanced synapsids, and include the ancestors of mammals. Many of the traits today seen as unique to mammals had their origin within early therapsids, including hair, lactation, and an erect posture. Therapsids evolved from 'pelycosaurs' (specifically sphenacodonts) 275 million years ago. They replaced the pelycosaurs as the dominant large land animals in the Middle Permian. They remained the dominant fauna until replaced by archosaurs and rhynchosaurs in the Middle Triassic although some therapsids, the kannemeyeriiforms for example, remained diverse in the Late Triassic. The therapsids included the cynodonts, the group that gave rise to mammals in the Late Triassic around 225 million years ago. Of the non-mammalian therapsids, only cynodonts and dicynodonts survived the Triassic-Jurassic extinction event. The last of the non-mammalian therapsids, the cynodont tritylodontids, became extinct in the Early Cretaceous, approximately 100 million years ago.
Similar in some respects to modern large ungulates were tapinocephalids. Basal theriodonts were similar to similar our terrestiral carnivorans, and many of the smaller forms filled niches that might correspond to our rodents. It was actually the small forms that were able to survive the extinction event that hit many therapsid groups. Ironically it was these small, insectivoran-style therapsids that would eventually give rise to mammals while the hippo- and carnivoran- mimics went extinct.
Basal therapsids may or may not have had "improved" metabolic conditioning. It seem likely that the relatively advanced forms like basal cynodonts might have been advanced well above the common basal amniote metabolism. If this is true, then it is not unreasonable to restore them with some form of insulatory structure; in this case, hair. Tenrecs and basal mammals (i.e., the duckbilled platypus) show the type of metabolism to which I refer. More basal forms may or may not have had more advanced metabolic rates than modern reptiles.
While the early therapsids had skulls very similar to those of their pelycosaurian ancestors, they differed in the post-cranial skeleton. Legs and feet
Their legs are positioned more vertically beneath their bodies than are the sprawling legs of reptiles and pelycosaurs. The feet were more symmetrical, with the first and last toes short and the middle toes longer, indication the foots axis was placed parallel to that of the animal, not sprawling out sideways. This would have given a more mammal-like gait than the lizard-like gait of the pelycosaurs. Jaw and teeth
Therapsids' temporal fenestrae are greater than those of the pelycosaurs. The jaws of therapsids are more complex and powerful, and the teeth are differentiated into frontal incisors for nipping, great lateral canines for puncturing and tearing, and molars for shearing and chopping food.
Biarmosuchia is a group of Permian therapsids. Biarmosuchians are the most basal group of therapsids. They were moderately sized, lightly built carnivores, intermediate in form between the sphenacodont pelycosaurs and more advanced therapsids. Biarmosuchus is an extinct genus of biarmosuchian therapsid that lived around 255 mya during the late Permian period. It is known from a single species, Biarmosuchus tener, and belongs to the monotypic family Biarmosuchidae. The first specimen was found in channel sandstone that was deposited by flood waters originating from the young Ural mountains. A large opening for the eye and a small temple opening common in primitive mammal-like reptiles, this lends to a weak bite but how it ate is pure speculation. Biarmosuchus grew up to 1.5-2 m in length.
The teeth contained eight small incisors on the palate, followed by a canine tooth and a further five canine teeth. So together the species contained fourteen upper teeth and twelve lower teeth of small size.
Theriodonts ("Beast Tooth", referring to more mammal-like teeth), are a major group of therapsids. They can be defined in traditional, Linnaean terms, in which case they are a suborder of mammal-like reptiles that lived from the Middle Permian to the Middle Cretaceous, or in Cladistic terms, in which case they include not only the traditional theriodonts but also their descendants the mammals as well (in the same way that, cladistically speaking, the theropod dinosaurs include the birds as a sub-clade).
Theriodonts appeared almost the same time as the anomodonts, about 265 million years ago, in the Middle Permian. Even these early theriodonts were more mammal-like than their Anomodont and Dinocephalian contemporaries.
Theriodonts fall into three main groups: Gorgonopsia, Therocephalia and Cynodontia. Early theriodonts may have been warm-blooded. Early forms were carnivorous, but several later groups became herbivorous during the Triassic.
Theriodont jaws were more mammal-like than was the case of other therapsids, because their dentary was larger, which gave them more efficient chewing ability. Furthermore, several other bones that were on the lower jaw (found in reptiles), moved into the ears, allowing the theriodonts to hear better and their mouths to open wider. This made the theriodonts the most successful group of synapsids.
Eutheriodonts refer to all theriodonts except the gorgonopsians (the most primitive group). They included the therocephalians, cynodonts and their descendants: the mammals. The name means "true beast tooth". The eutheriodonts have larger skulls, accommodating larger brains and improved jaw muscles.
The theriodonts (eutheriodonts) are one of the two synapsid survivors of the great Permian Triassic extinction event, the other being the dicynodonts. Therocephalians included both carnivorous and herbivorous forms; both died out after the Early Triassic. The remaining theriodonts, the cynodonts, also included carrnivores such as Cynognathus, as well as newly evolved herbivorous (Traversodonts). While Traversodonts for the most part remained medium-sized to reasonably large (length of largest species up to 2 meters), the carnivorous forms became progressively smaller as the Triassic progressed. By the Late Triassic the small cynodonts included the rodent-like tritylodonts (possibly related to or descended from travsersodonts), and the tiny, shrew-like, trithelodonts, which evolved into the first mammals. The trithelodonts died out during the Jurassic, and the tritylodonts survived in the Cretaceous, but the mammals continued to evolve. Many mammal groups managed to survive the Cretaceous Paleogene extinction event, which wiped out the non-avian dinosaurs, allowing the mammals to diversify and dominate the Earth.
Family tree - cynodonts to crown group mammals
Liu, J.; Olsen, P. (2010). "The Phylogenetic Relationships of Eucynodontia (Amniota: Synapsida)". Journal of Mammalian Evolution 17 (3): 151. doi:10.1007/s10914-010-9136-8
Cynodontia or cynodonts ("dog teeth") are a taxon of therapsids which first appeared in the Late Permian (approximately 260 Ma) and were eventually distributed throughout all seven continents by the Early Triassic (256 Ma). This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of therapsids. They are named after their dog-like teeth.
Cynodonts have nearly all the characteristics of mammals. Their teeth were fully differentiated, the braincase bulged at the back of the head, and many of them walked in an upright manner. Cynodonts still laid eggs, as all Mesozoic proto-mammals probably did. Their temporal fenestrae were much larger than in its ancestors, and the widening of the zygomatic arch allowed for more robust jaw musculature supporting the evidence of a more mammal-like skull. They also have the secondary palate that other primitive therapsids lacked, except the therocephalians, who were the closest relatives of cynodonts. Their dentary was the largest bone in their lower jaw, as other smaller bones moved into the ears. They were probably warm-blooded, and covered in hair.
Procynosuchus (Greek: "Before dog crocodile") was a cynodont from the Late Permian. It is considered to be one of the earliest and most basal cynodonts. Remains of Procynosuchus have been found in Germany, Zambia and South Africa. It was 60 cm (2 ft) long.
As one of the earliest cynodonts, Procynosuchus has many primitive features, but it also has features that distinguish it from all other early therapsids. Some of these features have been interpreted as adaptations for a semi-aquatic lifestyle. For example, the wide zygapophyses of the vertebrae allow for a high degree of lateral flexibility, and Procynosuchus may have used anguilliform locomotion, or eel-like undulation, to swim through the water. The tail of Procynosuchus is also unusually long for a cynodont. The long haemal arches would have given the tail a large lateral surface area for greater propulsion through the water. Relatively flat foot bones may also have been an adaptation toward swimming, as the feet may have been used like paddles. Ridges on the femur are an indication of strong flexor muscles that could have stabilized the leg during limb-driven swimming. When the thigh is pulled back in the water, the lower leg tends to bend forward. Strong flexor muscles would have pulled the lower leg back with the femur, providing the powerful backward thrust that is needed to swim.
Eucynodontia ("true dog teeth") is a grouping of animals that includes both mammals and mammal-like non-mammalian therapsids ("mammal-like reptiles") such as cynodonts ("dog teeth"). Its membership was and is made up of both carnivores and herbivores. The chronological range extends from at least the Lower Triassic, possibly the Upper Permian, until the present day.
Cynognathus is an extinct genus of large-bodied cynodont therapsid that lived in the Early and Middle Triassic. It is known from a single species, Cynognathus crateronotus. Cynognathus was a meter-long predator closely related to mammals and had an almost worldwide distribution. Fossils have so far been recovered from South Africa, South America, China and Antarctica.
Cynognathus was a heavily built animal, and measured around 1 metre (3.3 ft) in body length. It had a particularly large head, 30 centimetres (1.0 ft) in length, with wide jaws and sharp teeth. Its hind limbs were placed directly beneath the body, but the fore-limbs sprawled outwards in a reptilian fashion. This form of double (erect/sprawling) gait is also found in some primitive mammals alive today.
The dentary was equipped with differentiated teeth that show this animal could effectively process its food before swallowing. The presence of a secondary palate in the mouth indicates that Cynognathus would have been able to breathe and swallow simultaneously.
The lack of ribs in the stomach region suggests the presence of an efficient diaphragm: an important muscle for mammalian breathing. Pits and canals on the bone of the snout indicate concentrations of nerves and blood vessels. In mammals, such structures allow hairs (whiskers) to be used as sensory organs.
The Probainognathians are one of the two major clades of the infraorder Eucynodontia, the other being Cynognathians. They were mostly carnivorous, though some species may have evolved omnivorous traits. The Probainognathia form into four groups: Probainognathidae, Chiniquodontidae, Tritheledontidae, and Mammaliaformes. The earliest and most basal Probainognathian is Lumkuia, from South Africa. Non-mammalian probainognathians lived from Triassic to Jurassic, making this clade one of the longest lived therapsid family.
Lumkuia is an extinct genus of probainognathian cynodont. It is the earliest and most basal known member of Probainognathia.
The postcanines are similar to the later chiniquodontids, but the secondary palate is quite short in comparison, and the genus lacks the angulation of the ventral cranial margin seen in chiniquodontids. Lumkuia can be seen as more derived than other contemporary cynodonts such as Cynognathus with the crowns of its teeth high and narrow and having inwardly curving tops.
Prozostrodontia is a clade of cynodonts including mammals and their closest non-mammaliform relatives such as Tritheledontidae and Tritylodontidae. It was erected as a node-based taxon by Liu and Olsen (2010) and defined as the least inclusive clade containing Prozostrodon brasiliensis, Tritylodon langaevus, Pachygenelus monus, and Mus musculus (the house mouse). Prozostrodontia is diagnosed by several characters, including:
Reduced prefrontal and postorbital bones, with a reduction or disappearance of a strut of bone called the postorbital bar separating the eye socket from the temporal region
Unconnected dentary bones in the lower jaw
The presence of a small hole in the skull called the sphenopalatine foramen
A sagittal crest extending along the top of the skull and connecting with a lambdoidal crest at the back of the skull
Neural spines of the dorsal vertebrae angled backward
A convex-shaped iliac crest and a reduced posterior iliac spine on the hip
An acetabular notch on the ischium (a groove in the hip socket)
The position of a small projection called the lesser trochanter close to the head of the femur
Prozostrodontia includes tritylodontids, which have traditionally been placed within the more primitive cynodont group Cynognathia as distant mammal relatives. It also includes Tritheledontidae, which has long been placed close to mammals. Most previous studies considered Tritheledontidae a valid monophyletic grouping, meaning it was a true clade including all the descendants of a single common, but Liu and Olsen (2010) found Tritheledontidae to be a paraphyletic series of basal prozostrodontians.
Tritylodontids ("three knob teeth", named after the shape of animal's teeth) were small to medium-sized, highly specialized and extremely mammal-like cynodonts. They were the last known family of the non-mammalian synapsids. One of the last cynodont lines to appear, the Tritylodontidae descended from a Cynognathus-like cynodont. The Tritylodontids were herbivorous, chewing through vegetation, such as stems, leaves, and roots. Some scientists believe that the mammals arose from this group of cynodonts, however, some say that mammals arose from the Tritheledontidae, another group of specialized cynodonts.
The Tritylodontids are the longest living of all the non-mammalian therapsids. They appeared in the latest Triassic period, and persisted through the Jurassic until the Middle Cretaceous. This shows that the Tritylodontids were a very successful group of therapsids, even though they lived right beneath the ruling dinosaurs' feet. No one knows why the Tritylodontids went extinct by the Middle Cretaceous. Perhaps the Tritylodontids were outcompeted by their relatives, the mammals. Some mammals have developed herbivory during the Late Jurassic and Cretaceous. Or, the Tritylodontids may have gone extinct because of the new plant group, the angiosperms or flowering plants because they weren't used to eating new type of plants. It is very clear that the Tritylodontids were warm-blooded. The Tritylodontid fossils were found in the Americas, South Africa, and Eurasia. They may have managed to live worldwide, including Antarctica.
Oligokyphus (meaning "small curved animal"), was a small animal, around 50 centimetres (20 in) in length, belonging to the herbivorous Tritylodontidae family. It resembled a weasel in appearance, with a long and slim body. The limbs sat directly under the body, like modern mammals, but unlike other known synapsids. Oligokyphus was found widely across North America, Europe and China. This indicates that there were substitutes with the terrestrial vertebrates.
The teeth of the upper and lower jaw contain bump rows that fit together perfectly in order to maintain an accurate bite. Oligokyphus had a face similar to that of modern mammals, although there were differences in the cheekbones and eyesockets. It had a bony secondary palate and double-rooted cheek teeth. Unlike mammals, the teeth of Oligokyphus did not occlude. The jaw was double jointed, and the neck was flexible, with an atlas and axis and a double occipital condyle.
The teeth were different from those of related cynodonts; there were no canine teeth, and unusually large, rodent-like incisors. There is a large gap, or diastema, separating the cheek teeth from the incisors. The lower jaw of these animals moved back and forth when the mouth was shut so that the food could be chopped up. Oligokyphus had no premaxilla, but did have a lateral extension of the maxilla.
While the postcanines in non-mammalians, such as Oligokyphus, are difficult to differentiate from canines, the lower postcanines of Oligokyphus (also considered to be pre-molars) are defining from other Tritylodonts. On lower postcanine teeth of Trityldonts, two cusps can be found per row; however, Oligokyphus have two rows with three cusps in each row. These cusps, specific to Oligokyhpus Tritylodonts, allowed for a well-fitting bite that was particularly good at shredding plant material dense in fiber. The foremost incisors are similar to those of today's rodents, extremely intensified and enlarged. The typical location of canine teeth is left empty with Oligokyphus. Instead, a gap is inserted in this area of the jaw as Oligokyphus lack the teeth commonly known as canines.
Oligokyphus is in the family Trityledontidae. The members of the family were all small to medium sized advanced synapsids with combined specialized structures for herbivorous eating. They were the last members of the non-mammalian synapsids. The first Trityledont was found in South Africa in the upper Jurassic rocks. It was first thought to be on of the earliest mammals. This classification has since been adjusted. These non-mammals became progressively more mammal-like. They are now classified as the closest relatives to the mammals and this is supported by their high, flat crested jaw, large zygomatic arches, well developed secondary palate, and dentition.
There have also been comparisons between the cranial nerves of Trityledonts and mammals. The shoulder girdle and forelimb structures were suggestive of these animals digging. These animals are extremely active and burrow in leaf litter and dirt, which suggests characteristics of rodents and rabbits. They naturally have a metabolism that is partially or completely endothermic. They were thought to be driven out by relatives such as mammals, which were competitors for the same territory. Another reason that this animal could have gone extinct is due to new plant development. Some flowering plants, or angiosperms, can be detrimental to these animals since they may not be used to eating new plants.
Oligokyphus were small tetrapod, terrestrial animals. It is believed these animals were primarily land dwelling, living amongst small shrubs or bushes. It is also thought that Oligokyphus fed on seeds or nuts, as their teeth resemble those of modern animals that also feed on seeds and nuts. It is a rather difficult to estimate the social behaviors of Oligokyphus as most of it does not preserve in the fossil record. However, considering the conditions on the planet during the times that Oligokyphus was alive and thriving (late Triassic and early Jurassic) and also the locations of which fossils of these animals were found, some educated predictions can be made about their metabolism and feeding habits. Oligokyphus, with its conveniently placed leg and hip structures, likely was quick-moving and fed off of low-lying plant life. With its long weasel-like body, it may have even been possible for Oligokyphus to reach higher vegetation simply by standing on its hind legs. It probably had good use of its hands to manipulate seeds and other digestively pleasing foods. There has not been any support showing Oligokyphus had the ability to climb vertically, as some rodents are capable of doing today.
Mammaliaformes ("mammal-shaped") is a clade that contains the crown group mammals and their closest extinct relatives. It is defined as the clade originating from the most recent common ancestor of Morganucodonta and the crown group mammals; the latter is the clade originating with the most recent common ancestor of extant Monotremata, Marsupialia, and Placentalia.
Early mammaliaforms were generally shrew-like in appearance and size, and most of their distinguishing characteristics were internal. In particular, the structure of the mammaliform (and mammal) jaw and arrangement of teeth is nearly unique. Instead of having many teeth that are frequently replaced, mammals have one set of baby teeth and later one set of adult teeth which fit together precisely. This is thought to aid in the grinding of food to make it quicker to digest. Warm-blooded animals require more calories than those that are cold-blooded, so quickening the pace of digestion is a necessity. The drawback to the fixed dentition is that worn teeth cannot be replaced, as was possible for the reptilian ancestors of mammaliforms.
However, as small mammals are generally very short-lived compared to reptiles of the same size, this was not much of a problem during the early phase of their evolution, in which the trait was set.
Lactation, along with other characteristically mammalian features, is also thought to characterize the Mammaliaformes, but these traits are difficult to study in the fossil record. While the early mammaliforms likely had some form of lactation, their mammary glands probably were not associated with distinct mammae with nipples but rather were distributed in patches on the belly side with the young licking milk from the fur. Prior to hatching, the same glands would provide moisture to the leathery eggs, a situation still found in monotremes. Some early mammaliaforms did have fur. An insulative covering is necessary to keep a homeothermic animal warm if it is very small, less than 5 cm (1.97 in) long. Scientists still debate if the morganucodonts should be classified as true mammals or classified as a clade outside of mammalia. An argument that is often used to classify them as non-mammals is the fact that they did not possess the three middle ear bones, they were equipped with a double jaw-joint instead which meant that the jaw articulation would be made up of the dentary-squamosal joint as well as a quadrate-articular one. Both the articular and quadrate would eventually become the malleus and incus. There is a trough at the back of the jaw that houses postdentary bones, such bones are absent today in mammals (all living mammals today have a jaw that is composed of a single bone, one of the defining features of Mammalia. Morganucodonts may be tagged as "prototherians".
Morganucodon ("Glamorgan tooth") is an early mammalian genus that lived during the Late Triassic. It first appeared about 205 million years ago.
Like most of the early mammals, Morganucodon was a small, furry, plantigrade animal. The tail was moderately long. According to Kemp (2005) "the skull was 2-3 cm in length and a presacral body length of about 10 cm [4 inches]. In general appearance it would have looked like a shrew or mouse".
Like all primitive mammals, the gait was somewhat sprawling, and the hind feet likely bore a spur similar to those found in the platypus and echidnas. Such a spur would have been connected to a venom gland for protection or mating competition.
Morganucodon was likely nocturnal, and spent the day in a burrow. The diet appears to have been insects and other small animals, again much like a modern shrew. Like most modern mammal insectivores, it grew fairly quickly to adult size. The teeth also appear to have grown in mammalian fashion, with deciduous teeth being replaced by permanent teeth that were retained throughout the rest of the animal's life. This pattern of rapid growth and permanent teeth indicate Morganucodon, unlike its therapsid ancestors, lived a fairly short life, similar to those of most small mammals. As it evolved earlier than marsupial and placental mammals, it likely laid eggs.
Although unknown bifore the Middle Jurassic and therefore not among the oldest known mammals, Docodonts are considered to be one of the most archaic mammalian group.
Most Docodonts are known solely from the dentition, which includes complex, broad cheeck theeth. In Docodon the lower molars are rectangular and the buccal cups are heigher than the lingual cups; the upper molars are hourglass shaped and transversely wider then long.
The theeth of Docodonts have been cited as evidence that the group evolved from Morganucodonts. According to this hypothesis, the wide molars of Docodonts evolved by expansion of the lingual cingula of typical morganicodon molars. Cusps on the cingula eventually enlarged, and transverse crests formed, joining them to the original (lateral) cusps.
These derived conditions are superficially similar to those characterizing therians, but they are different enough to indicate that they arose indipendently.
Insight on the phylogenetic positions of Docodonts is afforded by the best-known docodont, Haldanodon. Haldanodon was a ducodont living in the Jurassic period, around 145 million years ago.
Haldanodon is represented by dozens of jaws, severals skulls and a skeleton.
Based on its robust limb skeleton, especially the scapula with a postscapular fossa, the broad humerus with a prominent deltopectoral crest, an elongate ulnar olecranon process, and short and robust phalanges, Haldanodon appears to have been fossorial. However, its occurence un lignite deposits suggests that it may also have been semi-aquatic, similar to extant desman moles.
Haldanodon resembles cynodonts in several plesiomorphous cranial features that are present in more derived states in Morganucodontids. The presence in Haldanodon of a large septomaxilla in the nasal region, retention of larger accessory ("postdentary") jaw bones and a larger stapes than in Morganucodontids, and several other cynodont-like features could indicate that this genus was more primitive than Morganucodon and diverged even earlier from the mammalian stem.
At the same time, several other cranial characters of Haldanodon are derived, like those of other early mammals.
Lillegraven and Krusat seggested that Haldanodon could have acquired many of its "mammalian" traits earlier than, and indipendently from, Morganucodontids, which would suggest that Mammalia is polyphyletic. Subsequent phylogenetic analyses, however, support a monophyletic Mammalia thet includes Haldanodon.
A new Docodont recently reported from the Middle Jurassic of China provides additional evidence on the relationship and behavior of these archaic mammals.
Based on a partial skeleton, Castorocauda is the largest known docodont, almost half a meter long from the snout to te end of the tail.
Castorocauda was a genus of small, semi-aquatic relative of mammals living in the Jurassic period, around 154 million years ago. It was highly specialized, with adaptations evolved convergently with those of modern semi-aquatic mammals such as beavers, otters, and the platypus.
Castorocauda was a genus of small, semi-aquatic relative of mammals living in the Jurassic period, around 154 million years ago, found in lakebed sediments of the Daohugou Beds of Inner Mongolia. It was highly specialized, with adaptations evolved convergently with those of modern semi-aquatic mammals such as beavers, otters, and the platypus.
Castorocauda is not considered to be a mammal by the crown group-definition, as belonging to the group containing the most recent common ancestor of all living mammals (including monotremes, placentals, and marsupials) and its descendants. Castorocauda is a member of the order Docodonta, which is a wholly extinct group of Mammaliaformes, or proto-mammals. It has no known modern descendants. The crown group concept is however not universally accepted. Kielan-Jaworowska et al. (2002), for example, continued to include the mammaliaformes in Mammalia.
An important goal of paleontologists is to track the origin and evolution of certain characteristics. Hard anatomy characters such as teeth and bones preserve well in the fossil record and are the main source of information about how fossil animals are related to their modern counterparts. Soft anatomy features such as internal organs do not preserve readily.
A fossil discovery was made in 2004 in the fossil-rich beds of Liaoning province, China; it was reported in the journal Science by an international team led by Qiang Ji of Nanjing University. The fossil of Castorocauda was so well preserved that an important feature of its soft anatomy‹ hair‹ was preserved. Hair is present in all modern mammals and was therefore assumed, under principles of maximum parsimony, to have been present in all fossil true mammals. The presence of hair in Castorocauda indicates that hair was not only present in mammals, but also in their closest relatives, the docodonts. In fact, the hair appears to have been a very advanced dense pelage including guard hairs and underfur. The tiny auditory ossicles of the middle ear and associated areas were also well preserved in this Castorocauda fossil. Features of these bones confirms the evolutionary position of docodonts as less closely related to true mammals than Hadrocodium, but more closely related to mammals than other mammaliaforms such as Morganucodon.
The name Castorocauda is derived from the Latin castor- meaning "beaver", -"cauda" meaning "tail". The tail was broad with scales interspersed with hairs that grew less frequent toward the tip. Overall it was very similar to the tails of modern beavers and was presumably used for locomotion in water in a similar fashion. The caudal vertebrae were flattened dorso-ventrally and similar overall to those in a beaver or otter. Fossilized impressions of some webbing is also present between the toes.
Features of the limbs suggested that it may have been adapted for digging. The forelimbs are robust, with enlarged olecranon and other processes associated with strong muscle attachment. The limbs are similar to the modern platypus, an animal that both digs and swims. Castorocauda, Haldanodon and perhaps other docodonts were fossorial. These early specializations were also present in the unique early true mammal Fruitafossor.
Docodonts in general have distinctive teeth, and the teeth of Castorocauda have the distinguishing features of the group. The teeth of Castorocauda are different in many ways from all other docodonts, presumably due to a difference in diet. Most docodonts had teeth specialized for an omnivorous diet. The teeth of Castorocauda suggest that the animal was a piscivore, feeding on fish and small invertebrates. The first two molars had cusps in a straight row, eliminating the grinding function suggesting that they were strictly for gripping and not for chewing. This feature of three cusps in a row is similar to the ancestral condition in mammal relatives (as seen in triconodonts), but is almost certainly a derived character in Castorocauda. These first molars were also recurved in a manner designed to hold slippery prey once grasped. These teeth are very similar to the teeth seen in mesonychids, an extinct group of semi-aquatic carnivorous ungulates, and resemble, to a lesser degree, the teeth of seals.
The complete dental formula was not recoverable, but the lower jaw contained 4 incisors, 1 canine, 5 premolars, and 6 molars.
The animal probably weighed about 500-800 grams (1 pound to nearly 2 pounds) and grew to at least 42.5 cm (17 inches) in length. This makes it the largest mammaliaform (including true mammals) of the Jurassic. The previous record holder was Sinoconodon which was thought to weigh up to 500 g.
Mammals hear sounds after they are transmitted from the outside world to their inner ears by a chain of three bones, the malleus, incus, and stapes. Two of these, the malleus and incus, are derived from bones involved in jaw articulation in most other vertebrates.
Mammals have hair. Adults of somespecies lose most of their hair, but hair is present at least during some phase of the ontogeny of all species. Mammalian hair, made of a protein called keratin, serves at least four functions. First, it slows the exchange of heat with the environment (insulation). Second, specialized hairs (whiskers or "vibrissae") have a sensory function, letting the owner know when it is in contact with an object in its external environment. These hairs are often richly innervated andwell-supplied with muscles that control their position. Third, through their color and pattern, hairs affect the appearance of amammal. They may serve to camouflage, to announce the presence of especially good defense systems (for example, the conspicuous color pattern of a skunk is a warning to predators), or to communicate social information (for example, threats, such as the erect hair onthe back of a wolf; sex, such as the different colors of male and female capuchin monkeys; presence of danger, such as the white underside of the tail of a white tailed deer). Fourth, hair provides some protection, either simply by providing an additional protective layer (against abrasion or sunburn, for example) or by taking on the form of dangerous spines that deter predators (porcupines, spinyrats, others).
Mammals feed their newborn young with milk, a substance rich in fats and protein that is produced by modified sweat glands called mammary glands. These glands, which take a variety of shapes, are usually located on the ventral surface of females along paths that run from the chest region to the groin. They vary in number from two (one right, one left, as in humans) to a dozen or more.
Other characteristics found in most mammals include highly differentiated teeth; teeth are replaced just once during an individual's life (this condition is called diphyodonty, and thefirst set is called "milk teeth); a lower jaw made up of asingle bone, the dentary; four-chambered hearts, a secondary palate separating air and food passages in the mouth; a muscular diaphragm separating thoracic and abdominal cavities; highly developed brain; endothermy and homeothermy; separate sexes with the sex of an embryobeing determined by the presence of a Y or 2 X chromosomes; and internal fertilization.
The Class Mammalia includes around 5000 species placed in 26 orders (systematists do not yet agree on the exact number or on how some orders are related to others). Mammals can be found in all continents and seas. In part because of their high metabolic rates (associated with homeothermy and endothermy), they often play an ecological role that seems disproportionately large compared to their numerical abundance.
Megazostrodon is an extinct Mammaliaform that is widely accepted as being one of the first mammals and which appeared in the fossil record approximately 200 million years ago. It did have some non-mammalian characteristics but they were sufficiently minor to warrant the analysis that this animal probably represents the final stage of the transition between cynodont, or "mammal-like" reptiles and true mammals.
Megazostrodon was a small, furry, shrew-like animal between 10 to 12 centimetres (4 to 5 in) long which probably ate insects and small lizards. It is thought that it was nocturnal as it had a much larger brain than its cynodont relatives and the enlarged areas of its brain were found to be those that process sounds and smells. This was probably in order to avoid being in competition with the reptiles or becoming prey to the dinosaurs.
These early mammals developed many traits which were to make them well-suited for a very active lifestyle. They developed four types of teeth (as opposed to the uniform teeth of the reptiles), incisors, canines, premolars and molars, which enabled them to chew and therefore process their food more thoroughly than their reptilian cousins. There is evidence that the inward-closing movement of the mandible suggests a shearing action to chew food. Their skeletons changed so that their limbs were more flexible (they became less laterally splayed, allowing for faster forward motion) and they developed a shorter ribcage and larger lungs to allow for faster respiration. The structure of their jaw bones changed, the lower jaw becoming a single bone ‹ the dentary (as opposed to the seven different bones found in reptilian lower jaws). The other bones which once made up the jaw moved to the middle ear to create a hearing system.
Probably the most important aspect of change in the evolution that led to these first mammals was that their direct ancestors (the cynodonts) had become warm-blooded. This meant that they relied on the food they ate to help sustain their body temperature rather than depending on their surrounding environment, which enabled them to maintain higher activity levels during the day than reptiles could (as reptiles frequently have to perform temperature regulation activities such as sun basking and seeking shade) and even to become nocturnal ‹ a major advantage in a world where most predators were active during the day.
Despite all the mammalian traits that Megazostrodon acquired, it is thought that they still laid leathery eggs like their reptilian cousins, similar to the extant monotremes. Live birth and the placenta would be later evolutions for mammals.
Fruitafossor was a termite-eating mammal endemic to North America during the Late Jurassic epoch (155.7-150.8 mya), existing for approximately 4.9 million years.
The description is based on a surprisingly complete skeleton of a chipmunk-sized animal. It was discovered on March 31, 2005, in Fruita, Colorado. It resembled an armadillo (or anteater) and probably ate colonial insects in much the same manner as these animals do today. Other skeletal features clearly show that Fruitafossor was not related to armadillos, anteaters, or any modern group of mammal. This indicates that specializations associated with feeding on ants or termites have independently evolved many times in mammals: in Fruitafossor, anteaters, numbats, aardvarks, pangolins, and spiny anteaters.
The teeth of Fruitafossor bear a striking resemblance to modern armadillos and aardvarks. They were open-rooted, peg-like teeth without enamel. This type of tooth is present today in insectivorous mammals, particularly those that are highly specialized to feed on colonial insects. This is termed "myrmecophagy." Since ants had not yet evolved at the time of Fruitafossor, it is assumed that these animals fed on termites, which were abundant along with their relatives the cockroaches.
Fruitafossor has been nicknamed Popeye, after the cartoon sailor, because of its large front limbs. The features of the front limb indicate that the animal was fossorial, employing scratch digging like modern moles, gophers, and spiny anteaters. The olecranon process was highly enlarged indicating the forelimb had powerful muscles. This feature also supports the idea that they were myrmecophagous, as modern mammals employ this technique to break into termite mounds.
Its vertebral column is also very similar to armadillos, sloths, and anteaters (order Xenarthra). It had extra points of contact among vertebrae similar to the xenarthrous process that are only known in these modern forms. These processes generate a rigid and relatively inflexible backbone, which is good for digging.
This find is an important discovery in mammal evolution, because of where it fits in the evolutionary tree of mammals and because of its ecological niche. Most mammals of the Mesozoic were omnivores or unspecialized insectivores. Fruitafossor is unique in the degree of specialization, both for digging and in regard to how specialized it was on insects. This fossil, along with others such as Repenomamus, Volaticotherium, and Castorocauda, challenge the notion that early mammals and mammaliaforms were restricted to a single niche and demonstrate that at least some early specialization occurred.
Fruitafossor has no modern relatives. It is an early offshoot of mammal related to therians (the subclass containing marsupials and placentals). It has a unique suite of therian and prototherian characteristics. Its shoulder-girdle is similar to a platypus or reptile, but many other features are more similar to most other modern mammals. This has led researchers to suggest that it may have been the earliest known relative to the evolutionary line leading to Theria.
Allotheria (Holotheria) (meaning "other beasts", from the Greek αλλος, allos-other and θεριον, therion-wild animal) was a branch of successful Mesozoic mammals. The most important characteristic was the presence of lower molariform teeth equipped with two longitudinal rows of cusps. Allotheria includes Multituberculata, probably Haramiyida, and possibly the enigmatic Gondwanatheria.
Allotheres also had a narrow pelvis, indicating that they gave birth to tiny helpless young like marsupials do.
The Multituberculata were a group of rodent-like mammals that existed for approximately 120 million years, the longest fossil history of any mammal lineage, but were eventually outcompeted by rodents, becoming extinct during the early Oligocene. At least 200 species are known, ranging from mouse-sized to beaver-sized. These species occupied a diversity of ecological niches, ranging from burrow-dwelling to squirrel-like arborealism. Multituberculates are usually placed outside either of the two main groups of living mammals Theria, including placentals and marsupials, and Monotremata, but closer to Theria than to monotremes.
The multituberculates are often considered the most successful, diversified, and long-lasting mammals in natural history. They first appeared in the Jurassic, or perhaps even the Triassic, survived the mass extinction in the Cretaceous, and became extinct in the early Oligocene epoch, some 35 million years ago.
The multituberculates had a cranial and dental anatomy similar to rodents, with cheek-teeth separated from the chisel-like front teeth by a wide tooth-less gap (the diasteme). Each cheek-tooth displayed several rows of small cusps (or tubercles, hence the name) that operated against similar rows in the teeth of the jaw. As in modern rodents, this masticatory apparatus formed an efficient chopping device.
During the Cretaceous and Paleocene, the multituberculates radiated into a wide variety of morphotypes, including the squirrel-like arboreal ptilodonts. The peculiar shape of their last lower premolar is their most outstanding feature. These teeth were larger and more elongated than the other cheek-teeth and had an occlusive surface forming a serrated slicing blade. Though it can be assumed that this was used for crushing seeds and nuts, it is believed that most small multituberculates also supplemented their diet with insects, worms, and fruits.
Ptilodus was a relatively large multituberculate of 30 to 50 centimetres (12 to 20 in) in length, which is about the same size as a squirrel. Its feet, legs and long tail suggest it was a good climber, so it very possibly lead a squirrel-like lifestyle.
There are seven species, and others have been proposed at one time or another.
Triconodonta (also known as Eutriconodonta) is the generic name for a group of early mammals which were close relatives of the ancestors of all present-day mammals. Triconodonts lived in the Jurassic and Cretaceous periods. They are one of the groups that can be classified as mammals by any definition. Several other extinct groups of Mesozoic animals that are traditionally considered to be mammals (such as Morganucodonta and Docodonta) are now placed just outside Mammalia by those who advocate a 'crown-group' definition of the word "mammal".
Their name, meaning "Three conical teeth", is based on one of their fundamental characteristics. They had the typical morphology of the proto-mammals: small, furry, tetrapod animals with long tails. They probably had a nocturnal lifestyle to avoid dinosaur predators, coming out from their burrows after dusk to hunt for small reptiles and insects. However, recent evidence from China suggests that some triconodonts such as Repenomamus were indeed able to take on small dinosaurs.
Repenomamus is the largest mammal known from the Cretaceous period of Manchuria, and it is the mammal for which there is the best evidence that it fed on dinosaurs. It is not possible to determine if Repenomamus actively hunted live dinosaurs or scavenged dead dinosaurs.
Repenomamus was probably not a fast runner. The humerus and femur left their joints at a somewhat splayed angle, and the legs were relatively short compared to the body. The feet were plantigrade. Repenomamus's behavior and overall body shape may have resembled those of modern day Tasmanian devils.
Repenomamus was carnivorous. A specimen of Repenomamus robustus has been discovered with the fragmentary skeleton of a juvenile Psittacosaurus preserved in its stomach. This is the strongest evidence that Mesozoic mammals fed on dinosaurs, creating interest in the popular press. There were, however, earlier indications that Mesozoic mammals fed on dinosaurs.
Repenomamus giganticus was more than 1 m. (3.3 f) long and weighed about 12-14 kg. (26-31 lb). Its skull measures 16 cm. (6.25 in) long, its body 52 cm. (20.5 in), and the preserved part of its tail 36 cm. (14 in).
These finds are considered important, because they expand the ecological niches known to be inhabited by mammals during the 150 million year reign of the dinosaurs. Previously, the only known mammals of this time period were small nocturnal insectivores, not unlike modern-day shrews. It had been assumed the niches of animals more than 1 m. (3.3 f) long were filled entirely by dinosaurs and reptiles like crocodilians, and were off limits to mammals until after the CretaceousPaleogene extinction event wiped out the dinosaurs and allowed the diversification of mammals during the Cenozoic. Repenomamus disproves this.
Symmetrodonta is a basal group of Mesozoic mammals characterized by the triangular aspect of the molars when viewed from above and the absence of a well-developed talonid. The traditional group of symmetrodonts ranges in age from the latest Triassic to the Late Cretaceous. Symmetrodonta are generally rare and poorly represented in the fossil record. It remains entirely possible they do not represent a discrete phylogenetic category, but with a series of intermediates between triconodonts, on the one hand, and dryolestoids and therians, on the other. At least some genera of symmetrodonts may even be true therians and part of the (disputed) clade Trituberculata.
Hadrocodium wui is an extinct basal mammal species that lived during the Lower Jurassic (approx. 195 million years ago, during the Sinemurian stage) in what is now the Yunnan province of China. Hadrocodium was a mere 3.2 cm (1.35 in) in length (about 2 grams), making it one of the smallest mammals ever.
Hadrocodium is the earliest known example of several features possessed only by mammals, including mammal-like mandible, middle-ear structures, and a relatively large brain cavity. These features had been considered limited to crown group mammals; the discovery of Hadrocodium suggests that they appeared much earlier (45 million years earlier) than previously thought.
Whether Hadrocodium was endothermic or cold-blooded has not been settled, although its apparent nocturnal features would seem to place it in the endotherm group.
Dryolestida is an extinct order of mammals; most of the members are known from the Jurassic to Tertiary, with one possible member, Necrolestes, surviving as late as early Miocene. It has been suggested that these mammals are either the possible ancestors of therian mammals or an offshoot from the same evolutionary line. It is also believed that they developed a fully mammalian jaw and also had the three middle ear bones. Other than that, not much is known about them, this is because their fossils are made up mostly of jaw and tooth remains.
The Dryolestids were formerly considered part of Pantotheria and/or Eupantotheria. The clade Quirogatheria, erected by José Bonaparte in 1992, is often used as a synonym for Dryolestida.
Theria is a subclass of mammals that give birth to live young without using a shelled egg, consisting of the Eutherians (including the placental mammals) and the Metatherians (including the marsupials). The only omitted extant mammal group is the egg-laying monotremes (Prototheria: the other subclass of Mammals): Platypus and Echidna
Eutherians are a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupials (such as kangaroos).
There are no living nonplacental eutherians, and so knowledge of their synapomorphies ("defining features") is entirely based on a few fossils, which means the reproductive features that distinguish modern placentals from other mammals cannot be used in defining Eutheria.
The features of Eutheria that distinguish them from metatherians, a group that includes modern marsupials, are:
an enlarged malleolus ("little hammer") at the bottom of the tibia, the larger of the two shin bones.
the joint between the first metatarsal bone and the entocuneiform bone in the foot is offset further back than the joint between the second metatarsal and mesocuneiform bones - in metatherians these joints are level with each other.
various features of jaws and teeth.
Reproductive features are also of no use in identifying fossil placental mammals, which are distinguished from other eutherians by:
the presence of a malleolus at the bottom of the fibula, the smaller of the two shin bones.
a complete mortise and tenon upper ankle joint, where the rearmost bones of the foot fit into a socket formed by the ends of the tibia and fibula.
a wide opening at the bottom of the pelvis, which allows the birth of large, well-developed offspring. Marsupials have and nonplacental eutherians had a narrower opening that allows only small, immature offspring to pass through.
the absence of epipubic bones extending forward from the pelvis, which are not found in any placental, but are found in all other mammals - nonplacental eutherians, marsupials, monotremes and mammaliformes - and even in the cynodont therapsids that are closest to mammals. Their function is to stiffen the body during locomotion. This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.
Juramaia is an extinct genus of very basal eutherian mammal from late Middle Jurassic (Callovian to Bathonian stage) deposits of western Liaoning, China. Juramaia is known from the holotype BMNH PM1343, an articulated and nearly complete skeleton including incomplete skull preserved with full dentition. It was collected in the Daxigou site, Jianchang, from the Tiaojishan Formation about 160 million years ago.The type species is Juramaia sinensis. The discovery of Juramaia provides new insight into the evolution of placental mammals by showing that a new milestone in mammal evolution was reached 35 million years earlier than previously thought. Furthermore, its discovery fills gaps in the fossil record and helps to calibrate modern DNA-based methods of dating the evolution.
Understanding the beginning point of placentals is a crucial issue in the study of all mammalian evolution. The date of an evolutionary divergence, when an ancestor species splits into two descendant lineages, is among the most important pieces of information an evolutionary scientist can have. Modern molecular studies, such as DNA-based methods, can calculate the timing of evolution by a "molecular clock." But the molecular clock needs to be cross-checked and tested by the fossil record. Prior to the discovery of Juramaia, the divergence point of eutherians from metatherians posed a quandary for evolutionary historians: DNA evidence suggested that eutherians should have shown up earlier in the fossil record around 160 million years ago.
The oldest known eutherian was Eomaia (down mother) dated 125 million years ago. The discovery of Juramaia gives much earlier fossil evidence to corroborate the DNA findings, filling an important gap in the fossil record of early mammal evolution and helping to establish a new milestone of evolutionary history.
Juramaia also reveals adaptive features that may have helped the eutherian newcomers to survive in a tough Jurassic environment. Juramaia's forelimbs are adapted for climbing; since the majority of the Jurassic mammals lived exclusively on the ground, the ability to escape to the trees and explore the canopy might have allowed eutherian mammals to exploit an untapped niche.
The divergence of eutherian mammals from marsupials eventually led to placental birth and reproduction that are so crucial for the evolutionary success of placentals. But it is their early adaptation to exploit niches on the tree that paved their way toward this success.
Citation: Zhe-Xi Luo, Chong-Xi Yuan, Qing-Jin Meng&Qiang Ji, 'A Jurassic eutherian mammal and divergence of marsupials and placentals', Nature 476, 442445 (25 August 2011).
Epitherians comprise all the placental mammals except the Xenarthra (Anteaters, tree Sloths, and Armadillos). They are primarily characterized by having a stirrup-shaped stapes in the middle ear, which allows for passage of a blood vessel. This is in contrast to the column-shaped stapes found in marsupials, monotremes, and xenarthrans. They are also characterized by having a shorter fibula relative to the tibia.
Epitheres are thought to have appeared in the early part of the Late Cretaceous age. Before the end of the Mesozoic Era, ancestral forms of most of the living orders (such as the ungulates and the insectivores) had already appeared. After the extinction of non-avian dinosaurs (in the Early Paleocene), the epitheres' diversity exploded, and by the end of the Eocene, all living orders of Epitheria had appeared. Epitheres are one of the most successful groups of animals.
Monophyly of Epitheria has been challenged by molecular phylogenetic studies. While preliminary analysis of a set of retroposons shared by both Afrotheria, and Boreoeutheria (presence/absence data) supported the Epitheria clade, more extensive analysis of such transposable element insertions around the time of the divergence of Xenarthra, Afrotheria, and Boreoeutheria strongly support the hypothesis of a near-concomitant origin (trifurcation) of these three superorders of mammals.
Another analysis suggests that the root of this clade lies between the Atlantogenata and Boreoeutheria.
Alternative hypotheses place either Atlantogenata and Boreoeutheria, or Afrotheria and Exafroplacentalia (Notolegia) at the base of the tree:
Boreoeutheria (synonymous with Boreotheria) (Gk: βορεο=North + θεριο=Beast) is a clade (Magnorder) of Placental Mammals that is composed of the sister taxa Laurasiatheria and Euarchontoglires (Supraprimates). It is now well supported by DNA sequence analyses as well as retrotransposon presence/absence data.
Most of the male members of the clade share the distinction of external testicles, with the exceptions of Rhinoceroses and Cetacea.
The following diagram is representative of the new, and now standard, eutherian phylogeny:
Reconstruction of the placental mammalian (eutherian) evolutionary tree has undergone diverse revisions, and numerous aspects remain hotly debated. Initial hierarchical divisions based on morphology contained many misgroupings due to features that evolved independently by similar selection processes. Molecular analyses corrected many of these misgroupings and the superordinal hierarchy of placental mammals was recently assembled into four clades. However, long or rapid evolutionary periods, as well as directional mutation pressure, can produce molecular homoplasies, similar characteristics lacking common ancestors. Retroposed elements, by contrast, integrate randomly into genomes with negligible probabilities of the same element integrating independently into orthologous positions in different species. Thus, presence/absence analyses of these elements are a superior strategy for molecular systematics. By computationally scanning more than 160,000 chromosomal loci and judiciously selecting from only phylogenetically informative retroposons for experimental high-throughput PCR applications, we recovered 28 clear, independent monophyly markers that conclusively verify the earliest divergences in placental mammalian evolution. Using tests that take into account ancestral polymorphisms, multiple long interspersed elements and long terminal repeat element insertions provide highly significant evidence for the monophyletic clades Boreotheria (synonymous with Boreoeutheria), Supraprimates (synonymous with Euarchontoglires), and Laurasiatheria. More importantly, two retropositions provide new support for a prior scenario of early mammalian evolution that places the basal placental divergence between Xenarthra and Epitheria, the latter comprising all remaining placentals. Due to its virtually homoplasy-free nature, the analysis of retroposon presence/absence patterns avoids the pitfalls of other molecular methodologies and provides a rapid, unequivocal means for revealing the evolutionary history of organisms."
Kriegs, Jan Ole, Gennady Churakov, Martin Kiefmann, Ursula Jordan, Juergen Brosius, Juergen Schmitz. (2006) Retroposed Elements as Archives for the Evolutionary History of Placental Mammals. PLoS Biol 4(4): e91.
Family tree of placental mammals according to molecular phylogenetics
Erinaceomorpha (hedgehogs, gymnures)
Soricomorpha (moles, shrews, solenodons)
Cetartiodactyla (camels and llamas, pigs and peccaries, ruminants whales and hippos)
Primates (tarsiers, lemurs, monkeys, apes including humans)
Euarchontoglires (synonymous with Supraprimates) is a clade of mammals, the living members of which are rodents, lagomorphs, treeshrews, colugos and primates (including humans).
The Euarchontoglires clade is based on DNA sequence analyses and retrotransposon presence/absence data, combining the Glires clade, which consists of Rodentia and Lagomorpha, with that of Euarchonta, a clade consisting of Scandentia, Primates (which includes humans) and Dermoptera.
Euarchontoglires is now recognized as one of four major groups within Eutheria (containing placental mammals). These four clades are usually discussed without a Linnaean rank, but has been assigned the rank of cohort or magnorder, and superorder. Relations within the four cohorts, Euarchontoglires, Xenarthra, Laurasiatheria, and Afrotheria, and the identity of the placental root, remain somewhat controversial.
Euarchontoglires probably split from the Laurasiatheria sister group about 85 to 95 million years ago during the Cretaceous, developing in the Laurasian island group which would later become Europe. This hypothesis is supported by fossil as well as molecular evidence. The clade of Euarchontoglires and Laurasiatheria is recognized as Boreoeutheria.
The hypothesized relationship among the Euarchontoglires is as follows:
Lagomorpha (rabbits, hares, pikas)
The Euarchonta are a grandorder of mammals containing four orders: the Dermoptera or colugos, the Scandentia or treeshrews, the extinct Plesiadapiformes, and the Primates.
The term "Euarchonta" (meaning "true ancestors") first appeared in the general scientific literature in 1999, when molecular evidence suggested that the morphology-based Archonta should be trimmed down to exclude Chiroptera. Major DNA sequence analyses of predominantly nuclear sequences (Murphy et al., 2001) support the Euarchonta hypothesis, while a major study investigating mitochondrial sequences supports a different tree topology (Arnason et al., 2002). A study investigating retrotransposon presence/absence data has claimed strong support for Euarchonta (Kriegs et al., 2007). Some interpretations of the molecular data link Primates and Dermoptera in a clade (mirorder) known as Primatomorpha, which is the sister of Scandentia. In some the Dermoptera are a member of the primates rather than a sister. Other interpretations link the Dermoptera and Scandentia together in a group called Sundatheria as the sister group of the Primates. Together, the three are known as Euarchonta, the "True Founders".
Plesiadapiformes ("near Adapid-like" or "almost Adapiformes") is an extinct order of Mammals. It is either closely related to the primates or a precursor to them. Many are too derived to be ancestral to Primates, but the earliest Plesiadapiformes have teeth that are strongly indicative of a common ancestor. Purgatorius is believed to be close to the last common ancestor of Primates and Plesiadapiformes.
Plesiadapiformes first appear in the fossil record in the Cretaceous period (145-66 Mya), though many were extinct by the beginning of the Eocene. It is possible that they are the first mammals to have developed finger nails in place of claws.
Purgatorius Illustration courtesy Doug Boyer, Duke University
The primate lineage is thought to go back at least 65 million years ago (mya), even though the oldest known primate from the fossil record is Plesiadapis (circa 55-58 mya) from the Late Paleocene. Other studies, including molecular clock studies, have estimated the origin of the primate branch to have been in the mid-Cretaceous period, around 85 mya.
Plesiadapis is one of the oldest known primate-like mammal species which existed about 58-55 (85?) million years ago in North America and Europe. Plesiadapis literally means "near-Adapis", which is a reference to the Eocene lemuriform, Adapis. Plesiadapis tricuspidens, the type specimen, is so named because of the three cusps present on its upper incisors.
Plesiadapis' dentition shows a functional shift toward grinding and crushing in the cheek teeth as an adaptation towards increasing omnivority and herbivority. The dental formula for Plesiadapis is . The skull of Plesiadapis is relatively broad and flat, with a long snout with rodentlike jaws and teeth and long, gnawing incisors separated by a gap from its molars. Orbits are still directed to the side, unlike the forward-facing eyeballs of modern primates that enable three-dimensional vision. Although its braincase was small according to today's standards, it was larger than in the contemporary hoofed mammals, for instance. Plesiadapis had mobile limbs that terminated in strongly curved claws, and it sported a long bushy tail which is beautifully preserved in the Menat skeletons. The way of life of Plesiadapis has been much debated in the past. Climbing habits could be expected in a relative of the primates, but tree-dwelling animals are rarely found in such high numbers. Based on this and other evidence, some paleontologists have concluded that these animals were mainly living on the ground, like today's marmots and ground squirrels. However, more recent investigations have confirmed that the skeleton of Plesiadapis is that of an adept climber, which can be best compared to tree squirrels or to tree-dwelling marsupials such as possums. The short, robust limbs, the long, laterally compressed claws, and the long, bushy tail indicate that it was an arboreal quadruped. Remains found showed that it had a body mass of around 2.1 kilograms.
The Primates are an ancient and diverse eutherian group, with
around 233 living species placed in 13 families. Most dwell in
tropical forests. The smallest living primate is the pygmy mouse
lemur, which weighs around 30 g. The largest is the gorilla, weighing
up to around 175 kg.
Primates radiated in arboreal habitats, and many of the
characteristics by which we recognize them today (shortened rostrum and forwardly directed orbits, associated with stereoscopic vision; relatively large braincase; opposable hallux
and pollex; unfused and
highly mobile radius and ulna in the forelimb and tibia and fibula in the hind) probably arose as adaptations for life in the trees or areprimitive traits that were retained for the same reason. Severalspecies, including our own, have left the trees for life on theground; nevertheless, we retain many of these features.
Primates are usually recognized based on a suite of primitive
characteristics of the skull, teeth, and limbs. Some of these are
listed above, including the separate and well-developed radius and
ulna in the forearm and tibia and fibula in the hindleg. Others
include pentadactyl feet
and presence of a clavicle.
Additional characteristics (not necessarily unique to primates)
include first toe with a nail, while other digits bear either nails or claws, and stomach
simple in most forms (sacculated in some leaf-eating cercopithecids).
Within primates, there is a tendency towards reduction of the
olfactory region of the brain and expansion of the cerebrum
(especially the cerebral cortex), correlated with an increasing
reliance on sight and increasingly complex social behavior.
The teeth of primates vary considerably. The dental
formula for the order is 0-2/1-2, 0-1/0-1, 2-4/2-4, 2-3/2-3 =
18-36. The incisors
are especially variable. In some forms, most incisors have been lost,
although all retain at least 1 lower incisor. In others, the incisors
are intermediate in size and appear to function as pincers or
nippers, as they commonly do in other groups of mammals. In some,
including most strepsirhines (see next paragraph), the lower incisors
form a toothcomb used in grooming and perhaps foraging. In the
the incisors are reduced to 1 in each jaw and are rodent-like in form
and function. Canines
are usually (but not always) present; they vary in size, including
within species between males and females. Premolars
are usually bicuspid
(bilophodont), but sometimes canine-like or molar-like. Molars
have 3-5 cusps, commonly 4. A hypocone
was added early in primate history, and the paraconid
was lost, leaving both upper and lower teeth with a basically
pattern. Primitively, primate molars were brachydont
but they have become bunodont and quadrate in a number of modern
Living primates are divided into two great groups, the
Strepsirhini and the Haplorhini. Strepsirhines have naked noses,
lower incisors forming a toothcomb,
and no plate separating orbit from temporal fossa. The second digit
on the hind foot of many strepsirhines is modified to form a
"toilet claw" used in grooming. Strepsirhines include mostly arboreal
species with many primitive characteristics, but at the same time,
some extreme specializations for particular modes of life.
Most primate species live in the tropics or subtropics, although a
few, most notably humans, also inhabit temperate regions. Except for
a few terrestrial species, primates are arboreal. Some species eat
leaves or fruit; others are insectivorous or carnivorous.
Here, we follow Anderson and Jones (1984) in formally dividing
living primates into two suborders, the Strepsirhini and the
Haplorhini. We differ, however, in that we place humans and their
close relatives, the chimpanzee, gorilla, and orang in the
Cercopithecoidea(Old World monkeys)
Platyrrhini(New World monkeys)
Lorisiformes(Lorises and Allies)
The Haplorhines, the "dry-nosed" primates (the Greek name means "simple-nosed"), are members of the Haplorhini clade: the prosimian tarsiers and the anthropoids. The anthropoids are the catarrhines (Old World monkeys and apes, including humans) and the platyrrhines (New World monkeys).
The omomyids are an extinct group of prosimians, believed to be more closely related to the tarsiers than to any strepsirrhines, and are considered the most primitive haplorhines.
Haplorhines share a number of derived features that distinguish them from the strepsirrhine "wet-nosed" primates (whose Greek name means "curved nose"), the other suborder of primates from which they parted in evolution some 63 million years ago. The haplorhines, including tarsiers, have all lost the function of the terminal enzyme which manufactures vitamin C, while the strepsirrhine prosimians, like most other orders of mammals, have retained this enzyme and the ability to manufacture vitamin C. The haplorhine upper lip, which has replaced the ancestral rhinarium found in strepsirrhines, is not directly connected to their nose or gum, allowing a large range of facial expressions. Their brain to body ratio is significantly greater than the strepsirrhines, and their primary sense is vision. Haplorhines have a postorbital plate, unlike the postorbital bar found in strepsirhines. Most species are diurnal (the exceptions being the tarsiers and the night monkeys).
All anthropoids have a single-chambered uterus; tarsiers have a bicornate uterus like the strepsirrhines. Most species typically have single births, although twins and triplets are common for marmosets and tamarins. Despite similar gestation periods, haplorhine newborns are relatively much larger than strepsirrhine newborns, but have a longer dependence period on their mother. This difference in size and dependence is credited to the increased complexity of their behavior and natural history.
The Old and New World Primates but Tarsius.
Aegyptopithecus, which means Egyptian Primate, from Greek αιγυπτος (Egypt) and πιθικος (primate), is an early fossil catarrhine that predates the divergence between hominoids (apes) and Old World monkeys. It is known from a single species Aegyptopithecus zeuxis and lived some 35-33 million years ago in the early part of the Oligocene epoch. It likely resembled modern-day New World monkeys (it is about the same size as a modern howler monkey). Aegyptopithecus is believed to be a basal catarrhine, a crucial link between Eocene and Miocene fossil hominoids.
Aegyptopithecus zeuxis has become one of the best known extinct primates based on craniodental and postcranial remains.
Aegyptopithecus zeuxis was a species that had a dental formula of 2:1:2:3 on both the upper and lower jaws, with the lower molars increasing in size posteriorly. The molars showed an adaptation called compartmentalizing shear, which is where the cutting edges involved in the buccal phase serve to surround basins in such a way that food is cut into fragments that are trapped and then ground during the lingual phase.
The canines of this species were sexually dimorphic. The ascending mandibular ramus of this species is relatively broad. The orbits are dorsally oriented and relatively small which suggested that this was a diurnal species. This species showed some postorbital constriction. The interorbital distance of Aegyptopithecus zeuxis is large much like that found in colobines.
A sagittal crest developed in older individuals and extends over the brow ridges. This species had an auditory region which is similar to that found in platyrrhines, having no bony tube and the tympanic fused to the lateral surface of the bulla.
The humerus has a head which faces posteriorly and is narrower than primates that practice suspensory behavior. The humerus also shares some features with extinct hominoids: a large medial epicondyle and a comparatively wide trochlea. This species had an ulna that compares to the extinct members of the genus Alouatta.
On the foot bones, this species had a grasping hallux. Aegyptopithecus zeuxis shares characteristics with haplorrhines such as a fused mandibular and frontal symphyses, postorbital closure, and superior and inferior transverse tori.
Based on dental dimensions and femoral remains the body mass of A. zeuxis is estimated to be 6.708 kg.
Based on estimated femoral neck angle (120-130 degrees) of aforementioned remains, the femur is similar to that of a quadrupedal anthropoid. The greater trochanter's morphology is inconsistent with that of leaping primates, serving as further evidence of the animal's quadrupedalism.
Aegyptopithecus is thought to have been an arboreal quadruped due to the distal articular region of the femur, which is deeper than that of "later" catarrhines. Also, based on overall femoral morphology, A. zeuxis is thought to have been robust.
The phalanges of the hands and feet suggest powerful grasping consistent with arboreal quadrupedalism.
In conjunction with the femur, the humerus suggests arboreal quadrupedalism. This is based on the pronounced brachialis flange and stabilizing muscles on brachial flexors rather than extensors.
In addition, the ulna and distal articular surface of the humerus indicate that A. zeuxis was not only an arboreal quadruped, but also large and slow. This is consistent with evidence extrapolated from femoral morphology.
Platyrrhines have flat noses, outwardly directed nasal
openings, 3 premolars in upper and lower jaws, anterior upper molars
with 3 or 4 major cusps, and are found only in the New World
(families Cebidae and Callitrichidae). Catarrhines have paired downwardly directed nasal openings, which are
close together; usually 2 premolars in each jaw, anterior upper
molars with 4 cusps, and are found only in the Old World (Cercopithecidae,
Like the Platyrrhines (with the exception of the genus Aotus), the Catarrhines are diurnal.
Apes do not have tails. The tails of Old World Monkeys are not prehensile, but serve as balancing organs. Catarrhines have flat finger- and toenails. They have prehensile (grasping) hands, and all but humans also have prehensile feet.
Most species show considerable sexual dimorphism and do not form a pair bond. Most, but not all, species live in social groups.
Proconsul is an extinct genus of primates that existed from 23 to 5 million years ago during the Miocene epoch. Four species have been classified to date: P. africanus, P. heseloni, P. major and P. nyanzae. The four species differ mainly in body size. Environmental reconstructions for the Early Miocene Proconsul sites are still tentative and range from forested environments to more open, arid grasslands.
They had a mixture of Old World monkey and ape characteristics, so their placement in the ape superfamily Hominoidea is tentative, with some scientists placing Proconsul outside it, before the split of the apes and Old World monkeys.
Proconsul's monkey-like features include pronograde postures, indicated by a long flexible back, curved metacarpals, and an above-branch arboreal quadrupedal positional repertoire. The primary feature linking Proconsul with extant apes is its lack of a tail; other "ape-like" features include its enhanced grasping capabilities, stabilized elbow joint and facial structure. Proconsul was definitely not suspensory like modern apes.
Proconsul africanus had a dental formula of 2:1:2:3 on both the upper and lower jaws. The molars of this species had thin enamel and there was a prominent molar cingula. This species also possessed a robust zygomatic bone and a pronounced snout. This species had a broad interorbital region and small frontoethmoidal sinuses. The maxillary sinus was restricted. This species had an auditory region which would be similar to that of extant apes and cercopithecoid monkeys. The ectotympanic tube was well-developed. This species lacked a tail and the canines of this species were sexually dimorphic. The skull lacks supraorbital tori and can be considered somewhat prognathous. This species has a cranial capacity of 167 cc and an encephalization quotient of 1.5. Based on the cranium, this species had an external brain surface much like that of gibbons and cercopithecoid monkeys. The wrist of this species has been described as monkey-like. This species has a talus in which the trochlear surface is highly curved and deeply grooved. The foot of this species possessed a transverse arch. Proconsul africanus had a brachial index of 96 which is comparable to the extant genus Pan. Overall the skeleton of this species can be described as being robust. This species had an average body mass of around 18 kilograms.
Based upon the dental morphology, it is conjectured that Proconsul africanus was a frugivorous species.
Based on postcranial pieces, Proconsul africanus was likely an arboreal quadruped.
Afropithecus is one of the better known primates of Miocene era in Africa (about 17 Mya ago) and the current fossils for this primate suggest that it was similar to the even more numerous Proconsul. However despite a body similarity to Proconsul, Afropithecus is thought to have had a head more like Aegyptopithecus (which may actually be Propliopithecus) and teeth similar to Heliopithecus, the latter of which has actually been considered by some to be a synonym of Afropithecus.
Most of the study done about Afropithecus has been about its teeth, particularly the thickness of the tooth enamel. Enamel thickness is seen to vary considerably over different parts of the teeth, but generally Afropithecus is regarded as having thick enamel. Since Afropithecus predates the previous oldest known thick enamelled primate, Kenyapithecus, it is now regarded as the oldest known primate to have this feature.
From dentition its known that the palate, which is almost completely calcified, of Afropithecus is shallow, long and narrow with tooth rows that converge posteriorly, and it is probable the tooth rows were originally nearly parallel. Afropithecus had a 6.5mm diastema between its very procumbent second incisor and the canine.
Combined with the primitive features of the face, Afropithecus is thought to have had a diet composed of mostly hard fruits.
Oreopithecus is an extinct primate from the Miocene epoch whose fossils have been found in today's Tuscany and Sardinia in Italy. Oreopithecus (from the Greek ορος, oros and πιθικος, pithekos, meaning "hill-ape") existed 9 to 7 million years ago in the Tusco-Sardinian area when this territory was an isolated island in a chain of islands stretching from central Europe to northern Africa.
Oreopithecus' hominid affinities remained controversial for decades until new analyses in the 1990s reasserted Oreopithecus as directly related to Dryopithecus; the peculiar cranial and dental features explained as consequences of insular isolation. These new evidences confirmed that Oreopithecus was bipedal but also revealed that its peculiar form of bipedalism was much different from that of Australopithecus? - the hallux formed a 100° angle with the other toes enabling the foot to act as a tripod in erect postures - and could not enable Oreopithecus to develop a fast bipedal locomotion. When a land bridge finally broke the isolation of the Tusco-Sardinian area 6.5 million years ago, truly large predators were present among the new generation of European immigrants and Oreopithecus faced quick extinction together with other endemic genera.
Oreopithecus bambolii is estimated to have weighed 30-35 kg (66-77 lb). It possessed a relatively short snout, elevated nasal bones, small and globular neurocranium, vertical orbital plane, and gracile facial bones. The shearing crests on its molars suggest a diet specializing in plant leaves. The very robust lower face, with a large attachment surface for the masseter muscle and a sagittal crest for attachment of the temporal muscle, indicates a heavy masticatory apparatus.
Its teeth were small relative to body size. The lack of a diastema (gap) between the second incisor and first premolar of the mandible indicates that Oreopithecus had canines of size comparable to the rest of its dentition. In many primates, small canines correlate with reduced inter-male competition for access to mates and less sexual dimorphism.
Its habitat appears to have been swampy, and not savanna or forest. The postcranial anatomy of Oreopithecus features adaptations to both suspensory arborealism and bipedalism. Functional traits related to suspensory locomotion include its broad thorax, short trunk, high intermembral index, long and slender digits, and extensive mobility in virtually all joints. At the same time, it also features adaptations to upright walking such as the presence of a lumbar curve, in distinction to otherwise similar species known from the same period. Since the fossils have been dated to about 8 million years ago, this represents an unusually early appearance of upright posture. How adapted it was for bipedal walking is not known, but its fingers and arms also seems to show adaptations for climbing and swinging.
Morphological and functional studies indicate a significant part of Oreopithecus positional behavior was bipedal. Its foot has been described as chimp-like, but is different from those of extant primates. The habitual line of leverage of the primate foot is parallel to the third metatarsal bone. In Oreopithecus, the lateral metatarsals are permanently abducted so that this line falls between the first and second metatarsals instead. Furthermore, the shape of the tarsus indicate loads on the foot were transmitted to the medial side of the foot instead of the lateral, like in other primates. The metatarsals are short and straight, but their lateral orientation increased the area of support during bipedal standing and walking. (Source: National Academy Of Sciences).
The lack of predators and the limitation of space and resources in Oreopithecus' insular environment favored a locomotor system optimized for low energy expenditure rather than speed and mobility. Its peculiar feet and short legs were optimized for stable postural harvesting and bipedal shuffling rather than walking or running. The postcranial anatomy of Oreopithecus is therefore considered autapomorphic rather than synapomorphic. Its foot proportions are close to the unusual proportions of Gorilla and Homo but are distinct from those found in specialized climbers. It is not comparable to Gorilla due to low body mass but is similar to Homo; it is not our ancestor but a key species for understanding human bipedality.
Oreopithecus had hominid-like hand proportions that allowed a firm, pad-to-pad precision grip. Features not present in the hands of extant or fossil apes include hand length, relative thumb length, a deep and large insertion for the flexor pollicis longus, and the shape of the carpometacarpal joint between the metacarpal bone of the index finger and the capitate bone. At the base of the second metacarpal bone, the facet for the capitate is oriented transversally, like in hominids. The capitate, on the other hand, lacks the waisting associated with apes and climbing, and still present in Australopithecus. Oreopithecus share the specialised orientation at the carpometacarpal joint with A. afarenis and the marked groove for the flexor pollicis longus with A. africanus. It is thus likely that the hand morphology of Oreopithecus is derived for apes and convergent for early hominids.
The history of hominoid classification in the second half of the 20th century is sufficiently complex to warrant a few books itself. Most of the palaeoanthropologists have changed their minds at least once as new fossils have come to light and new observations have made, and will probably continue to do so. The classifications found in the literature of one decade are not generally the same as those of another.
Apes are Old World anthropoid mammals, more specifically a clade of tailless catarrhine primates, belonging to the biological superfamily Hominoidea. The apes are native to Africa and South-east Asia. Apes are the world's largest primates; the orangutan, an ape, is the world's largest living arboreal animal. Hominoids are traditionally forest dwellers, although chimpanzees may range into savanna, and the extinct australopithecines are famous for being savanna inhabitants, inferred from their morphology. Humans inhabit almost every terrestrial habitat.
Hominoidea contains two families of living (extant) species:
Hylobatidae consists of four genera and sixteen species of gibbon, including the lar gibbon and the siamang. They are commonly referred to as lesser apes.
Hominidae consists of orangutans, gorillas, bonobos, common chimpanzees and humans. Alternatively, the hominidae family are collectively described as the great apes.
Members of the superfamily are called hominoids (not to be confused with "hominids" or "hominins").
Some or all hominoids are also called "apes". However, the term "ape" is used in several different senses. It has been used as a synonym for "monkey" or for any tailless primate with a humanlike appearance.Thus the Barbary macaque, a kind of monkey, is popularly called the "Barbary ape" to indicate its lack of a tail. Biologists have used the term "ape" to mean a member of the superfamily Hominoidea other than humans, or more recently to mean all members of the superfamily Hominoidea, so that "ape" becomes another word for "hominoid".
Except for gorillas and humans, hominoids are agile climbers of trees. Their diet is best described as vegetarian or omnivorous, consisting of leaves, nuts, seeds and fruits, including grass seeds, and in most cases other animals, either hunted or scavenged (or farmed in the case of humans), along with anything else available and easily digested.
Morphological species differences between human relatives.(Larger) Source
Until recently, most classifications included only humans in this
family; other apes were put in the family Pongidae (from which the
gibbons were sometimes separated as the Hylobatidae). The evidence
linking humans to gorillas and chimps has grown dramatically in the
past two decades, especially with increased use of molecular
techniques. It now appears that chimps, gorillas, and humans form a
clade of closely related species; orangutans are slightly less close
phylogenetically, and gibbons are a more distant branch. Here we
follow a classification reflecting those relationships. Chimps,
gorillas, humans, and orangutans make up the family Hominidae;
gibbons are separated as the closely related Hylobatidae.
Thus constituted, the Hominidae includes 4 genera and 5 species.
Its nonhuman members are restricted to equatorial Africa, Sumatra and
Borneo. Hominid fossils date to the Miocene and are known from Africa
Hominids range in weight from 48 kg to 270 kg. Males are larger
than females. Hominids are the largest primates, with robust bodies
and well-developed forearms. Their pollex
and hallux are
opposable except in humans, who have lost opposability of the big
toe. All digits have flattened nails. No hominid has a tail, and none
has ischial callosities. Numerous skeletal differences between
hominids and other primates are related to their upright or
All members of this family have large
braincase. Most have a prominent face and prognathous jaw; again,
humans are exceptional. All are catarrhine, with nostrils close
together and facing forward and downward. The dental
formula is the same for all members of the group: 2/2, 1/1, 2/2,
3/3 = 32. Hominids have broad
incisors and their canines
are never developed into tusks. The upper
molars are quadrate
and bunodont; the
lowers are bunodont
and possess a hypoconulid.
The uppers lack lophs
connecting labial and lingual cusps and thus, in contrast to
cercopithecids, are not bilophodont.
Hominids are omnivorous, primarily frugivorous or folivorous. All
but humans are good climbers, but only the orangutan is really
Members of this family are well-known for the complexity of their
social behavior. Facial expression and complex vocalizations play an
important role in the behavior of hominids. All make and use nests.
Hominids generally give birth to a single young, and the period of
parental care is extended.