Temporal range: Early Devonian - Holocene, 395–0Ma
|Representatives of the four classes of extant tetrapods, (clockwise from upper left): a frog (an amphibian), a hoatzin (a bird), a skink (a reptile), and a mouse (a mammal)|
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The superclass Tetrapoda (Ancient Greek τετραπόδηs tetrapodēs, "four-footed"), or the tetrapods, comprises the first four-limbed vertebrates and their descendants, including the living and extinct amphibians, reptiles, birds, and mammals. The tetrapods evolved from the lobe-finned fishes about 395 million years ago in the Devonian Period. The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear, and are areas of active research and debate among palaeontologists at present.
While most species today are terrestrial, little evidence supports the idea that any of the earliest tetrapods could move about on land, as their limbs could not have held their midsections off the ground and the known trackways do not indicate they dragged their bellies around. Presumably, the tracks were made by animals walking along the bottoms of shallow bodies of water. Amphibians today generally remain semiaquatic, living the first stage of their lives as fish-like tadpoles. Several groups of tetrapods, such as the snakes and cetaceans, have lost some or all of their limbs. In addition, many tetrapods have returned to partially aquatic or fully aquatic lives throughout the history of the group (modern examples of fully aquatic tetrapods include cetaceans and sirenians). The first returns to an aquatic lifestyle may have occurred as early as the Carboniferous Period whereas other returns occurred as recently as the Cenozoic, as in cetaceans, pinnipeds, and several modern amphibians.
A quadruped actually uses four limbs for locomotion; not all tetrapods are quadrupeds.
- 1 Biodiversity
- 2 Evolution
- 2.1 Origin
- 2.2 Palaeozoic tetrapods
- 2.3 Mesozoic tetrapods
- 2.4 Cenozoic tetrapods
- 3 Extant (living) tetrapods
- 4 Classification
- 5 Phylogeny of early tetrapod diversification
- 6 Anatomical features of early tetrapods
- 7 See also
- 8 References
- 9 Further reading
Tetrapoda includes four classes: amphibians, reptiles, mammals, and birds. Overall, the biodiversity of lissamphibians, as well as of tetrapods generally, has grown exponentially over time; more than 30,000 species living today are descended from a single amphibian group in the Early to Middle Devonian. However, that diversification process was interrupted at least a few times by major biological crises such as the Permian–Triassic extinction event, which at least affected amniotes. The overall composition of biodiversity was driven primarily by amphibians in the Palaeozoic, dominated by reptiles in the Mesozoic and expanded by the explosive growth of birds and mammals in the Cenozoic. As biodiversity has grown so has the number of niches tetrapods have occupied. The first tetrapods were aquatic and fed primarily on fish. Today, the Earth supports a great diversity of tetrapods that live in many habitats and subsist on a variety of diets.
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The change from a body plan for breathing and navigating in water to a body plan enabling the animal to move on land is one of the most profound evolutionary changes known. It is also one of the best understood, largely thanks to a number of significant fossil finds in the late 20th century combined with improved phylogenetic analysis.
"Age of Fishes"
The Devonian period is traditionally known as the "Age of Fishes", marking the diversification of numerous extinct and modern major fish groups. Among them were the early bony fishes, who diversified and spread in freshwater and brackish environments at the beginning of the period. The early types resembled their cartilaginous ancestors in many features of their anatomy, including a shark-like tailfin, spiral gut, large pectoral fins stiffened in front by skeletal elements and a largely unossified axial skeleton.
They did, however, have certain traits separating them from cartilaginous fishes, traits that would become pivotal in the evolution of terrestrial forms. With the exception of a pair of spiracles, the gills did not open singly to the exterior as they do in sharks; rather, they were encased in a gill chamber stiffened by membrane bones and covered by a bony operculum, with a single opening to the exterior. The cleithrum bone, forming the posterior margin of the gill chamber, also functioned as anchoring for the pectoral fins. The cartilaginous fishes do not have such an anchoring for the pectoral fins. This allowed for a movable joint at the base of the fins in the early bony fishes, and would later function in a weight bearing structure in tetrapods. As part of the overall armour of rhomboid cosmin scales, the skull had a full cover of dermal bone, constituting a skull roof over the otherwise shark-like cartilaginous inner cranium. Importantly, they also had a swim bladder/lung, a feature lacking in sharks and rays.
Lungs before land
The lung/swim bladder originated as an outgrowth of the gut, forming a gas-filled bladder above the digestive system. In its primitive form, the air bladder was open to the alimentary canal, a condition called physostome and still found in many fish. The primary function is not entirely certain. One consideration is buoyancy. The heavy scale armour of the early bony fishes would certainly weigh the animals down. In cartilaginous fishes, lacking a swim bladder, the open sea sharks need to swim constantly to avoid sinking into the depths, the pectoral fins providing lift. Another factor is oxygen consumption. Ambient oxygen was relatively low in the early Devonian, possibly about half of modern values. Per unit volume, there is much more oxygen in air than in water, and vertebrates are active animals with high energy requirement compared to invertebrates of similar sizes. The Devonian saw increasing oxygen levels, opening up ecological niches as active, large-bodied animals for groups able to exploit aerial oxygen. Particularly in tropical swampland habitats, atmospheric oxygen is much more stable, and may have prompted a reliance of lungs rather than gills for primary oxygen uptake. In the end, both buoyancy and breathing may have been important, and some modern physostome fishes do indeed use their bladders for both.
To function in gas exchange, lungs required a blood supply. In cartilaginous fishes and teleosts, the heart lies low in the body and pumps blood forward through the ventral aorta, which splits up in a series of paired aortic arches, each corresponding to a gill arch. The aortic arches then merge above the gills to form a dorsal aorta supplying the body with oxygenated blood. In lungfishes, bowfin and bichirs, the swim bladder is supplied with blood by paired pulmonary arteries branching off from the hindmost (6th) aortic arch. The same basic pattern is found in the lungfish Protopterus and in terrestrial salamanders, and was likely the pattern found in the tetrapods' immediate ancestors as well as the first tetrapods. In most other bony fishes the swim bladder is supplied with blood by the dorsal aorta.
External and internal nares
The nostrils in most bony fish differ from those of tetrapods. Normally, bony fish have four nares (nasal openings), one naris behind the other on each side. As the fish swims, water flows into the forward pair, across the olfactory tissue, and out through the posterior openings. This is true not only of ray-finned fish but also of the coelacanth, a fish included in the Sarcopterygii, the group that also includes the tetrapods. In contrast, the tetrapods have only one pair of nares externally but also sport a pair of internal nares, called choanae, allowing them to draw air through the nose. Lungfish are also sarcopterygians with internal nostrils, but these are sufficiently different from tetrapod choanae that they have long been recognized as an independent development.
The evolution of the tetrapods' internal nares was hotly debated in the 20th century. The internal nares could be one set of the external ones (usually presumed to be the posterior pair) that have migrated into the mouth, or the internal pair could be a newly evolved structure. To make way for a migration, however, the two tooth-bearing bones of the upper jaw, the maxilla and the premaxilla, would have to separate to let the nostril through and then rejoin; until recently, there was no evidence for a transitional stage, with the two bones disconnected. Such evidence is now available: a small lobe-finned fish called Kenichthys, found in China and dated at around 395 million years old, represents evolution "caught in mid-act", with the maxilla and premaxilla separated and an aperture—the incipient choana—on the lip in between the two bones. Kenichthys is more closely related to tetrapods than is the coelacanth, which has only external nares; it thus represents an intermediate stage in the evolution of the tetrapod condition. The reason for the evolutionary movement of the posterior nostril from the nose to lip, however, is not well understood.
Into the shallows
The relatives of Kenichthys soon established themselves in the waterways and brackish estuaries and became the most numerous of the bony fishes throughout the Devonian and most of the Carboniferous. The basic anatomy of group is well known thanks to the very detailed work on Eusthenopteron by Erik Jarvik in the second half of the 20th century. The bones of the skull roof were broadly similar to those of early tetrapods and the teeth had an infolding of the enamel similar to that of labyrinthodonts. The paired fins had a build with bones distinctly homologous to the humerus, ulna, and radius in the fore-fins and to the femur, tibia, and fibula in the pelvic fins.
There were a number of families: Rhizodontida, Canowindridae, Elpistostegidae, Megalichthyidae, Osteolepidae and Tristichopteridae. Most were open-water fishes, and some grew to very large sizes; adult specimens are several meters in length. The Rhizodontid Rhizodus is estimated to have grown to 7 meters (23 feet), making it the largest freshwater fish known.
While most of these were open-water fishes, one group, the Elpistostegalians, adapted to life in the shallows. They evolved flat bodies for movement in very shallow water, and the pectoral and pelvic fins took over as the main propulsion organs. Most median fins disappeared, leaving only a protocercal tailfin. Since the shallows were subject to occasional oxygen deficiency, the ability to breath atmospheric air with the swim bladder became increasingly important. The spiracle became large and prominent, enabling these fishes to draw air.
The tetrapods have their root in the early Devonian tetrapodomorph fish. Primitive tetrapods developed from an osteolepid tetrapodomorph lobe-finned fish (sarcopterygian-crossopterygian), with a two-lobed brain in a flattened skull. The coelacanth group represents marine sarcopterygians that never acquired these shallow-water adaptations. The sarcopterygians apparently took two different lines of descent and are accordingly separated into two major groups: the Actinistia (including the coelacanths) and the Rhipidistia (which include extinct lines of lobe-finned fishes that evolved into the lungfish and the tetrapodomorphs).
From fins to feet
The oldest known tetrapodomorph is Kenichthys from China, dated at around 395 million years old. Two of the earliest tetrapodomorphs, dating from 380 Ma, were Gogonasus and Panderichthys. They had choanae and used their fins as to move through tidal channels and shallow waters choked with dead branches and rotting plants. Their fins could have been used to attach themselves to plants or similar while they were lying in ambush for prey. The universal tetrapod characteristics of front limbs that bend forward from the elbow and hind limbs that bend backward from the knee can plausibly be traced to early tetrapods living in shallow water. Pelvic bone fossils from Tiktaalik shows, if representative for early tetrapods in general, that hind appendages and pelvic-propelled locomotion originated in water before terrestrial adaptations.
Another indication that feet and other tetrapod traits evolved while the animals were still aquatic is how they were feeding. They did not have the modifications of the skull and jaw that allowed them to swallow prey on land. Prey could be caught in the shallows, at the water's edge or on land, but had to be eaten in water where hydrodynamic forces from the expansion of their buccal cavity would force the food into their esophagus.
It has been suggested that the evolution of the tetrapod limb from lobe-finned fishes is related to the loss of the proteins actinodin 1 and actinodin 2, which are involved in fish fin development Robot simulations indicate the necessary nervous circuitry for walking evolved from the nerves governing swimming, utilizing the sideways oscillation of the body with the limbs primarily functioning as anchoring points and providing limited thrust.
A 2012 study using 3d reconstructions of Ichthyostega concluded that it was incapable of typical quadrupedal gaits. The limbs could not move alternately as they lacked the necessary rotary motion range. In addition, the hind limbs lacked the necessary pelvic musculature for hindlimb-driven land movement. Their most likely method of terrestrial locomotion is that of synchronous "crutching motions", similar to modern mudskippers.
Denizens of the swamp
The first tetrapods probably evolved in coastal and brackish marine environments, and in shallow and swampy freshwater habitats. Formerly, researchers thought the timing was towards the end of the Devonian. In 2010, this belief was challenged by the discovery of the oldest known tetrapod tracks, preserved in marine sediments of the southern coast of Laurasia, now Świętokrzyskie (Holy Cross) Mountains of Poland. They were made during the Eifelian stage at the end of the Middle Devonian. The tracks, some of which show digits, date to about 395 million years ago—18 million years earlier than the oldest known tetrapod body fossils. Additionally, the tracks show that the animal was capable of thrusting its arms and legs forward, a type of motion that would have been impossible in tetrapodomorph fish like Tiktaalik. The animal that produced the tracks is estimated to have been up to 2.5 metres (8.2 ft) long with footpads up to 26 centimetres (10 in) wide, although most tracks are only 15 centimetres (5.9 in) wide. The new finds suggest that the first tetrapods may have lived as opportunists on the tidal flats, feeding on marine animals that were washed up or stranded by the tide. Currently, however, fish are stranded in significant numbers only at certain times of year, as in alewife spawning season; such strandings could not provide a significant supply of food for predators. There is no reason to suppose that Devonian fish were less prudent than those of today. According to Melina Hale of University of Chicago, ancient trackways are not necessarily made by early tetrapods, but could also be created by relatives of the tetrapods who used their fleshy appendages in a similar substrate-based locomotion.
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Research by Jennifer A. Clack and her colleagues showed that the very earliest tetrapods, animals similar to Acanthostega, were wholly aquatic and quite unsuited to life on land. This is in contrast to the earlier view that fish had first invaded the land — either in search of prey (like modern mudskippers) or to find water when the pond they lived in dried out — and later evolved legs, lungs, etc.
By the late Devonian, land plants had stabilized freshwater habitats, allowing the first wetland ecosystems to develop, with increasingly complex food webs that afforded new opportunities. Freshwater habitats were not the only places to find water filled with organic matter and choked with plants with dense vegetation near the water's edge. Swampy habitats like shallow wetlands, coastal lagoons and large brackish river deltas also existed at this time, and there is much to suggest that this is the kind of environment in which the tetrapods evolved. Early fossil tetrapods have been found in marine sediments, and because fossils of primitive tetrapods in general are found scattered all around the world, they must have spread by following the coastal lines — they could not have lived in freshwater only.
One analysis from the University of Oregon suggests no evidence for the "shrinking waterhole" theory - transitional fossils are not associated with evidence of shrinking puddles or ponds - and indicates that such animals would likely not have survived short treks between depleted waterholes. The new theory suggests instead that proto-lungs and proto-limbs were useful adaptations to negotiate the environment in humid, wooded floodplains.
The Devonian tetrapods went through two major bottlenecks during what is known as the Late Devonian extinction; one at the end of the Frasnian stage, and one twice as large at the end of the following Famennian stage. These events of extinctions led to the disappearance of primitive tetrapods with fish-like features like Ichthyostega and their primary more aquatic relatives.  When tetrapods reappear in the fossil record again after the Devonian extinctions, the adult forms are all fully adapted to a terrestrial existence, with later species secondary adapted to an aquatic lifestyle.
Excretion in tetrapods
The common ancestor of all present gnathostomes lived in freshwater, and later migrated back to the sea. To deal with the much higher salinity in sea water, they evolved the ability to turn the nitrogen waste product ammonia into harmless urea, storing it in the body to give the blood the same osmolarity as the sea water without poisoning the organism. This is the system currently found in cartilaginous fishes. Ray-finned fishes (Actinopterygii) later returned to freshwater and lost this ability, while the fleshy-finned fishes (Sarcopterygii) retained it. Since the blood of ray-finned fishes contains more salt than freshwater, they could simply get rid of ammonia through their gills. When they finally returned to the sea again, they did not recover their old trick of turning ammonia to urea, and they had to evolve salt excreting glands instead. Lungfishes do the same when they are living in water, making ammonia and no urea, but when the water dries up and they are forced to burrow down in the mud, they switch to urea production. Like cartilaginous fishes, the coelacanth can store urea in its blood, as can the only known amphibians that can live for long periods of time in salt water (the toad Bufo marinus and the frog Rana cancrivora). These are traits they have inherited from their ancestors.
If early tetrapods lived in freshwater, and if they lost the ability to produce urea and used ammonia only, they would have to evolve it from scratch again later. Not a single species of all the ray-finned fishes living today has been able to do that, so it is not likely the tetrapods would have done so either. Terrestrial animals that can only produce ammonia would have to drink constantly, making a life on land impossible (a few exceptions exist, as some terrestrial woodlice can excrete their nitrogenous waste as ammonia gas). This probably also was a problem at the start when the tetrapods started to spend time out of water, but eventually the urea system would dominate completely. Because of this it is not likely they emerged in freshwater (unless they first migrated into freshwater habitats and then migrated onto land so shortly after that they still retained the ability to make urea), although some species never left, or returned to, the water could of course have adapted to freshwater lakes and rivers.
It is now clear that the common ancestor of the bony fishes (Osteichthyes) had a primitive air-breathing lung—later evolved into a swim bladder in most actinopterygians (ray-finned fishes). This suggests that crossopterygians evolved in warm shallow waters, using their simple lung when the oxygen level in the water became too low.
Fleshy lobe-fins supported on bones rather than ray-stiffened fins seem to have been an ancestral trait of all bony fishes (Osteichthyes). The lobe-finned ancestors of the tetrapods evolved them further, while the ancestors of the ray-finned fishes (Actinopterygii) evolved their fins in a different direction. The most primitive group of actinopterygians, the bichirs, still have fleshy frontal fins.
Fossils of early tetrapods
Nine genera of Devonian tetrapods have been described, several known mainly or entirely from lower jaw material. All but one were from the Laurasian supercontinent, which comprised Europe, North America and Greenland. The only exception is a single Gondwanan genus, Metaxygnathus, which has been found in Australia.
The first Devonian tetrapod identified from Asia was recognized from a fossil jawbone reported in 2002. The Chinese tetrapod Sinostega pani was discovered among fossilized tropical plants and lobe-finned fish in the red sandstone sediments of the Ningxia Hui Autonomous Region of northwest China. This finding substantially extended the geographical range of these animals and has raised new questions about the worldwide distribution and great taxonomic diversity they achieved within a relatively short time.
These earliest tetrapods were not terrestrial. The earliest confirmed terrestrial forms are known from the early Carboniferous deposits, some 20 million years later. Still, they may have spent very brief periods out of water and would have used their legs to paw their way through the mud.
Why they went to land in the first place is still debated. One reason could be that the small juveniles who had completed their metamorphosis had what it took to make use of what land had to offer. Already adapted to breathe air and move around in shallow waters near land as a protection (just as modern fish (and amphibians) often spent the first part of their life in the comparative safety of shallow waters like mangrove forests), two very different niches partially overlapped each other, with the young juveniles in the diffuse line between. One of them was overcrowded and dangerous while the other was much safer and much less crowded, offering less competition over resources. The terrestrial niche was also a much more challenging place for primary aquatic animals, but because of the way evolution and the selection pressure works, those juveniles who could take advantage of this would be rewarded. Once they gained a small foothold on land, thanks to their preadaptations and being at the right place at the right time, favourable variations in their descendants would gradually result in continuing evolution and diversification.
At this time the abundance of invertebrates crawling around on land and near water, in moist soil and wet litter, offered a food supply. Some were even big enough to eat small tetrapods, but the land was free from dangers common in the water.
From water to land
Initially making only tentative forays onto land, tetrapods adapted to terrestrial environments over time and spent longer periods away from the water. It is also possible that the adults started to spend some time on land (as the skeletal modifications in early tetrapods such as Ichthyostega suggests) to bask in the sun close to the water's edge, while otherwise being mostly aquatic.
Until the 1990s, there was a 30 million year gap in the fossil record between the late Devonian tetrapods and the reappearance of tetrapod fossils in recognizable mid-Carboniferous amphibian lineages. It was referred to as "Romer's Gap", which now covers the period from about 360 to 345 million years ago (the Devonian-Carboniferous transition and the early Mississippian), after the palaeontologist who recognized it.
During the "gap", tetrapod backbones developed, as did limbs with digits and other adaptations for terrestrial life. Ears, skulls and vertebral columns all underwent changes too. The number of digits on hands and feet became standardized at five, as lineages with more digits died out. The very few tetrapod fossils found in the "gap" are all the more precious.
The transition from an aquatic lobe-finned fish to an air-breathing amphibian was a momentous occasion in the evolutionary history of the vertebrates. For an animal to live in a gravity-neutral, aqueous environment and then invade one that is entirely different required major changes to the overall body plan, both in form and in function. Eryops is an example of an animal that made such adaptations. It retained and refined most of the traits found in its fish ancestors. Sturdy limbs supported and transported its body while out of water. A thicker, stronger backbone prevented its body from sagging under its own weight. Also, by utilizing vestigial fish jaw bones, a rudimentary ear was developed, allowing Eryops to hear airborne sound.
By the Visean age of mid-Carboniferous times the early tetrapods had radiated into at least three main branches. Recognizable basal-group tetrapods are representative of the temnospondyls (e.g. Eryops) lepospondyls (e.g. Diplocaulus) and anthracosaurs, which were the relatives and ancestors of the Amniota. Depending on whichever authorities one follows, modern amphibians (frogs, salamanders and caecilians) are derived from either temnospondyls or lepospondyls (or possibly both, although this is now a minority position).
The first amniotes are known from the early part of the Late Carboniferous, and during the Triassic counted among their number the earliest mammals, turtles, crocodiles (lizards and birds appeared in the Jurassic, and snakes in the Cretaceous), and a fourth Carboniferous group, the baphetids, which are thought related to temnospondyls, left no modern survivors.
Amphibians and reptiles were affected by the Carboniferous Rainforest Collapse (CRC), an extinction event that occurred ~300 million years ago. The sudden collapse of a vital ecosystem shifted the diversity and abundance of major groups. Several large groups, labyrinthodont amphibians were particularly devastated, while the first reptiles fared better, being ecologically adapted to the drier conditions that followed. Amphibians must return to water to lay eggs, in contrast, reptiles - whose amniote eggs have a membrane ensuring gas exchange out of water and can therefore be laid on land - were better adapted to the new conditions. Reptiles invaded new niches at a faster rate and began diversifying their diets, developing herbivory and carnivory, previously only having been insectivores and piscivores.
In the Permian period, in addition to temnospondyl and anthracosaur clades among the early "amphibia" (labyrinthodonts), there were two important clades of amniotes, the Sauropsida and the Synapsida. The latter were the most important and successful Permian animals.
The end of the Permian saw a major turnover in fauna during the Permian–Triassic extinction event. There was a protracted loss of species, due to multiple extinction pulses. Many of the once large and diverse groups died out or were greatly reduced.
Life on Earth seemed to recover quickly after the Permian extinctions, but this was mostly in the form of disaster taxa, such as the hardy Lystrosaurus; specialized animals that formed complex ecosystems, with high biodiversity, complex food webs and a variety of niches, took much longer to recover. Current research indicates that this long recovery was due to successive waves of extinction, which inhibited recovery, and to prolonged environmental stress to organisms that continued into the Early Triassic. Recent research indicates that recovery did not begin until the start of the mid-Triassic, 4M to 6M years after the extinction; and some writers estimate that the recovery was not complete until 30M years after the P-Tr extinction, i.e. in the late Triassic.
A small group of reptiles, the diapsids, began to diversify during the Triassic, notably the dinosaurs. By the late Mesozoic, the large labyrinthodont groups that first appeared during the Paleozoic such as temnospondyls and reptile-like amphibians had gone extinct. All current major groups of sauropsids evolved during the Mesozoic, with birds first appearing in the Jurassic as a derived clade of theropod dinosaurs. Many groups of synapsids such as anomodonts and therocephalians that once comprised the dominant terrestrial fauna of the Permian also became extinct during the Mesozoic; during the Triassic, however, one group (Cynodontia) gave rise to the descendant taxon Mammalia, which survived through the Mesozoic to later diversify during the Cenozoic.
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Extant (living) tetrapods
Following the great faunal turnover at the end of the Mesozoic, only six major groups of tetrapods were left, all of which also include many extinct groups:
- Lissamphibia: frogs and toads, newts and salamanders, and caecilians
- Testudines: turtles and tortoises
- Lepidosauria: tuataras, lizards, amphisbaenians and snakes
- Crocodilia: crocodiles, alligators, caimans and gharials
- Neornithes: modern birds
- Mammalia: mammals
Classification of tetrapods has a long history. Traditionally, tetrapods are divided into four classes based on gross anatomical and physiological traits. Snakes and other legless reptiles are considered tetrapods because they are sufficiently like other reptiles that have a full complement of limbs. Similar considerations apply to caecilians and aquatic mammals. Newer taxonomy is frequently based on cladistics instead, giving a variable number of major "branches" (clades) of the tetrapod family tree.
As is the case throughout evolutionary biology today, there is debate over how to properly classify the groups within Tetrapoda. Traditional biological classification recognizes evolutionary transitions between older groups and descendant groups with markedly different characteristics. For example, the birds, which evolved from the dinosaurs, are defined as a separate group from them, because they represent a distinct new type of physical form and functionality. In phylogenetic nomenclature, in contrast, the newer group is always included in the old. For this school of taxonomy, dinosaurs and birds are not groups in contrast to each other, but rather birds are a sub-type of dinosaurs.
History of classification
The tetrapods, including all large- and medium-sized land animals, have been among the best understood animals since earliest times. Already in Aristotle's days, the basic division between mammals, birds and egg-laying tetrapods (the "herptiles") was well known, and the inclusion of the legless snakes into this group was likewise recognized. With the birth of modern biological classification in the 18th century, Linnaeus used the same division, with the tetrapods occupying the first three of his six classes of animals. While reptiles and amphibians can be quite similar externally, the French zoologist Pierre André Latreille recognized the large physiological differences at the beginning of the 19th century and split the herptiles into two classes, giving the four familiar classes of tetrapods: amphibians, reptiles, birds and mammals.
With the basic classification of tetrapods settled, a half a century followed where the classification of living and fossil groups predominately was done by experts working within classes. In the early 1930s, American vertebrate palaeontologist Alfred Romer (1894–1973) produced an overview, drawing together taxonomic work from the various subfields to create an orderly taxonomy in his Vertebrate Paleontology. This classical scheme with minor variations is still used in works where systematic overview is essential, e.g. Benton (1998) and Knobill and Neill (2006). While mostly seen in general works, it is also still used in some specialist works like Fortuny & al. (2011). The taxonomy down to subclass level shown here is from Hildebrand and Goslow (2001):
- Superclass Tetrapoda - four-limbed vertebrates
- Class Amphibia - amphibians
- Class Reptilia - reptiles, (ancestors of both birds and mammals)
- Class Aves - birds
- Class Mammalia - mammals
This classification is the one most commonly encountered in school textbooks and popular works. While orderly and easy to use, has come under critique from cladistics. The earliest tetrapods are grouped under Class Amphibia, although several of the groups are more closely related to amniotes than to modern day amphibians. Traditionally, birds are not considered a type of reptile, but crocodiles are more closely related to birds than they are to other reptiles like lizards. Basal non-mammalian synapsids ("mammal-like reptiles") traditionally also sort under Class Reptilia as a separate subclass but they are more closely related to mammals than to living reptiles. Considerations like these have led some authors to argue for a new classification based purely on phylogeny, disregarding the anatomy and physiology.
Phylogeny of early tetrapod diversification
- Note: The origin of the subclass Lissamphibia, to which all extant amphibians belong, is disputed. This cladogram is the result of one analysis conducted by Ruta, Jeffery & Coates (2003) that placed Lissamphibia within Lepospondyli, with the latter clade being within the crown group Tetrapoda. A second analysis by the authors placed Lissamphibia within Temnospondyli, thus placing Lepospondyli outside crown group Tetrapoda and Temnospondyli within. Another prevailing theory not represented by either of the cladograms is a diphyletic grouping of Lissamphibia with both Lepospondyli and Temnospondyli, with caecilians belonging to the former clade and frogs and salamanders belonging to the latter.
Anatomical features of early tetrapods
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The tetrapod's ancestral fish must have possessed similar traits to those inherited by the early tetrapods, including internal nostrils (to separate the breathing and feeding passages) and a large fleshy fin built on bones that could give rise to the tetrapod limb. The rhipidistian crossopterygians fulfill every requirement for this ancestry. Their palatal and jaw structures were identical to those of early tetrapods, and their dentition was identical too, with labyrinthine teeth fitting in a pit-and-tooth arrangement on the palate. The crossopterygian paired fins were smaller than tetrapod limbs, but the skeletal structure was very similar in that the crossopterygian had a single proximal bone (analogous to the humerus or femur), two bones in the next segment (forearm or lower leg), and an irregular subdivision of the fin, roughly comparable to the structure of the carpus / tarsus and phalanges of a hand.
The major difference between crossopterygians and early tetrapods was in relative development of front and back skull portions; the snout is much less developed than in most early tetrapods and the post-orbital skull is exceptionally longer than an amphibian's.
A great many kinds of early tetrapods lived during the Carboniferous period. Therefore, their ancestor would have lived earlier, during the Devonian period. Devonian ichthyostegans were among the earliest of the tetrapods, with a skeleton that is directly comparable to that of rhipidistian ancestors. Early temnospondyls (Late Devonian to Early Mississippian) still had some ichthyostegid features such as similar skull bone patterns, labyrinthine tooth structure, the fish skull-hinge, pieces of gill structure between the cheek and shoulder, and the vertebral column. They had, however, lost several other fish features such as the fin rays in the tail.
To propagate in the terrestrial environment, animals had to overcome certain challenges. Their bodies needed additional support, because buoyancy was no longer a factor. They needed a new method of respiration to extract atmospheric oxygen, instead of oxygen dissolved in water. Animals had to develop new means of locomotion to traverse distances between waterholes. Water retention was now important, since it was no longer the living matrix, and could be lost easily to the environment. Finally, animals needed new sensory input systems to have any ability to function reasonably on land.
The most notable characteristics that make a tetrapod's skull different from a fish's are the relative frontal and rear portion lengths. The fish had a long rear portion while the front was short; the orbital vacuities were thus located towards the anterior end. In the tetrapod, the front of the skull lengthened, positioning the orbits farther back on the skull. The lacrimal bone was not in contact with the frontal anymore, having been separated from it by the prefrontal bone. Also of importance is that the skull was now free to rotate from side to side, independent of the spine, on the newly forming neck.
A diagnostic character of temnospondyls is that the tabular bones (which formed the posterior corners of the skull-table) were separated from the respective left and right parietals by a sutural junction between the postparietals and supratemporals. Also at the rear of the skull, all bones dorsal to the cleithrum were lost.
The lower jaw of, for example, Eryops resembled its crossopterygian ancestors in that on the outer surface lay a long dentary that bore teeth. There were also bones below the dentary on the jaw: two splenials, the angular and the surangular. On the inside were usually three coronoids that bore teeth and lay close to the dentary. On the upper jaw was a row of marginal labyrinthine teeth, located on the maxilla and premaxilla. In Eryops, as in all early amphibians, the teeth were replaced in waves that traveled from the front of the jaw to the back in such a way that every other tooth was mature, and the ones in between were young.
The "labyrinthodonts" had a peculiar tooth structure, from which their name derives—and though not exclusive to the group, the labyrinthine dentition is a useful indicator as to proper classification. The important feature of the tooth is that the enamel and dentine fold into a complicated corrugated pattern when viewed in cross section. This infolding strengthened the tooth and increased wear resistance. Such teeth survived for 100 Ma, first among crossopterygian fish, then stem reptiles. Modern amphibians no longer have this type of dentition, but rather pleurodont teeth, in fewer numbers of the whole group.
The difference in density between air and water causes smells (certain chemical compounds detectable by chemoreceptors) to behave differently. An animal first venturing out onto land would have difficulty in locating such chemical signals if its sensory apparatus was designed for aquatic detection.
Fish have a lateral line system that detects pressure fluctuations in the water. Such pressure is non-detectable in air, but grooves for the lateral line sense organs were found on the skull of labyrinthodonts, suggesting a partially aquatic habitat. Modern amphibians, which are semi-aquatic, exhibit this feature whereas it has been retired by the higher vertebrates. The olfactory epithelium would also have to change to detect airborne odors.
In addition to the lateral line organ system, the eye had to change. This change came about because the refractive index of light differs between air and water, so the focal length of the lens altered to function in air. The eye was now exposed to a relatively dry environment rather than being bathed by water, so eyelids developed and tear ducts evolved to produce a liquid to moisten the eyeball.
Animals retained the balancing function of the middle ear from fish ancestry. However, delicate air vibrations could not set up pulsations through the skull as in a proper auditory organ. Typical of most labyrinthodonts, the spiracular gill pouch was retained as the otic notch, closed in by the tympanum, a thin, tight membrane.
The hyomandibula of fish migrated upwards from its jaw supporting position, and was reduced in size to form the stapes. Situated between the tympanum and braincase in an air-filled cavity, the stapes was now capable of transmitting vibrations from the exterior of the head to the interior. Thus the stapes became an important element in an impedance matching system, coupling airborne sound waves to the receptor system of the inner ear. This system had evolved independently within several different amphibian lineages.
The impedance matching ear had to meet certain conditions to work. The stapes had to be perpendicular to the tympanum, small and light enough to reduce its inertia, and suspended in an air-filled cavity. In modern species that are sensitive to over 1 kHz frequencies, the footplate of the stapes is 1/20th the area of the tympanum. However, in early amphibians the stapes was too large, making the footplate area oversized, preventing the hearing of high frequencies. So it appears they could only hear high intensity, low frequency sounds—and the stapes more probably just supported the brain case against the cheek.
The pectoral girdle of early tetrapods such as Eryops was highly developed, with a larger size for both increased muscle attachment to it and to the limbs. Most notably, the shoulder girdle was disconnected from the skull, resulting in improved terrestrial locomotion. The crossopterygian cleithrum was retained as the clavicle, and the interclavicle was well-developed, lying on the underside of the chest. In primitive forms, the two clavicles and the interclavical could have grown ventrally in such a way as to form a broad chest plate, although such was not the case in Eryops. The upper portion of the girdle had a flat, scapular blade, with the glenoid cavity situated below performing as the articulation surface for the humerus, while ventrally there was a large, flat coracoid plate turning in toward the midline.
The pelvic girdle also was much larger than the simple plate found in fishes, accommodating more muscles. It extended far dorsally and was joined to the backbone by one or more specialized sacral ribs. The hind legs were somewhat specialized in that they not only supported weight, but also provided propulsion. The dorsal extension of the pelvis was the ilium, while the broad ventral plate was composed of the pubis in front and the ischium in behind. The three bones met at a single point in the center of the pelvic triangle called the acetabulum, providing a surface of articulation for the femur.
The main strength of the ilio-sacral attachment of Eryops was by ligaments, a condition structurally, but not phylogenetically, intermediate between that of the most primitive embolomerous amphibians and early reptiles. The condition that is more usually found in higher vertebrates is that cartilage and fusion of the sacral ribs to the blade of the ilium are utilized in addition to ligamentous attachments.
The humerus was the largest bone of the arm, its head articulating with the glenoid cavity of the pectoral girdle, distally with the radius and ulna. The radius resided on the inner side of the forearm and rested directly under the humerus, supporting much of the weight, while the ulna was located to the outside of the humerus. The ulna had a head, which muscles pulled on to extend the limb, called the olecranon that extended above the edge of the humerus.
The radius and the ulna articulated with the carpus, which was a proximal row of three elements: the radiale underlying the radius, the ulnare underneath the ulna and an intermedium between the two. A large central element was beneath the last and may have articulated with the radius. There were also three smaller centralia lying to the radial side. Opposite the head of each toe lay a series of five distal carpals. Each digit had a first segment, the metacarpal, lying in the palm region.
The pelvic limb bones were essentially the same as in the pectoral limb, but with different names. The analogue to the humerus was the femur, which was longer and slimmer. The two lower arm bones corresponded to the tibia and fibula of the hind leg, the former being the innermost and the latter the outermost bones. The tarsus is the hind version of the carpus and its bones correspond as well.
In typical early tetrapod posture the upper arm and upper leg extended nearly straight horizontal from its body, and the forearm and the lower leg extended downward from the upper segment at a near right angle. The body weight was not centered over the limbs, but was rather transferred 90 degrees outward and down through the lower limbs, which touched the ground. Most of the animal's strength was used to just lift its body off the ground for walking, which was probably slow and difficult. With this sort of posture, it could only make short broad strides. This has been confirmed by fossilized footprints found in Carboniferous rocks.
Early tetrapods had a wide gaping jaw with weak muscles to open and close it. In the jaw were fang-like palatal teeth that, when coupled with the gape, suggests an inertial feeding habit. This is when the amphibian would grasp the prey and, lacking any chewing mechanism, toss the head up and backwards, throwing the prey farther back into the mouth. Such feeding is seen today in the crocodile and alligator. A study of these jaws shows they fed underwater, not on land. As it is taken that early tetrapods were not very active, this suggests that they were not predatory. It is more likely that it fed on fish either in the water or on those that became stranded at the margins of lakes and swamps.
The tongue of modern adult amphibians is quite fleshy and attached to the front of the lower jaw, so it is reasonable to speculate that it was fastened in a similar fashion in primitive forms, although it was probably not specialized like it is in a frog.
Modern amphibians breathe by inhaling air into lungs, where oxygen is absorbed. They also breathe through the moist lining of the mouth and skin, known as cutaneous respiration. Eryops also inhaled, but its ribs were too closely spaced to suggest that it did this by expanding the rib cage. More likely, it breathed by buccal pumping in which it opened its mouth and nostrils, depressed the hyoid apparatus to expand the oral cavity, closed its mouth and nostrils finally and elevated the floor of the mouth to force air back into the lungs — in other words, it gulped, then swallowed. It probably exhaled by contraction of the elastic tissue in the lung walls. Other special respiratory methods probably existed.
Early tetrapods most likely had a three-chambered heart, as do modern amphibians and reptiles, in which oxygenated blood from the lungs and de-oxygenated blood from the respiring tissues enters by separate atria, and is directed via a spiral valve to the appropriate vessel — aorta for oxygenated blood and pulmonary vein for deoxygenated blood. The spiral valve is essential to keeping the mixing of the two types of blood to a minimum, enabling the animal to have higher metabolic rates, and be more active than otherwise.
Ligamentous attachments within the limbs were present in Eryops, being important because they were the precursor to bony and cartilaginous variations seen in modern terrestrial animals that use their limbs for locomotion.
Of all body parts, the spine was the most affected by the move from water to land. It now had to resist the bending caused by body weight and had to provide mobility where needed. Previously, it could bend along its entire length. Likewise, the paired appendages had not been formerly connected to the spine, but the slowly strengthening limbs now transmitted their support to the axis of the body.
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