Tetrapods (Greek τετραποδη tetrapoda, Latin quadruped, "four-footed") are vertebrate animals having four feet, legs or leglike appendages. Amphibians, reptiles, dinosaurs/birds, and mammals are all tetrapods, and even the limbless snakes are tetrapods by descent. The earliest tetrapods radiated from the Sarcopterygii, or lobe-finned fish, into air-breathing amphibians in the Devonian period.
[edit] Evolution
•Panderichthys, suited to muddy shallows;
•Tiktaalik with limb-like fins that could take it onto land;
•Early tetrapods in weed-filled swamps, such as;
•Acanthostega, which had feet with eight digits,
•Ichthyostega with limbs.
Descendants also included pelagic lobe-finned fish such as coelacanth species.
[edit] Devonian tetrapods
Research by Jennifer A. Clack and her colleagues showed that the earliest tetrapods, such as Acanthostega, were wholly aquatic and quite unsuited to life on land. This overturned 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.
The first tetrapods are now thought to have evolved in shallow and swampy freshwater habitats, towards the end of the Devonian, a little more than 365 million years ago. 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. [1] 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.
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 make the blood as salty as the sea water without poisoning the organism. Ray-finned fishes later returned to freshwater and lost this ability. Since their blood contained more salt than freshwater, they could simply get rid of ammonia through their gills. When they finally returned to the sea again, they could 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.
Primitive tetrapods developed from a lobe-finned fish (an "osteolepid sarcopterygian"), with a two-lobed brain in a flattened skull, a wide mouth and a short snout, whose upward-facing eyes show that it was a bottom-dweller, and which had already developed adaptations of fins with fleshy bases and bones (the "living fossil" coelacanth is a related marine lobe-finned fish without these shallow-water adaptations).
Even more closely related was Panderichthys, which even had a choana. These fishes used their fins as paddles in shallow-water habitats choked with plants and detritus. Their fins could also have been used to attach themselves to plants or similar while they were laying in ambush for prey. The universal tetrapod characteristics of front limbs that bend backward at the elbow and hind limbs that bend forward at the knee can plausibly be traced to early tetrapods living in shallow water.
It is now clear that the common ancestor of the bony fishes had a primitive air-breathing lung (later evolved into a swim bladder in most ray-finned fishes). This suggests that it evolved in warm shallow waters, the kind of habitat the lobe finned fishes were living and made use of their simple lung when the oxygen level in the water became too low.
The lungfishes are now considered as being the closest living relatives of the tetrapods, even closer than the coelacanth.
Fleshy lobe fins supported on bones rather than ray-stiffened fins seems to have been an original trait of the bony fishes (Osteichthyes). The lobe-finned ancestors of the tetrapods evolved them further, while the ancestors of the ray-finned (Actinopterygii) fishes evolved their fins in the opposite direction. The most primitive group of the ray-fins, the bichirs, still have fleshy frontal fins.
Nine genera of Devonian tetrapods have been described, several known mainly or entirely from lower jaw material. All of them were from the European-North American 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.
It is plausible that at first adults would be too heavy and slow and have greater needs for large prey. Small juveniles would be much lighter, faster and could subsist on relatively small invertebrates. Modern mudskippers are said to be able to snap insects in flight while on land, and the early juvenile tetrapods might also have shown formidable ablilities.[citation needed]
Initially making only tentative forays onto land, as the generations went by they adapted to terrestrial environments and spent longer periods away from the water, also spending a longer part of their childhood on land before returning to the water for the rest of their life. It is possible also the adults started to spend some time on land as the skeletal modifications in early tetrapods as Ichthyostega suggests, but only to bask in the sun close to the water's edge, not to hunt or move around. It is a fact that the first true tetrapods adapted to terrestrial locomotion were small. Only later did they increase in size.
The fully grown obviously kept most of the anatomical and other forms of adaptations from their juvenile stage, giving them modified limbs and other traits of terrestrial properties. To be successful adults they first had to be successful juveniles. The adults of some of the smaller species were in that case probably able to move on land too when sufficiently evolved.
If some sort of neoteny or dwarfism occurred, making the animals sexually mature and fully grown while still living on land, they would only need to visit water to drink and reproduce.
- Cladistic analysis of osteolepiform Sarcopterygians
[edit] Carboniferous tetrapods
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", 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) and similarly primitive 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 one or the other (or possibly both, although this is now a minority position) of these two groups. 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, and crocodiles (lizards and birds appeared in the Jurassic, and snakes in the Cretaceous). As living members of the tetrapod clan — that is of the tetrapod "crown-group" — these varied tetrapods represent the phylogenetic end-points of these two divergent lineages. A third, more primitive, Carboniferous group, the baphetids, left no modern survivors. Finally, the Lepospondyli are an extinct Palaeozoic group of uncertain relationships.
[edit] Permian tetrapods
In the Permian period, as the separate tetrapod lineages each developed in their own way, the term "tetrapoda" becomes less useful. In addition to temnospondyl and anthracosaur clades among the early "amphibia" (labyrinthodonts), there were two important divergent clades of amniotes, the Sauropsida and the Synapsida, of which the latter were the most important and successful Permian animals. Each of these lineages, however, remains grouped with the tetrapoda, just as Homo sapiens could be considered a very highly-specialized kind of lobe-finned fish.
[edit] Living tetrapods
There are three main categories of living ("crown group") tetrapods, all of which also include many extinct groups:
- Amphibia
- frogs and toads, newts and salamanders, and caecilians
- Sauropsida
- birds and modern reptiles
- Synapsida
- mammals
Note that snakes and other legless reptiles are considered tetrapods because they are descended from ancestors who had a full complement of limbs. Similar considerations apply to caecilians and aquatic mammals.
