Tetrapod: Encyclopedia II - Tetrapod - Evolution

Tetrapod - Evolution

Tetrapod - Devonian tetrapods

The first tetrapods evolved in shallow and swampy freshwater habitats, towards the end of the Devonian, a little more than 360 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]

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 and choana, 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). 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 haibat the lobe finned fishes were living and made use of their simple lung when the oxygen level in the water became too low.

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 breath air and move around in shallow waters near land as a protection (just as modern ambhibians and fish often spent their first part of the live in shallow waters), two very different niches was partially overlapping each others, with the young juveniles in the diffuse line between. One of them was overcrowded and dangerous (there were most likely always some predators, vertebrates or big invertebrates, with the ability to hunt them even at very low water), the other much safer, much less crowded and nothing or very little competition about the resources. The terrestrial niche was also a much more challenging and different place for primary aquatic animals. But because of the way evolution and the selection pressure works, those juveniles who could take adavantage of this would be rewarded. All they needed were to gain a small foothold on land, and the evolution would take care of the rest. Thanks to all their preadaptations and being at the right place at the right time, hidden potentials could emerge. At this time there was a lot of invertebrates crawling around on land and near water, in moist soil and wet litter, more than big enough to give the small ones a good meal. Some were even big enough to eat small tetrapods, but land would still be a much safer place and offer more than the waters if they knew how to make use of it. Adults would be too heavy and slow and demand bigger prey. Small juveniles were much ligheter, faster and was satisfied with relatively small invertebrates. Modern mudskippers are said to be able to snap insects in flight while on land, so maybe we shouldn't underestimate the early juvenile tetrapods either. Only small trips to land now and then at start, but as the generations went by, they would have become more adapted to terrestrial enviroments and spend longer time away from the water, and also spend a longer part of their childhood on land before they permanently went back to the water for the rest of their life. The adults obvioulsy kept most of the anatomical and other forms of adaptations from their juvenile stage, giving them modified limbs and othere traits of terrestrial properties. The adults of some of the smaller species were in that case probably able to move on land too when the evolution had worked long enough. If some sort of neoteny occured, 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

Tetrapod - 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 Amphibia are representative of the labyrinthodonts, which are comprised of the temnospondyls (e.g. Eryops) and similarly primitive anthracosaurs, who 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 countered among their number the earliest mammals, turtles, and crocodiles (lizards and birds apeared 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.

Tetrapod - 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 amniots, 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.

Tetrapod - Living tetrapods

There are four main categories of living ("crown group") tetrapods:

Note that snakes are considered tetrapods because they are descended from ancestors who had a full complement of limbs. Similar considerations apply to aquatic mammals. Most tetrapods today are terrestrial, at least in their adult forms, but some species, such as the axolotl, remain aquatic. Tetrapods that returned to the sea include ichthyosaurs and modern whales and dolphins.

Adapted from the Wikipedia article "Evolution", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki