A tetrapod (Greek tetrapoda, "four-legged") is a vertebrate animal having four feet, legs or leglike appendages. Since amphibians, reptiles, dinosaurs and mammals are all tetrapods, and even birds and snakes are tetrapods by decline, the word is only truly useful in describing the earliest tetrapods, which radiated from the Sarcopterygii, or "lobe-finned" fishes, into air-breathing amphibians in the Devonian period.

The Latin version of "tetrapod" is "quadruped," meaning any four-legged creature.

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.

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 linked lobe-finned fish without these shallow-water adaptations. These fishes used their fins as paddles in shallow-water habitats choked with plants and detritus. The universal tetrapod individuality of front limbs that bend backward at the elbow and hind limbs that bend onward at the knee can plausibly be traced to early tetrapods living in shallow water.

The evolution of the air-breathing lung from the primitive swim bladder of lobe-finned fishes has not yet been worked out in specify. However, functioning internal gills were there in at least one late Devonian tetrapod, Acanthostega.

Nine genera of Devonian tetrapods have been described, several known mostly or entirely from lower jaw material. All of them were from the European-North American supercontinent that comprised Europe, North America and Greenland. The only exception is a single Gondwanan genus, Metaxygnathus, which has been establish in Australia.

The first Devonian tetrapod recognized from Asia was recognized from a fossil jawbone reported in 2002. The Chinese tetrapod Sinostega pani was discovered amongst fossilized tropical plants and lobe-finned fish in the red sandstone sediments of the Ningxia Hui Autonomous Region of northwest China. This finding considerably extended the geographical range of these animals and has raised new questions about the worldwide sharing 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 afterward. Still, they may have spent very brief periods out of water and would have used their legs to paw their way throughout the mud.

Cladistic analysis of osteolepiform Sarcopterygians

Carboniferous Tetrapods

Until the 1990s, there was a 30-million year gap in the fossil record among 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 documented it.

During the "gap", tetrapod backbones residential, 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 little tetrapod fossils found in the "gap" are all the more precious.

The evolution 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 main 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 establish 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 three major branches. Recognizable basal-group Amphibia (frogs, salamanders and caecilians) are envoy of the labyrinthodonts, which are comprised of the temnospondyls (e.g. Eryops) and likewise primitive Amniota that now include mammals, turtles, crocodiles, birds, lizards and snakes, represented in mid-Carboniferous times by the anthracosaurs. As living members of the tetrapod clan—that is of the tetrapod "crown-group"—these different tetrapods represent the phylogenetic end-points of these two divergent lineages. A third Palaeozoic group, the baphetids, left no modern survivors.

Permian Tetrapods

In the Permian period, as the part tetrapod lineages each developed in their own way, the term "tetrapoda" becomes less useful. Each lineage, however, remains grouped with the tetrapoda, just as Homo sapiens could be measured a very highly specialized kind of lobe-finned fish.

Most tetrapods today are terrestrial, at least in their mature forms, but some species, such as the axolotl, remain aquatic. Tetrapods that returned to the sea comprise ichthyosaurs and modern whales and dolphins.

Classification of Tetrapods

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

Amphibia
frogs and toads, newts and salamanders
Anapsida
Only extant examples are turtles
Synapsida
Many extinct species and all mammals
Diapsida
dinosaurs, most modern reptiles, and birds
Note that snakes are measured tetrapods because they are descended from ancestors who had a full complement of limbs. Similar considerations apply to aquatic mammals.