A tetrapod is a vertebrate animal having four feet, legs or leglike appendages. The technical term tetrapoda literally means 'four-legged' (from the Greek).
The first tetrapods evolved in shallow and swampy freshwater habitats, towards the end of the Devonian period, a bit more than 360 million years ago. The primitive tetrapods developed from osteolopiform lobe-fin fishes, the Sarcopterygii— the 'living fossil' coelacanth is a lobe-finned fish— which had already developed some adaptations of fins with fleshy bases and bones, which they used as paddles in shallow water habitats choked with plants and detritus. The universal tetrapod characters 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.
The evolution of the air-breathing lung from the primitive swim-bladder of lobe-finned fishes has not yet been worked out in detail. Functioning internal gills were present in at least one Late Devonian tetrapod, Acanthostega.
These earliest tetrapods were not terrestrial. The earliest confirmed terrestrial forms are known from the early Carboniferous deposits, some 20 million years later. At last they would have used their legs to paw their way through the mud.
Devonian tetrapods: external links
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 amphibian lineages, in the mid-Carboniferous. It was referred to as 'Romer's Gap' for the palaeontologist who recognized it.
During the 'gap' tetrapod backbones developed, limbs with digits and other adaptations for terrestrial life and walking gaits. But limbs were not the only organs involved. Ears, skulls and vertebral columns all underwent changes too. The number of digits on manes (hand) and (pes) foot became standardized. The very few tetrapod fossils in the 'gap' are all the more precious. Jennifer A. Clack, Gaining Ground: the origin and evolution of tetrapods analyzes them.
By the Visean era of mid-Carboniferous times, recognizable basal-group Amphibia (frogs, salamanders and caecilians) are represented by the temnospondyls and similarly primitive Amniota (which now include mammals, turtles, crocodiles, birds, lizards and snakes) are represented by the anthracosaurs. Living members of the tetrapod clan (that is, 'crown-group') tetrapods represent the phylogenetic end-points of these two divergent lineages. A third Palaeozoic group, the baphetids left no modern survivors.
Carboniferous tetrapods: external links
In the Permian period the term 'tetrapoda' becomes less useful, as the separate tetrapod lineages each developed in their own way. Each lineage, however, remained part of the tetrapoda, just as Homo sapiens could be considered a very highly-specialized kind of lobe-finned fish!
Today, most tetrapods are land-dwelling, at least in their adult forms, but some species, such as the axolotl remain aquatic. whales and ichthyosaurs are tetrapods that have returned to the sea.
There are four main 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
;Dyapsida : dinosaurs, most modern reptiles, birds
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.