Tetrapod

A tetrapod (; from Ancient Greek τετρα (tetra) 'four' and πούς (poús) 'foot') is any vertebrate animal of the clade Tetrapoda (). The main diagnostic and defining trait (synapomorphy) of tetrapods is having four limbs, connected to the shoulder- and pelvic girdles, for which the group is named. As a clade, Tetrapoda is defined such that it includes every organism which descended from four-limbed ancestors, including those which have lost some or all of these limbs, e.g. snakes. Tetrapods include all extant and extinct amphibians and amniotes, with the latter in turn evolving into two major clades, the sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (extinct "pelycosaurs", therapsids and all extant mammals, including humans). Hox gene mutations have resulted in some tetrapods becoming limbless (snakes, legless lizards, and caecilians) or two-limbed (cetaceans, sirenians, some lizards, kiwis, and the extinct moa and elephant birds). Nevertheless, they still qualify as tetrapods through their ancestry, and some retain a pair of vestigial spurs that are remnants of the hindlimbs. Tetrapods evolved from a group of semiaquatic animals within the tetrapodomorphs which, in turn, evolved from ancient lobe-finned fish (sarcopterygians) around 390 million years ago in the Middle Devonian period. Early tetrapodomorphs were transitional between lobe-finned fishes and true four-limbed tetrapods, though most still fit the body plan expected of other lobe-finned fishes. The oldest fossils of four-limbed vertebrates (tetrapods in the broad sense of the word) are trackways from the Middle Devonian, and body fossils became common near the end of the Late Devonian, around 370–360 million years ago. These Devonian species all belonged to the tetrapod stem group, meaning that they did not belong to any modern tetrapod group. Limbs evolved prior to terrestrial locomotion, but by the start of the Carboniferous Period, 360 million years ago, a few stem-tetrapods...