Tetrapods (/ˈtɛtrəˌpɒdz/;[4] from Ancient Greek τετρα- (tetra-) 'four', and πούς (poús) 'foot') are four-limbed animals constituting the superclass Tetrapoda (/tɛˈtrɒpədə/). It includes extant and extinctamphibians, reptiles (including dinosaurs and therefore birds), and synapsids (including mammals). Tetrapods evolved from a group of animals known as the Tetrapodomorpha which, in turn, evolved from ancient sarcopterygian fish around 390 million years ago in the middle Devonian period;[5] their forms were transitional between lobe-finned fishes and the four-limbed tetrapods. The first crown-tetrapods (from a traditional, apomorphy-based perspective) appeared by the very early Carboniferous, 350 million years ago.[6] The specific aquatic ancestors of the tetrapods and the process by which they colonized Earth's land after emerging from water remains unclear. The change from a body plan for breathing and navigating in water to a body plan enabling the animal to move on land is one of the most profound evolutionary changes known.[7][8] The first tetrapods (stem) or "fishapods" were primarily aquatic. Modern amphibians, which evolved from earlier groups, are generally semiaquatic; the first stage of their lives is as fish-like tadpoles, and later stages are partly terrestrial and partly aquatic. However, most tetrapod species today are amniotes, most of which are terrestrial tetrapods whose branch evolved from earlier tetrapods about 340 million years ago (crown amniotes evolved 318 million years ago).[citation needed] The key innovation in amniotes over amphibians is the amnion, which enables the eggs to retain their aqueous contents on land, rather than needing to stay in water. (Some amniotes later evolved internal fertilization, although many aquatic species outside the tetrapod tree had evolved such before the tetrapods appeared, e.g. Materpiscis.)
Amniote tetrapods began to dominate and drove most amphibian tetrapods to extinction.[clarification needed][citation needed][opinion] One group of amniotes diverged into the reptiles, which includes lepidosaurs, dinosaurs (which includes birds), crocodilians, turtles, and extinct relatives; while another group of amniotes diverged into the mammals and their extinct relatives. Amniotes include the tetrapods that further evolved for flight—such as birds from among the dinosaurs, and bats from among the mammals.
Some tetrapods, such as the snakes, have lost some or all of their limbs through further speciation and evolution; some have only concealed vestigial bones as a remnant of the limbs of their distant ancestors. Others returned to being amphibious or otherwise living partially or fully aquatic lives, the first during the Carboniferous period,[9] others as recently as the Cenozoic.[10][11]
Tetrapods have numerous anatomical and physiological features that are distinct from their aquatic ancestors. These include the structure of the jaw and teeth for feeding on land, limb girdles and extremities for land locomotion, lungs for respiration in air, a heart for circulation, and eyes and ears for seeing and hearing in air.
Tetrapods can be defined in cladistics as the nearest common ancestor of all living amphibians (the lissamphibians) and all living amniotes (reptiles, birds, and mammals), along with all of the descendants of that ancestor. This is a node-based definition (the node being the nearest common ancestor). The group so defined is the crown group, or crown tetrapods. The term tetrapodomorph is used for the stem-based definition: any animal that is more closely related to living amphibians, reptiles, birds, and mammals than to living dipnoi (lungfishes). The group so defined is known as the tetrapod total group.[12]