Category:Tetrapods

Tetrapods (/ˈtɛtrəpɒd/; from Greek: τετρα- "four" and πούς "foot") are four-limbed (with a few exceptions, such as snakes) animals constituting the superclass Tetrapoda. It includes extant and extinct amphibians, reptiles (including dinosaurs and therefore birds), and mammals. Tetrapods evolved from a group of animals known as the Tetrapodomorpha which, in turn, evolved from ancient Sarcopterygii (lobe-finned fishes) around 390 million years ago in the middle Devonian period;[3] their forms were transitional between lobe-finned fishes and the four-limbed tetrapods. The first tetrapods (from a traditional, apomorphy-based perspective) appeared by the late Devonian, 367.5 million years ago;[1] 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.[4][5] The first tetrapods 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 those are terrestrial tetrapods whose branch evolved from earlier tetrapods about 340 million years ago (crown amniotes evolved 318 million years ago). The key innovation in amniotes over amphibians is laying of eggs on land or having further evolved to retain the fertilized egg(s) within the mother.

Amniote tetrapods began to dominate and drove most amphibian tetrapods to extinction. 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 amphibians or otherwise living partially or fully aquatic lives, the first during the Carboniferous period,[6] others as recently as the Cenozoic.[7][8]

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.