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The superclass Tetrapoda (Ancient Greek τετραπόδηs tetrapodēs, "four-footed"), or the tetrapods , comprises the first four-limbed vertebrates and their descendants, including the living and extinct amphibians, reptiles, mammals, birds, and some extinct fish. Tetrapods evolved from the lobe-finned fishes around 390 million years ago in the middle Devonian Period,〔 with modern tetrapod groups having appeared by the late Devonian, 367.5 million years ago.〔 The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear, and are areas of active research and debate among palaeontologists at present. While most species today are terrestrial, the first tetrapods were fully aquatic. Amphibians today generally remain semiaquatic, living the first stage of their lives as fish-like tadpoles. Amniotes evolved about 340 million years ago (crown amniotes 318 mya), and their descendants drove most amphibians to extinction. One population of amniotes diverged into lizards, dinosaurs, birds and their relatives, while another diverged into mammals and their extinct relatives. Several groups of tetrapods, such as the caecilians, snakes, cetaceans, sirenians, and moas have lost some or all of their limbs. In addition, many tetrapods have returned to partially aquatic or fully aquatic lives throughout the history of the group (modern examples of fully aquatic tetrapods include cetaceans and sirenians). The first returns to an aquatic lifestyle may have occurred as early as the Carboniferous Period, whereas other returns occurred as recently as the Cenozoic, as in cetaceans, pinnipeds, and several modern amphibians. 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.〔 (as PDF )〕 It is also becoming increasingly well-understood as a result of more transitional fossil finds and improved phylogenetic analysis. == Definitions == 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. Stegocephalia is a larger group equivalent to some broader uses of the word "tetrapod", used by scientists who prefer to reserve the word "tetrapod" for the crown group (based on the nearest common ancestor of living forms). Such scientists use the term "stem-tetrapod" to refer to those tetrapod-like vertebrates that are not members of the crown group, including the tetrapodomorph fishes The two subclades of crown tetrapods are Batrachomorpha and Reptiliomorpha. Batrachomorphs are all animals sharing a more recent common ancestry with living amphibians than with living amniotes (reptiles, birds, and mammals). Reptiliomorphs are all animals sharing a more recent common ancestry with living amniotes than with living amphibians. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「tetrapod」の詳細全文を読む スポンサード リンク
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