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Freebase Commons Common /common

  • The superclass Tetrapoda, or the tetrapods, comprises the first four-limbed vertebrates and their descendants, including the living and extinct amphibians, reptiles, birds, and mammals. The tetrapods evolved from the lobe-finned fishes about 395 million years ago in the Devonian Period. 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, little evidence supports the idea that any of the earliest tetrapods could move about on land, as their limbs could not have held their midsections off the ground and the known trackways do not indicate they dragged their bellies around. Presumably, the tracks were made by animals walking along the bottoms of shallow bodies of water. Amphibians today generally remain semiaquatic, living the first stage of their lives as fish-like tadpoles. Several groups of tetrapods, such as the snakes and cetaceans, 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. 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. Wikipedia

Freebase Commons Biology /biology

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