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Tetrapods (Greek τετραποδη tetrapodē, equivalent to Latin quadruped, "four-feet") are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian. They are now a dominant part of the terrestrial fauna, representing all known larger land animals. Some groups have even returned to an aquatic existence, including the largest animal known, the blue whale.

Tetrapods share a variety of derived features. Many of these have been seen in tetrapod skeletons: the distinctive morphologies of the girdles and limbs, as well as the fixed pattern of skull roofing bones.The hypothesis that all of these shared similarities evolved separately in distantly related organisms is not parsimonious; for this reason, these characters reaffirm Tetrapoda as a monophyletic group.

Some transitionals tetrapods:

Transition Species Restoration
From fish to amphibian:
  • Acanthostega gunnari.
File:Acanthostega gunnari.jpg
From amphibian to reptile:
  • Chroniosuchus paradoxus.
Chroniosuchus paradoxus
From reptile to dinosaur:
  • Euparkeria capensis.
Euparkeria capensis
From dinosaur to bird:
  • Rahonavis ostromi.
Rahonavis ostromi
From reptile to mammal:
  • Thrinaxodon liorhinus.
Thrinaxodon liorhinus
From mammal to human:
  • Australopithecus afarensis.
Australopithecus afarensis

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