Tetrapoda

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 (in the sense that a tetrapod is an 4-limbed vertebrate) 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.

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Some transitional tetrapods:[]

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