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A tetrapod is a vertebrate animal having four feet, legs or leglike appendages. The technical term tetrapoda literally means 'four-legged' (from the Greek).

According to the theory of evolution, the first tetrapod was a fish, probably descended from a coelacanth or similar creature, that inhabited tidal mudflats. It would have used its legs to paw its way through the mud. Gradually, this ancestral species developed lungs and became what we would call an amphibian. Today, most tetrapods are land-dwelling, at least in their adult forms, but some species (such as the axolotl, whale and ichthyosaur[?]) have returned to the sea. There are four main categories of tetrapods:

frogs and toads, newts and salamanders
only extant examples are turtles
many extinct species and all mammals

dinosaurs, most modern reptiles, birds

Note that snakes are considered tetrapods because they are descended from ancestors who had a full complement of limbs. Similar considerations apply to aquatic mammals.

UCMP Taxonomy page: http://www.ucmp.berkeley.edu/vertebrates/tetrapods/tetraintro

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