A crocodile is a large amphibious reptile. It lives mostly in large tropical rivers, where it is an ambush predator.
The Crocodylia is an order of mostly large, predatory, semiaquatic reptiles. Their ancestors, the clade Pseudosuchia, appeared about 250 million years ago in the early Triassic period.
Crocodiles as we know them first appeared 94 million years ago in the Upper Cretaceous period. They are the closest living relatives of birds: the two groups are the only survivors of the Archosauria.
One species, the Australian saltie, travels in coastal salt water as well as rivers. In very dry climates, crocodiles may aestivate and sleep out the dry season.