Crocodilia (or Crocodylia) is an order of mostly large, predatory, semiaquatic archosaurian reptiles, known as crocodilians. They first appeared 83.5 million years ago in the Late Cretaceous period (Campanian stage) and are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria. Members of the order's total group, the clade Pseudosuchia, appeared about 250 million years ago in the Early Triassic period, and diversified during the Mesozoic era. The order Crocodilia includes the true crocodiles ( family Crocodylidae), the alligators and caimans ( family Alligatoridae), and the gharial and false gharial ( family Gavialidae). Although the term ' crocodiles ' is sometimes used to refer to all of these, crocodilians is a less ambiguous vernacular term for members of this group.
Large, solidly built, lizard - like reptiles, crocodilians have long flattened snouts, laterally compressed tails, and eyes, ears, and nostrils at the top of the head. They swim well and can move on land in a "high walk " and a "low walk ", while smaller species are even capable of galloping. Their skin is thick and covered in non-overlapping scales. They have conical, peg- like teeth and a powerful bite. They have a four -chambered heart and, somewhat like birds, a unidirectional looping system of airflow within the lungs, but like other non-avian reptiles they are ectotherms.
