The Crocodilia are an order of large reptiles that appeared 83.5 million years ago in the Late Creataceousperiod . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria.Members of the crocodilian total group, the clade Crurotarsi, appeared about 220 million years ago in the Triassic period, and exhibited a wide diversity of forms during the Mesozoic era.
The order Crocodilia includes the true crocodiles , the alligators and caimans and the gharials . Although the term 'crocodiles' is sometimes used to refer to all of these, a less ambiguous vernacular term for this group is 'crocodilians'.