Crocodylia or Crocodilia is an order of semiaquatic, predatory reptiles that are known as crocodilians. They appeared 83.5 million years ago in the Late Cretaceous period and 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 Pseudosuchia, appeared about 250 million years ago in the Early Triassic period, and diversified during the Mesozoic era. The order includes the true crocodiles, the alligators and caimans, and the gharial and false gharial. Although the term "crocodiles" is sometimes used to refer to all of these families, the term "crocodilians" is less ambiguous.