Crocodilia is an order of large, predatory, semiaquatic reptiles commonly known as crocodilians. Their ancestors first emerged around 230 million years ago during the Late Triassic period (Norian stage) and they themselves emerged 200 million years ago. They are the closest living relatives of birds, as both groups are the only surviving members of the Archosauria. The broader lineage of Crocodilia, known as Pseudosuchia, dates back approximately 250 million years to the Early Triassic period but they themselves experienced minimal diversification throughout the Mesozoic era. This order includes three families: true crocodiles (Crocodylidae), alligators and caimans (Alligatoridae), and the gharial and false gharial (Gavialidae). While the term "crocodiles" is informally used to describe all members of this group, "crocodilians" is the more precise term.