The Archosauria (the "ruling reptiles") is a major group of diapsids (all reptiles living and extinct, except turtles).
The ancestral archosaurs probably originated some 250 million years or so ago, in the late Permian period. Their descendants (such as the dinosaurs) dominated the realm of the terrestrial vertebrates for a majority of the Mesozoic Era. Today, only the birds and crocodilians exist to provide a glimpse into the past glory of archosaurs.