Archosauria
Archosaurs (Greek: "ruling reptiles") are a group of diapsid reptiles that first appeared on earth roughly 250 million years ago. There are two primary groups - the ornithosuchians, which include dinosaurs and pterosaurs, and the pseudosuchians, which include the crocodylians. They are set apart by having socketed teeth and four-chambered hearts, among other characteristics.
Most archosaur groups became extinct in mass extinctions occurring 195 and 65 million years ago. They are today represented by the birds, descendants of dinosaurs, and crocodylians. The birds are traditionally treated as a class Aves, and the others as a subclass Archosauria within the class Reptilia. Biologists who prefer monophyletic groups, however, may treat them all in a separate class Archosauria.
