The Archosaurs were a group of amniotes whose living representatives consist of birds and crocodiles [amniotes are four-limbed vertebrates comprising the reptiles, birds and mammals which lay their eggs on land or retain the fertilized egg within the mother]. The Archosaurs also include all extinct dinosaurs, extinct crocodilian relatives, and pterosaurs. The Archosauria group includes the most recent common ancestor of living birds and crocodilians. They are differentiated from the other four-limbed vertebrates by the presence of single openings in each side of the skull, in front of the eyes (antorbital fenestrae), among other characteristics.

The first archosauromorphs (relatives of the true archosaurs) appeared in the fossil record during the Early Triassic, about 245 million years ago, just after the great end-Permian extinction. They include hippo-size beaked herbivores called rhynchosaurs, long-necked reptiles called prolacertiforms, terrestrial predators like the erythrosuchians and proterosuchians, and close relatives of the true archosaurs, including Euparkeria. Many of these early groups are limited to the Triassic period, and did not endure the extinctions in the Late Triassic that the dinosaurs and other taxa survived.

The Late Triassic (about 225 million years ago) witnessed the appearance of several new groups of archosaurs, some of which have living descendants today. Most other archosaurs and archosauromorphs did not make it into the Jurassic, but the crocodilian lineage did, as did the dinosaurs and the champsosaurs, which all appeared at about the same time in the Late Triassic. Close relatives of the dinosaurs, such as the pterosaurs and the possibly dinosaurian Eoraptor and Herrerasaurus, also show up in the Late Triassic.

The survivors of the Late Triassic-Early Jurassic extinctions went on to dominate the rest of the Mesozoic Era. The Late Cretaceous period was a time of global change and "ecological reshuffling," when many taxa did not survive. Pterosaurs, many dinosaurs (including some major bird groups), and some crocodilians vanished around this time.

As the Cenozoic Era dawned 65 million years ago, things were different—crocodiles and champsosaurs were still doing well, but the dinosaurs did not survive, and some birds persevered to continue the massive diversification that began in the Cretaceous period. Today we are left with only the crocodilians and the birds as extant (living) archosaurs.