Archosaurs are a group of diapsid amniotes whose living true representatives consist of crocodilians (dinosaurs, including birds, are descended relatives of the archosaurs). This group also includes extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most recent common ancestor of living dinosaurs/birds and crocodilians. It includes two main clades: Pseudosuchia, which includes crocodilians and their extinct relatives, and Ornithosuchia, which includes the ancestors of birds and their extinct relatives.
They date back to the Early Triassic.