An Archosaur is a group of reptiles that includes modern birds and crocodilians, as well as their extinct relatives like dinosaurs and pterosaurs. They are characterized by specific features in their skulls and limbs, which distinguish them from other reptiles. Archosaurs first appeared during the late Permian period and became dominant during the Mesozoic era. The term "Archosaur" means "ruling lizard," reflecting their status as top predators in many ecosystems. This group is divided into two main lineages: Pseudosuchia, which includes crocodiles, and Ornithodira, which encompasses birds and pterosaurs. Archosaurs played a significant role in the evolution of terrestrial vertebrates.