Archosaurs are a group of reptiles that include modern birds and crocodilians, as well as their extinct relatives like dinosaurs and pterosaurs. They are characterized by specific features such as a hole in the skull in front of the eyes and a unique ankle structure. Archosaurs first appeared during the late Permian period and became dominant during the Mesozoic era. This group is divided into two main lineages: Pseudosuchia, which includes crocodiles and their ancestors, and Ornithodira, which encompasses birds and pterosaurs. Archosaurs played a significant role in the evolution of terrestrial vertebrates and have left a lasting impact on the planet's biodiversity.