Archosauria

Archosauria is a major clade of diapsid reptiles that includes the living crocodilians and birds, as well as all extinct relatives such as non-avian dinosaurs and pterosaurs. In many classifications, Archosauria is defined as the crown group arising from the most recent common ancestor of extant crocodilians and birds and all its descendants. The group first appeared in the late Permian, about 250 million years ago, and survived the Permian–Triassic extinction, giving rise to a series of radiations in the Triassic and beyond.

Archosaurs are distinguished by a suite of skeletal features, including an opening in the skull in front of the eye (the antorbital fenestra) and other adaptations that supported more erect limb posture in many members. The early archosaur lineage diversified into two major branches: Crurotarsi, the crocodile-line archosaurs and their extinct relatives, and Avemetatarsalia (the bird-line), which includes pterosaurs and dinosaurs. Dinosaurs later radiated in a variety of ecological roles, while pterosaurs became the first vertebrates capable of powered flight. Birds are theropod dinosaurs and are the only surviving archosaurs from this broad lineage.

Today, the sole living archosaurs are birds and crocodilians, with birds representing the most diverse vertebrate group. The archosaur fossil record spans a wide range of forms, from small, bipedal predators in the Triassic to enormous sauropods and a variety of flying and aquatic lineages in the Mesozoic, illustrating a long and influential evolutionary history within the reptile lineage.