archosaur

Archosaur is a group of diapsid reptiles that includes all living crocodilians and birds, as well as all extinct relatives more closely related to them than to other reptiles. In modern classifications, Archosauria is divided into two major lineages: Pseudosuchia (the crocodile-line) and Avemetatarsalia (the bird-line). Pterosaurs and dinosaurs are part of Avemetatarsalia, with birds arising from theropod dinosaurs. Archosaurs originated in the Late Permian, about 250 to 260 million years ago, and became especially widespread and diverse during the Mesozoic Era.

A defining feature of archosaurs is a skull structure that often includes an antorbital fenestra in front of the eye socket and, in many taxa, a mandibular fenestra in the jaw. They typically possess thecodont dentition (teeth set in sockets) and a diapsid skull with two temporal openings. The limb posture ranges from sprawling in early forms to more upright in many later lineages, facilitating different locomotor lifestyles.

The archosaur radiation produced a broad range of body plans, from agile predators to large herbivores. Non-avian dinosaurs and pterosaurs dominated many ecosystems during the Triassic and Jurassic, while crocodilians persisted and diversified in various habitats. Birds, the living descendants of theropod dinosaurs, originated within the dinosaur lineage and became the most diverse group of vertebrates today.

Today archosaurs are represented by birds and crocodilians, with all other archosaurian lineages known only from fossil records. The group remains a central focus of comparative anatomy, phylogenetics, and paleontology to understand the evolution of major reptile traits and the origins of flight in birds.