archosaurs

Archosaurs are a major clade of diapsid reptiles that includes the living birds (Aves) and crocodilians (Crocodylia) and many extinct groups, such as non-avian dinosaurs and pterosaurs. In current classifications, Archosauria comprises two primary lineages: the crocodile-line Pseudosuchia and the bird-line Ornithodira, which further includes dinosaurs and pterosaurs.

Origin and diversification: Archosaurs first appear in the late Permian or early Triassic. After the Permian-Triassic crisis, archosaurs diversified rapidly during the Triassic, giving rise to diverse terrestrial, aquatic, and aerial forms. Non-avian dinosaurs became the dominant terrestrial vertebrates during much of the Mesozoic, while pterosaurs occupied aerial ecosystems and crocodilians thrived in freshwater and coastal habitats.

Key anatomical features: Archosaurs typically have skull openings in front of the eye (antorbital fenestra) and behind the teeth (mandibular fenestra), along with thecodont dentition set in sockets. Many archosaurs show an upright gait with limbs beneath the body, an adaptive trend that supported efficient locomotion in varied environments. In ornithodirans, an ankle structure supports rapid movement and coordination for both terrestrial and aerial activity, reflecting their distinctive limb mechanics.

Diversity and survival: Birds are the only living archosaurs that originated from the dinosaur radiation. Crocodilians represent another enduring lineage, occupying diverse freshwater and coastal environments worldwide. Extinct archosaurs include pseudosuchians such as rauisuchians and a broad array of early archosauriforms that diversified across the Triassic.

Fossil record and significance: Archosaurs played a central role in Mesozoic ecosystems, shaping terrestrial and aerial communities. The end-Cretaceous mass extinction ended most lineages except birds. Ongoing research into archosaurs informs understanding of evolution, anatomy, flight, and the distribution of major reptile groups.