Ornithodirans

Ornithodira is a major clade within Archosauria that includes the common ancestor of Pterosauria (pterosaurs) and Dinosauria and all of its descendants. In most classifications, it is defined as the most recent common ancestor of Pterosauria and Dinosauria and all of that ancestor’s descendants, thereby making birds living members of Ornithodira through their place in Dinosauria.

Anatomy and locomotion are key in distinguishing ornithodirans from crocodile-line archosaurs (Crurotarsi). Ornithodirans typically show an upright, parasagittal limb posture and a distinctive mesotarsal ankle joint that supports more efficient locomotion. The hind limb configuration often features an elongated metatarsus. The forelimbs are highly variable: in pterosaurs they became wings, while in many non-pterosaur dinosaurs they adapted for diverse forms of locomotion and feeding.

Diversity and evolution: The clade includes both non-avian dinosaurs and pterosaurs. Birds are living dinosaurs, arising from theropod dinosaurs, and thus are part of Ornithodira. The fossil record indicates an origin of ornithodirans in the late Permian to early Triassic, with pterosaurs diversifying in the Jurassic and Cretaceous and dinosaurs remaining prominent on land for much of the Mesozoic.

Taxonomic context: Ornithodira sits within Avemetatarsalia, the bird-line archosaurs, and is often treated as the sister group to Crurotarsi within Archosauria. While definitions and boundaries can vary among authors, the core concept centers on the shared ancestry of pterosaurs and dinosaurs, including birds.