Archosauria (lit. 'ruling reptiles') is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs /ˈɑːrkəˌsɔːr/[2] are broadly classified as reptiles, in the cladistic sense of term which includes birds. Extinct archosaurs include non-avian dinosaurs, pterosaurs, and extinct relatives of crocodilians. Modern paleontologists define Archosauria as a crown group that includes the most recent common ancestor of living birds and crocodilians, and all of its descendants. The base of Archosauria splits into two clades: Pseudosuchia, which includes crocodilians and their extinct relatives, and Avemetatarsalia, which includes birds and their extinct relatives (such as non-avian dinosaurs and pterosaurs).[3]
Older definitions of the group Archosauria rely on shared morphological characteristics, such as an antorbital fenestra in the skull, serrated teeth, and an upright stance. Some extinct reptiles, such as proterosuchids and euparkeriids, possessed these features yet originated prior to the split between the crocodilian and bird lineages. The older morphological definition of Archosauria nowadays roughly corresponds to Archosauriformes, a group named to encompass crown-group archosaurs and their close relatives.[3] The oldest true archosaur fossils are known from the Early Triassic period, though the first archosauriforms and archosauromorphs (reptiles closer to archosaurs than to lizards or other lepidosaurs) appeared in the Permian. Archosaurs quickly diversified in the aftermath of the Permian-Triassic mass extinction (~252 Ma), becoming the largest and most ecologically dominant terrestrial vertebrates from the Middle Triassic period up until the K-Pg mass extinction (~66 Ma).[4] Birds and several crocodyliform lineages were the only archosaurs to survive the K-Pg extinction, rediversifying in the subsequent Cenozoicera. Birds in particular have become among the most species-rich groups of terrestrial vertebrates in the present day.
Archosaurs can traditionally be distinguished from other tetrapods on the basis of several synapomorphies, or shared characteristics, which were present in their last common ancestor. Many of these characteristics appeared prior to the origin of the clade Archosauria, as they were present in archosauriforms such as Proterosuchus and Euparkeria, which were outside the crown group.[3]
The most obvious features include teeth set in deep sockets, antorbital and mandibular fenestrae (openings in front of the eyes and in the jaw, respectively),[5] and a pronounced fourth trochanter (a prominent ridge on the femur).[6] Being set in sockets, the teeth were less likely to be torn loose during feeding. This feature is responsible for the name "thecodont" (meaning "socket teeth"),[7] which early paleontologists applied to many Triassic archosaurs.[6] Additionally, non-muscular cheek and lip tissue appear in various forms throughout the clade, with all living archosaurs lacking non-muscular lips, unlike most non-avian saurischian dinosaurs.[8] Some archosaurs, such as birds, are secondarily toothless. Antorbital fenestrae reduced the weight of the skull, which was relatively large in early archosaurs, rather like that of modern crocodilians. Mandibular fenestrae may also have reduced the weight of the jaw in some forms. The fourth trochanter provides a large site for the attachment of muscles on the femur. Stronger muscles allowed for erect gaits in early archosaurs, and may also be connected with the ability of the archosaurs or their immediate ancestors to survive the catastrophic Permian-Triassic extinction event.[citation needed]
