Archosaurs are a group of diapsid amniotes whose living representatives consist of birds and crocodilians. This group also includes all extinct non-avian dinosaurs, extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most recent common ancestor of living birds and crocodilians. It includes two main 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).
Archosaurs can be distinguished from other tetrapods on the basis of several synapomorphies, or shared characteristics, first found in a common ancestor. The simplest and most widely agreed synapomorphies of archosaurs include teeth set in sockets, antorbital and mandibular fenestrae (openings in front of the eyes and in the jaw, respectively), and a fourth trochanter (a prominent ridge on the femur). 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"), which paleontologists used to apply to many Triassic archosaurs. 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.
There is some debate about when archosaurs first appeared. Those who classify the Permian reptiles Archosaurus rossicus and/or Protorosaurus speneri as true archosaurs maintain that archosaurs first appeared in the late Permian. Some taxonomists classify both Archosaurus rossicus and Protorosaurus speneri as archosauriforms (not true archosaurs but very closely related); these taxonomists maintain that archosaurs first evolved from archosauriform ancestors during the Olenekian stage of the Early Triassic. The earliest archosaurs were rauisuchians, such as Scythosuchus and Tsylmosuchus, both of which have been found from Russia and date back to the Olenekian.
Synapsids (a group including mammals and their extinct relatives, which are often referred to as "mammal-like reptiles") were the dominant land vertebrates throughout the Permian, but most perished in the Permian-Triassic extinction event. Very few large synapsids survived the event, although one form, Lystrosaurus (an herbivorous dicynodont), attained a widespread distribution soon after the extinction.
But archosaurs quickly became the dominant land vertebrates in the early Triassic. The two most commonly suggested explanations for this are: