Archosaurus rossicus

Archosaurs are a group of diapsid amniotes whose living representatives consist of birds and crocodilians. This group also includes all extinct 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 and all of its descendants. 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).

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.