Archosaurs

Origins

Archosaurs are a subgroup of archosauriforms, which themselves are a subgroup of archosauromorphs. Both the oldest archosauromorph (Protorosaurus speneri) and the oldest archosauriform (Archosaurus rossicus) lived in the late Permian. The oldest true archosaurs appeared during the Olenekian stage (247–251 Ma) of the Early Triassic. A few fragmentary fossils of large carnivorous crocodilian-line archosaurs (informally termed "rauisuchians") are known from this stage. These include Scythosuchus and Tsylmosuchus (both of which have been found in Russia), as well as the Xilousuchus, a ctenosauriscid from China. The oldest known fossils of bird-line archosaurs are from the Anisian stage (247–242 Ma) of Tanzania, and include Asilisaurus (an early silesaurid), Teleocrater (an aphanosaur), and Nyasasaurus (a possible early dinosaur).