Archosaurs are a group of diapsid amniotes and are broadly classified as reptiles. The living representatives of this group consist of birds and crocodilians. This group also includes all extinct dinosaurs, pterosaurs, and extinct relatives of crocodilians. 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).
"This clade includes the ancestors of dinosaurs, birds, and crocodiles. A clade represents a single branch on the tree of life."
Opisthopubic Pelvis (Speed Path)
"Some dinosaurs had a pubis bone that pointed down and backwards to support their abdomen, resulting in a four-pronged pelvic structure."
Propubic Pelvis (Payout Path)
"A three-pronged pelvic structure with a forward-pointing pubis."
Wikipedia's Info About Archosaurus
Archosaurus is an extinct genus of carnivorous archosauriform reptile. From the latest Permian of Russia and Poland, it is one of the earliest known archosauriforms. The type and only species is Archosaurus rossicus, known from several fragmentary specimens which cumulatively represent parts of the skull and cervical vertebrae.
When first described in 1960, Archosaurus was considered the oldest known archosaur and a close relative of Proterosuchus from the Early Triassic. However, Archosauria in modern terms is considered a more restricted group which Archosaurus lies outside of. The "classic" definition of archosaur utilized prior to the widespread use of cladistics is now roughly equivalent to the clade Archosauriformes. Archosaurus is still considered the oldest undisputed archosauriform, as well as one of the few valid members of the family Proterosuchidae.