The arkosaurs are a group of dipsid amniotes that evolved from the Archosauriformes during the Olenekiense (Early Triassic) period. They had enormous evolutionary success and diversified extraordinarily during the Mesozoic; the only current representatives of the group are crocodiles and birds.
The arcosaurs, or dominant reptiles, include the crocodiles and birds that exist today, in addition to the dinosaurs, pterosaurs, and tecodonts, a mixed group that includes the ancestors of all other arcosaurs.
Arcosaurs appeared about 250 million years ago. The first group, the proterosuquids, dispersed almost all over the world. Fossils have been found in Russia, South Africa, Antarctica, Australia, India, China and South America. They present the synapomorphs of the archosaurs; a anteorbital window, a determined orifice in the skull, recurved and flattened teeth and a fourth trochanter in the femur, a special rim.