Mesozoic, the "Age of Reptiles"

The close of the Permian saw the greatest mass extinction known (see the Permian–Triassic extinction event). Most of the earlier anapsid/synapsid megafauna disappeared, making room for the archosauromorph diapsids. The archosaurs was characterized by elongated hind-legs and an erect pose, the early forms looking somewhat like long legged crocodiles. The archosaurs became the dominant group during the Triassic, developing into the well known dinosaurs and pterosaurs, as well as crocodiles and phytosaurs. Some of the dinosaurs developed into the largest land animals ever to have lived, making the Mesozoic popularly known as the "Age of Reptiles". The dinosaurs also deveoped smaller forms, including the feather-bearing smaller theropds. In the mid Jurassic, these gave rise to the first birds.

The lepidosauromorph diapsids may have been ancestral to the sea reptiles. Developing into the ichthyosaurs and sauropterygians, they came to dominate the Mesozoic seas.

The Therpasids came under increasing pressure from the archosaurs the early Mesozoic and developed into increasingly smaller and more nocturnal forms, the first mammals being the only survivors of the line by late Jurassic