diapsiden

Diapsida, or diapsids, is a major clade within the amniotes that includes most living reptiles and birds. The defining feature is two temporal openings, or fenestrae, on each side of the skull behind the orbit. These upper and lower fenestrae allow for larger jaw muscles and lighter skull bones, and they distinguish diapsids from other skull types such as anapsids (no openings) and synapsids (one opening).

Diapsids comprise two principal living lineages: Lepidosauria, which includes tuataras, lizards, snakes, and amphisbaenians; and Archosauria, which comprises crocodilians, birds, and extinct groups such as non-avian dinosaurs and pterosaurs. Birds are living archosaurs and represent the theropod branch of dinosaurs. The vast majority of reptile diversity today is within Diapsida.

Fossil evidence shows diapsids first appeared in the late Paleozoic era, with early forms in the late Carboniferous to early Permian. They rapidly diversified during the Permian and Mesozoic, giving rise to a wide array of ecologies, from agile lizards to the giant dinosaurs, as well as numerous marine reptiles such as ichthyosaurs and plesiosaurs and flying reptiles such as pterosaurs. In the past, skulls with a single higher opening were grouped as Euryapsida, but such classifications are now considered paraphyletic and are generally absorbed into Diapsida.