diapsids

Diapsids are a major group of sauropsid amniotes defined by the skull having two temporal openings on each side, located behind the eye socket. These upper and lower temporal fenestrae provide attachment space for jaw muscles and are a distinguishing feature that separates diapsids from earlier anapsids and from long-vanished skull patterns. In modern usage, the term refers to a clade that includes most living reptiles and their extinct relatives.

The living diapsids fall into two large lineages: Lepidosauria, which includes tuataras, lizards, and snakes; and Archosauria, which includes crocodilians and birds, along with numerous extinct groups such as non-avian dinosaurs. Birds are widely accepted as living dinosaurs, descended from theropod dinosaurs within Archosauria. Diapsids, therefore, encompass a broad spectrum of terrestrial, aquatic, and aerial organisms, many of which exhibit considerable variation in skull architecture and jaw mechanics.

The diapsid skull has been associated with a wide range of ecological adaptations, contributing to diverse feeding strategies and skull kinesis. The group first appears in the late Carboniferous to early Permian, with early members such as Petrolacosaurus. Through the Permian and Triassic, diapsids diversified extensively, giving rise to many extinct lineages and, in the Mesozoic, to the dominance of lepidosaurs and archosaurs.

Historically, groups with a single high skull opening (the so-called Euryapsida) were once treated as a separate lineage. Modern understanding tends to place these members within Diapsida, interpreting their skull openings as modified rather than as a distinct, natural clade.