Diapsida

Diapsida is a major clade of reptiles defined by skull architecture featuring two temporal openings (fenestrae) behind each eye socket. These two fenestrae, the upper and lower temporal openings, allow for expanded jaw muscles and greater bite force. The diapsid condition distinguishes this group from reptiles with no temporal openings (anapsids) and from other skull configurations.

The fossil record places the earliest diapsids in the late Carboniferous to early Permian periods, roughly 315 to 300 million years ago. They diversified rapidly and eventually split into two living lineages: Lepidosauria, which includes lizards, snakes, and the tuatara, and Archosauria, which includes crocodilians and birds, along with numerous extinct groups such as non-avian dinosaurs and pterosaurs.

Today, diapsids are highly diverse. Lepidosaurs account for a substantial portion of reptile diversity, especially in deserts and forests around the world. Archosaurs include the extant crocodilians and birds; birds are the most species-rich group within diapsids, representing the descendants of theropod dinosaurs. The group also contains many extinct lineages that shaped the evolution of terrestrial ecosystems during the Mesozoic.

Taxonomic placement of turtles varies by classification. In many modern schemes, turtles are nested within Diapsida (as part of Archelosauria), despite possessing skulls that resemble the early anapsid condition. This reflects the complex and debated evolutionary history of diapsid reptiles.