diapsid In Reptilia, describes a skull with two temporal openings behind the eye. This type of skull is characteristic of two sub-classes of reptiles: the Lepidosauria, represented today by lizards, snakes, and the rhynchocephalian Sphenodon (tuatara); and the Archosauria, which includes the extinct (thecodonts, pterosaurs, dinosaurs, and the living crocodiles. The earliest known diapsids were lepidosaurs (e.g. Youngina) from the Upper Permian and Lower Triassic of S. Africa. Formerly, many authorities grouped all such reptiles in the subclass Diapsida, but the arrangement has been largely abandoned because the more primitive and more advanced forms are not clearly related to one another. Birds are considered to be closely related to the archosaurs. Although the skulls of birds have only one temporal opening, this is apparently derived from the fusion of the two diapsid apertures.

diapsid Applied to a type of skull that has two temporal openings behind the eye. This is typical of Archosauria, Lepidosauria, and Rhynchocephalia. See also DIAPSIDA.