Diapsids are by far the most speciose group of amniotes, with about 14 600 extant species. They have invaded all major habitats, from the polar circles (many migratory birds) to deserts (many lizards) and even the ocean (sea

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snakes, sauropterygians). Diapsids include most flying vertebrates (birds) and most poisonous chordates (snakes and the Gila Monster).

The early history of diapsids is poorly documented. Until the late seventies, the oldest known diapsids were the Upper Permian (250 Myr old) younginiforms from South Africa and Madagascar, and a few other contemporaneous diapsids of uncertain affinities. However, recent work has extended the fossil record of diapsids to the Pennsylvanian (about 300 Myr ago), and greatly increased our knowledge of the diversity of early diapsids. The oldest known crown-diapsids (saurians) date from the Late Upper Permian.

Extant diapsids are classified into either lepidosaurs (lizards and Sphenodon) or archosaurs (birds and crocodiles). Both of these clades are very successful and speciose, and archosaurs include some of the most fascinating vertebrates that ever lived, such as the pterosaurs (flying reptiles of the Mesozoic) and the many extinct groups of dinosaurs. Indeed, the present diversity of archosaurs, even though it compares favourably with many other clades, is a mere shadow of what it was in the mesozoic.

Diapsida was named after the two fenestrae (holes) found in the temporal region of the skull of most early and some extant diapsids. The lower temporal fenestra is between the jugal, postorbital, squamosal, and quadratojugal. The upper temporal fenestra is between the postorbital, parietal, and squamosal. Some diapsids have lost the lower fenestra (lizards) or even both fenestrae (snakes, amphisbaenids), but their early ancestors had both fenestrae.

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