Anapsida

Anapsida is a historical term in reptile zoology used to describe a grouping of reptiles characterized by anapsid skulls, meaning the skull lacks temporal openings behind the eye sockets. The term dates to early taxonomic classifications and referred to a range of primitive amniotes with this skull condition.

Historically, Anapsida included the parareptiles, a diverse group of Permian to Triassic reptiles, many of which were herbivorous or aquatic. In some classifications, turtles (Testudines) were also placed within Anapsida because their skulls appear to lack temporal openings. This inclusion is controversial and not widely accepted in modern systematics.

In contemporary phylogenetics, Anapsida is not regarded as a natural, monophyletic clade. Parareptiles are typically treated as a stem reptile lineage outside Diapsida, and turtles are generally placed within Diapsida, often in the clade Archelosauria, reflecting a diapsid affinity with secondary modification or loss of skull openings. Thus, Anapsida is considered largely obsolete as a formal evolutionary group.

The fossil record shows anapsid-like skulls in late Carboniferous to Permian reptiles, with the decline of parareptile lineages by the Triassic as diapsid lineages diversified. The concept illustrates how early paleontologists classified reptiles by skull anatomy before cladistic methods clarified evolutionary relationships among reptiles and their relatives.