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From top to bottom (A) a skull of an Anapsid, (B) a Synapsid (stem-mammal) skull, and (C) a Diapsid skull. [a] Temporal fenestrae are openings in the temporal region of the skull of some amniotes, behind the orbit (eye socket). These openings have historically been used to track the evolution and affinities of reptiles.
An anapsid is an amniote whose skull lacks one or more skull openings (fenestra, or fossae) near the temples. [1] Traditionally, the Anapsida are considered the most primitive subclass of amniotes, the ancestral stock from which Synapsida and Diapsida evolved, making anapsids paraphyletic .
A schematic of an anapsid skull showing the location of major dermal bones of the upper skull, including the quadrate bone (q).. The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids (reptiles, birds), and early synapsids.
The first reptiles had an anapsid type of skull roof, as seen in the Permian genus Captorhinus The synapsid/sauropsid division supplemented another approach, one that split the reptiles into four subclasses based on the number and position of temporal fenestrae , openings in the sides of the skull behind the eyes.
Massive skull roof, with openings only for the nostrils, eyes and a parietal eye, similar to the structure of the "anapsid" reptiles. With the exception of the later more reptile-like forms, the skull was rather flat and strongly ornamented with presumably tough dermal covering, accounting for an older term for the group: "Stegocephalia".
Though an analysis which included data from Pappochelys found weak support for the idea that Eunotosaurus was a parareptile, it found stronger support for the hypothesis that Eunotosaurus was itself a diapsid closely related to turtles, and that its apparently primitive, anapsid skull was probably developed as part of the turtle lineage ...
Paleothyris was an early sauropsid, yet it still had some features that were more primitive, more labyrinthodont-like than reptile-like, especially its skull, which lacked fenestrae, holes found in the skulls of most modern reptiles and mammals.
Acleistorhinus (ah-kles-toe-RYE-nuss) is an extinct genus of parareptile known from the Early Permian (middle Kungurian stage) of Oklahoma. [1] It is notable for being the earliest known anapsid reptile yet discovered.