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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 .
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.
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.
There are three main lines of amniotes, which may be distinguished by the structure of the skull and in particular the number of holes behind each eye. In anapsids , the ancestral condition, there are none; in synapsids (mammals and their extinct relatives) there is one; and in diapsids (including birds, crocodilians , squamates , and tuataras ...
Diagram of the diapsid skull with temporal openings, unlike in anapsids The name Diapsida means "two arches", and diapsids are traditionally classified based on their two ancestral skull openings ( temporal fenestrae ) posteriorly above and below the eye.
This placement within the diapsids suggests that the turtle lineage lost diapsid skull characteristics, since turtles possess an anapsid skull. This would make Parareptilia a totally extinct group with skull features that resemble those of turtles through convergent evolution.
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 ...
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".