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Characters and Anatomy

The most prominent feature of the anapsid skull is that it has no lateral temporal fenestrae - that is, no openings behind the eyes. This is the main difference between them and the other two subclasses of Reptilia, the diapsids and synapsids.

Schematic diapsid skullSchematic synapsid skull

Schematic anapsid skull

Schematic diagrams of diapsid, synapsid, and anapsid skulls, showing the general position of the temporal fenestrae, and with corresponding bones marked. Abbreviations: j, jugal; p, parietal; po, postorbital; sq, squamosal. Benton 2000

The purpose of fenestrae is not precisely known. Some theories are that they developed as a way of reducing weight and conserving calcium in the skull, or that their development allowed room for strong muscles to improve bite force. The lack of fenestrae is a primitive character for all amniotes, and cannot actually be used to define anapsids specifically (see note in major subgroups). In other words, a group cannot be identified reliably on something it lacks, as opposed to something it has - the lack of fenestrae is a condition ancestral to all reptiles. Indeed, the earliest confidently classified anapsid Acleistorhinus from the lower Permian (around 270 million years ago) possessed a lower temporal fenestra. Most, but not all, extinct anapsids lacked fenestrae.

As such, the characteristics of Anapsida are wide ranging. Most anapsids have a short jaw and a relatively strong bite force, due to large muscles at the back of the skull. All extinct anapsids had teeth (unlike their living representatives, turtles), and the shape of these reflects the differing diets of the animals. See major subgroups for more information on this. Additionally, most extinct anapsids are inferred to be oviparous, as all extant (living) anapsids are today.

All known groups of extinct anapsids were terrestrial, whereas the turtles have a full range of habitats - terrestrial, marine and freshwater.