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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 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.