#anapsid

Skull Classification *in early amniotes

The rise of terrestrial animals required the evolution of an egg capable of being laid on dry land. These animals are called “amniotes”. This is in contrast to fish and amphibians, whose eggs must be laid in water.

There are four general skull types seen in early amniotes. Each skull represents a different branch of animals and is classified by the number of temporal fenestra (holes in the skull behind the eye). There is still debate over the function of the temporal fenestra, but many believe the holes allow for more mobile jaw muscles.

[Illustration of a general skull with no holes behind the eye socket]

Anapsid. No temporal fenestra. The most basic amniotic skull. Modern turtles have anapsid skulls but are probably actually descended from diapsids whose fenestra closed.

[Arrow to another general skull with one low hole behind the eye socket]

Synapsid. A single temporal fenestra. Seen in extinct, mammalian-ancestor reptiles. Modern mammals are synapsids but no longer have temporal fenestra.

[Arrow from anapsid skull to another skull with two holes behind the eye socket, one over the other]

Diapsid. Two temporal fenestra. Seen in Lepidosaurs (snakes, lizards) and Archosaurs (birds, crocodilians, dinosaurs). Modern birds, lizards, and snakes have modified fenestra, but the diapsid condition can still be easily seen in tuatara and alligators.

[Arrow from diapsid skull to another skull with one high hole behind the eye socket]

Euryapsid. A single temporal fenestra like a synapsid, but positioned higher. Debated classification, as the euryapsid condition probably evolved multiple times and can therefore be seen in unrelated animals. Seen in extinct aquatic reptiles like Ichthyosaurs and Plesiosaurs.

References: http://www2.fiu.edu/~longoria/gly1101/Mz-reptiles.htm