The Difference Between...Anapsids, Synapsids, Diapsids and Euryapsids

Simplified skull diagrams of anapsid, synapsid, diapsid and euryapsid.
Image taken from Benton 2005

You will come across various references to amniote skull types such as synapsid and diapsid, these are the more commonly used ones. So for this post I will go through how to tell which is which by looking at the skull.

(a) Anapsid Skull: Skulls that lack openings, known as temporal fenestrae, are anapsids. These include turtles, modern and prehistoric, as well as extinct reptile species.

(b) Synapsid Skull: There is a single temporal fenestrae situated below the postorbital bone, in a similar position to the lower opening of a diapsid. Synapsid reptiles are now extinct but mammals are also synapsid and believed to be descendants of these reptiles.

Dimetrodon Skull, an example of a synapsid. Image credit:
mercyhurst.edu

(c) Diapsid Skull: Perhaps the most famous diapsids are the dinosaurs, but diapsid also covers snakes, crocodiles, lizards and birds. There are two temporal fenestrae behind the orbit. One is inferior (smaller) and one superior (bigger).

(d) Euryapsid Skull: This is probably the least known of the amniote skull. Although it appears to be similar to the synapsid skull it differs as it is positioned above the postorbital bone rather than beneath it. The Euryapsids include Ichthyosaurs and Plesiosaurs.