Euryapsida

Euryapsida is a polyphyletic (unnatural, as the various members are not closely related) group of reptiles that are distinguished by a single temporal fenestra, an opening behind the orbit, under which the post-orbital and squamosal bones articulate. They are different from Synapsida, which also have a single opening behind the orbit, by the placement of the fenestra. In synapsids, this opening is below the articulation of the post-orbital and squamosal bones. It is now commonly believed that euryapsids are in fact diapsids (which have two fenestrae behind the orbit) that lost the lower temporal fenestra.

Examples are:
 * Plesiosaurs
 * Ichthyosaurs
 * Placodonts