Page "Diapsid" Paragraph 3

Diapsids were originally classified as one of four subclasses of the class Reptilia, all of which were based on the number and arrangement of openings in the skull.

The other three subclasses were Synapsida ( one opening low on the skull, for the " mammal-like reptiles "), Anapsida ( no skull opening, including turtles and their relatives ), and Euryapsida ( one opening high on the skull, including many prehistoric marine reptiles ).

With the advent of phylogenetic nomenclature, this system of classification was heavily modified.

The Synapsids today are often not considered true reptiles, while the Euryapsida was found to be an unnatural assemblage of diapsids that had lost one of their skull openings.

Some studies have suggested that this is the case in turtles as well, and that turtles are actually heavily modified diapsids, which would leave only some prehistoric forms in the Anapsida.

In phylogenetic systems, birds ( descendants of traditional diapsid reptiles ) are also considered to be members of this group.