Diapsids are a class of reptiles which encompass all members of the group
Reptilia except for a few groups containing extinct animals and the
anapsids which are a class containing
turtles and related animals. What sets apart diapsids from other repitilia is the existence of openings on both sides of their
skull, including two for
eyes, and an upper and lower set of
temporal openings in the skull. The most notable group of diapsids are the dinosaurs from whom modern reptiles, including
birds, are descended from. Since the defining characteristics are so broad, there are diapsids of all sizes who utilize all different methods of movement and the only group of
vertebrates that is more diverse is the
Actinopterygii or the "ray-finned" fishes.
One fact of note about diapsid descended from the earliest members of the class is that none of them can hear high-frequency sounds through the air since they lack a tympanum. Since some of the saurians have a tympanum it is suggested that they evolved from turtles and mammals. This evolutionary fact has not been confirmed and the recent suggestion that turtles are saurians has only added to the confusion. Saurians that have no tympanum include snakes and a number of other lepidosaurs and all of these animals can only here low-frequency sounds transmitted through the ground.
The taxonomy of diapsids has still not been standardized although all common methods have some overlap. The following two are examples of diapsid classification.
Sources:
Gauthier, Jacques A. and Laurin, Michel. Diapsida. Tree of Life Web Project. January 16, 2004. <http://tolweb.org/tree?group=Diapsida>.
Introduction to the Diapsids UC Berkeley. January 16, 2004. <http://www.ucmp.berkeley.edu/diapsids/diapsids.html>.