Based on that, there are three groups of amniotes, as anapsids, synapsids and diapsids. They also have differentiated teeth (canines, molars, and incisors) and evolved a different palate shape from their reptilian ancestors. Anapsids have no temporal fenestrae.

Summary – Diapsid vs Synapsid.

Sauropsida is defined by Laurin and Reisz, 1995 as: "The last common ancestor of mesosaurs, testudines and diapsids, and all its descendents." Synapsid is within the scope of WikiProject Animals, ... and possibly merge Reptilia with Amniota or really intertwine them the way Reptilia is currently intertwined with Sauropsida, if we went back to the "traditional" reptilian subclasses. This anchors the definition on Mesosauridae - a basal group yet never appearing on the synapsid side.

Synapsids have one temporal fenestra behind each eye. Synapsida definition is - a subclass of Reptilia comprising extinct reptiles of the Pennsylvanian, Permian, and Triassic, having a single pair of lateral temporal openings in the skull, and usually held to be ancestral to the true mammals.

Synapsid - Simple English Wikipedia, the free encyclopedia The split between Synapsids and Sauropsids (the group which gave rise to reptiles) took place long before therapsids evolved. Amniotes have a temporal region in the skull which can either be solid or have openings called temporal fenestrae. Synapsid skulls contain a hole behind the eyes called the temporal fenestrae, which made their skulls lighter and less energy-intensive to grow and may have provided new attachment sites for jaw muscles. There are a number of characteristics which cannot be seen on fossils, but which are of great importance.

Synapsid definition is - any of a subclass (Synapsida) of terrestrial vertebrates (such as the pelycosaurs and therapsids) having a single pair of lateral temporal skull openings. But even then, some of those traditional subclasses are themselves multiply paraphyletic, like Euryapsida. sid (sĭ-năp′sĭd) n. Any of a group of amniote vertebrates that first emerged in the late Permian Period, characterized by a single opening in the temporal region on each side of the skull and including the mammals along with various extinct groups more closely related to mammals than to other amniotes. This makes it both stable and free of cultural association with traditional definitions of "reptiles."