Taxa such as Anapsida, Diapsida, Euryapsida, and Synapsida were named after their type of temporal fenestration. Temporal fenestra are large holes in the side of the skull. Many believe that they allow muscles to expand and to lengthen. An important distinction must be made between the type of fenestration found in certain taxa, and the taxa that were named after these types of fenestration. As such, it is primitive for amniotes because all their close relatives lack temporal fenestrae.
Amniote skulls in lateral view showing the four main types of fenestration.
Scale bars equal 1 cm. The synapsid condition is characterized by the presence of a single temporal fenestra bordered minimally by the jugal, postorbital, and squamosal. The quadratojugal and the parietal occasionally contribute to the edge of this fenestra. By comparison with diapsids, this fenestra can be called a lower temporal fenestra. All early members of Synapsida , the taxon that includes mammals and their extinct relatives, had a synapsid skull, but the temporal fenestra has been drastically modified in mammals by ventral processes of the frontal and the parietal that occlude the temporal fenestra.