Simplified phylogeny of the Reptilia highlighting diversity of their skull morphology. Paleontological evidence suggests that all reptiles, including extant lizards, snakes, tuatara, crocodiles, birds, and turtles, were derived from ancestor whose temporal region was completely roofed by bone. Earliest diapsid reptiles such as Petrolacosaurus (Araeoscelidia) acquired two temporal openings (fenestrae) on either side of the skull (red vertical bar). Recent molecular phylogenies indicate that turtles (Testudines) were derived from diapsid ancestor, which would require secondary closure of temporal fenestrae (green vertical bar). The bone surrounding anteroventral border of the upper temporal fenestra and anterodorsal border of the lower temporal fenestra in diapsids: the postorbital was colored in pink. The bone surrounding posteroventral border of the upper temporal fenestra and posterodorsal border of the lower temporal fenestra in diapsids: the squamosal was colored in blue. The bone surrounding the anteroventral margin of the lower temporal fenestra in diapsids: the jugal was colored in yellow. Lizards do not have the lower temporal bar. Both upper and lower temporal bars are absent in snakes and birds. Other extinct diapsid lineages such as Ichthyosauria and Sauropterygia were not included in phylogeny for simplicity.
Tokita et al. SpringerPlus 2013 2:336 doi:10.1186/2193-1801-2-336