Editing Synapsida (section)

===Temporal openings===
[[File:Skull synapsida 1.svg|thumb|right|200px|The synapsids are distinguished by a single hole, known as the [[temporal fenestra]], in the skull behind each eye. This schematic shows the skull viewed from the left side. The middle opening is the orbit of the eye; the opening to the right of it is the temporal fenestra.]]
Synapsids evolved a [[temporal fenestra]] behind each eye [[orbit (anatomy)|orbit]] on the lateral surface of the skull. It may have provided new attachment sites for jaw muscles. A similar development took place in the [[diapsid]]s, which evolved two rather than one opening behind each eye. Originally, the openings in the skull left the inner cranium covered only by the jaw muscles, but in higher therapsids and mammals, the [[sphenoid bone]] has expanded to close the opening. This has left the lower margin of the opening as an arch extending from the lower edges of the braincase.