The ancestor of diapsids is considered to have possessed a skull with two openings in the temporal region – upper and lower temporal fenestra on each side of the skull bounded by complete arches. The upper jaw is firmly attached to the posterior of skull. This makes for a very rigid, inflexible construction. The skull of the tuatara resembles this condition However, the lower temporal bar (sometimes called the cheek bone) is incomplete in some fossil Rhynchocephalia indicating that its presence in the tuatara is secondary