Patterns of temporal roofing in amniotic vertebrates

    The temporal region (the roof of the skull) is formed from the parietal (Par), squamosal (Sq), postorbital (PO), and jugal (Ju) bones (the eye orbit opening is shown as a black oval). Four patterns are distinguished, based on the number and position of temporal openings.  The ancestral anapsid pattern, found in turtles, has no openings [top right] . The synapsid pattern, found in the ancestors of mammals, has a single low lateral opening formed at the junction of the PO, Sq, & Ju bones (the arch formed by the Sq / Ju connection evolves into the zygomatic arch of Mammalia) [bottom right]. The diapsid pattern has a second opening at the junction of the PO, Sq & Par bones [bottom left]. The true diapsid pattern is seen today only in Rhyncocephalia (tuatara: Sphenodon); further reduction of the jugal in Archosauria (crocs and birds) and Lepidosauria results in a single temporal opening. (A fourth pattern, euryapsid, is found only in an extinct group of marine sauropsid reptiles, and consists of a single high lateral opening).