Phylogeny of the Amniota

    Living amniote groups are differentiated by the number and position of temporal openings in the skull. Relationships among the Anapsida (0 openings), Diapsida (2), and the extinct Synapsida (1 low) were formerly regarded as proceeding 0  1  2, but this is now uncertain. The absence of temporal openings in modern turtles is probably a secondary condition associated with strengthening the skull. The diapsid condition has been extensively modified in all living forms, where the two openings have been fused in the archosaurian Crocodilians and Aves. Therapsida are characterized by a Dentary-Squamosal jaw.