zygomatic arches, bone bridges in the postorbital region of the skull in terrestrial vertebrates.
When vertebrates first emerged on dry land, their skulls were covered with a continuous cap made of tegumentary bones with openings only for the eyes and nostrils (the amphibious Stegocephalia and the primitive reptiles Cotylosauria). Such a skull was called closed, or archless (anapsid). Further evolution of vertebrates was accompanied by the lightening of the skull: windows, or temporal fossae, appeared independently in the skull covering of various groups of animals; these openings, separated by the temporal arches, facilitated the development of the jaw musculature in the space between the brain case and the covering (a zygal skull).
Among crocodiles, dinosaurs, flying lizards, and rhynchocephalic reptiles (sphenodons), the skull is of the diapsid type; it has two temporal fossae divided by the superior temporal arch, which consists of the postorbital and squamous bones. The inferior temporal arch is formed by the cheekbone and the quadratus cheekbone. It is the lower boundary of the inferior temporal arch. The vertical bony bridge that separates the temporal fossae from the eye socket is sometimes called the postorbital arch, and the bone that borders the fossa at the back is called the posterior temporal arch. In a number of reptiles whose predecessors had diapsid skulls, the inferior temporal arch (in lizards) or superior temporal arch (in birds) has disappeared. Both arches are reduced in snakes. This fact is linked to the development of mobility of the quadratus bone (Streptostylia) and the in-creased mobility of individual parts of the skull relative to one another (kinetism of the skull). The parapsid skull of marine reptile plesiosaurs also had only one superior temporal fossa and only one temporal arch, formed by the postorbital and squamous bones. There was also one fossa in the synapsid skull of the therapsid reptiles, but it was lower. The single temporal arch in these animals consisted of elements which in the diapsid skull composed different arches (the cheekbone and squamous portion of the temporal arch; less frequently, the quadratus cheekbone).
The variants described above do not exhaust all the structures of the temporal region of the skulls of reptiles. For this reason, scholars have refused to make these types absolute or to divide reptiles into the four groups Anapsida, Diapsida, Parapsida, and Synapsida, all the more because animals with the same types of skull may have evolved from different origins.
In mammals, descendants of the therapsid reptiles, there is also only one temporal arch, usually called the zygomatic arch. It is formed by the cheekbone and a special zygomatic process of the squamous bone, which in man forms part of the complex temporal bone, in the function of its “squama.”
V. B. SUKHANOV