Zygomatic arch

The first time after vertebrates came to land , their skull was covered with a continuous roof of integumentary bones with openings only for the eyes and nostrils (amphibians — stegocephals and primitive reptiles — cotilosaurs ). Such a skull was called closed (quilted), or inanimate (anapsidic). Further evolution of the vertebrates was accompanied by relief of the skull: holes for different groups of animals appeared independently - temporal holes separated by zygomatic arches, which, without decreasing the strength of the skull, contributed to the development of the jaw muscles in the space between the brain skull and arch ( zygomatic skull ) ^[2] .

Crocodiles , dinosaurs , pterosaurs and beak-headed reptiles ( hatteria ) have a diapside type skull; it has two temporal openings, which are separated by an upper zygomatic arch, consisting of an ocular and scaly bone. The lower zygomatic arch is formed by the zygomatic and square-zygomatic bones. It borders the lower temporal foramen from below. In those zavropsids whose ancestors had a slab skull, the lower ( lizards ) or upper ( birds ) zygomatic arches disappeared. Both arcs were reduced in snakes , which is associated with the development of mobility of the square bone ( streptostyly ) and an increase in the mobility of individual parts of the skull relative to each other ( kinetism of the skull). The parapsid skull of marine reptiles ( plesiosaurs ) also had only one upper temporal opening and only one zygomatic arch ^[2] .

One hole was also in the skull of the synapsid . Their only zygomatic arch consisted of elements that, in the diapsid skull, are part of different arches (zygomatic and scaly bones). In mammals , descendants of synapsids, only one zygomatic arch, usually called the temporal arch, has also been preserved. It is formed by the zygomatic bone and a special temporal process of the scaly bone, which in humans is part of the complex temporal bone as its “scale” ^[2] .