Skull, Anapsid

a skull in which membrane bones form a continuous scute with openings only for nostrils and eyes. Anapsid skulls are characteristic of bony fishes, primitive amphibians, and the most primitive reptiles. In typical terrestrial vertebrates, temporal fossae separated by zygomatic, or temporal, arches formed during evolution in the roof of the skull, behind the orbits. As a result, the space provided for the musculature of the jaw increased, and the anapsid skull was transformed into a zygal skull. In snakes, the temporal part of the skull became uncovered when the temporal arches were reduced; this type of skull is called gymnocrotaphic. In Apoda the bones of the roof of the skull expanded secondarily, covering the temporal fossae and forming the anapsid skull again. The reduction of the anapsid skull may also occur by a loss of part of the membrane bones, especially in the area of the orbits.