The angular is a large
bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...
in the lower jaw (mandible) of amphibians and reptiles (
birds Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
included), which is connected to all other lower jaw bones: the dentary (which is the entire lower jaw in mammals), the splenial, the suprangular, and the articular. It is homologous to the tympanic bone in mammals, due to the incorporation of several jaw bones into the mammalian middle ear early in mammal evolution. In therapsids (mammal ancestors and their kin), the lower jaw is made up of the dentary (the
mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...
in mammals) and a group of smaller "postdentary" bones near the jaw joint. As the dentary increased in size over million of years, two of these postdentary bones, the articular and angular, became increasingly reduced and the dentary eventually made direct contact with the upper jaw. These postdentary bones, even before their articular function was lost, probably transmitted sound vibrations to the stapes and, in some therapsids, a bent plate that might have supported a membrane capable of detecting vibrations developed on the angular. Eventually, it developed into the ectotympanic ring which supports the tympanic membrane in the ears of modern mammals.


Vertebrate anatomy {{Musculoskeletal-stub