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The angular is a large
bone A bone is a rigid tissue that constitutes part of the vertebrate skeleton in animals. Bones protect the various organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, and enable mobili ...
in the
lower jaw 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 of ...
(mandible) of
amphibians Amphibians are ectothermic, tetrapod vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arboreal or fresh ...
and
reptiles Reptiles are tetrapod animals in the class or clade Reptilia . As a class in Linnean taxonomy, Reptilia refers to a paraphyletic grouping comprising all amniotes (vertebrates which encase their embryos in a series of protective sacs) except syn ...

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 lightweight skele ...

birds
included), which is connected to all other lower jaw bones: the
dentary 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 of ...
(which is the entire lower jaw in
mammals Mammals (from Latin , 'breast') are a group of vertebrate animals constituting the class Mammalia (), and characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region ...
), the
splenialThe splenial is a small bone in the lower jaw of reptiles, amphibians and birds, usually located on the lingual side (closest to the tongue) between the angular and suprangular.Watson, D. M. S. (1912). LXVII.—On some reptilian lower jaws. Journal ...
, the
suprangular The suprangular or surangular is a jaw bone found in most land vertebrates, except mammals. Usually in the back of the jaw, on the upper edge, it is connected to all other jaw bones: dentary, angular, splenial and articular. It is often a muscle at ...
, and the
articular The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals. Anatomy In most animals, the articular bone is connected to two other lower ...
. It is Homology (biology), homologous to the tympanic bone in mammals, due to the Evolution of mammalian auditory ossicles, incorporation of several jaw bones into the mammalian middle ear early in mammal evolution. In Therapsida, therapsids (mammal ancestors and their kin), the lower jaw is made up of the
dentary 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 of ...
(the mandible 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.


References

Vertebrate anatomy {{Musculoskeletal-stub