Jaw
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The jaw is either of the two opposable structures forming, or near the entrance to, the mouth. Their function is fundamentally for food acquisition, conveyance to the mouth, and/or initial processing (mastication or chewing). The term "jaws" is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it.
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Morphology
Vertebrates
In most vertebrates, the jaws are bony or cartilaginous and oppose vertically, comprising an upper jaw and a lower jaw. The lower jaw or mandible is the mobile component that articulates at its posterior processes, or rami (singular ramus), with the temporal bones of the skull on either side; the word jaw used in the singular typically refers to the lower jaw. The upper jaw or maxilla is more or less fixed with the skull and is composed of two bones, the maxillae, fused intimately at the median line by a suture; incomplete closure of this suture and surrounding structures may be involved in the malformation known as cleft palate. The maxillary bones form parts of the roof of the mouth, the floor and sides of the nasal cavity, and the floor of the orbit or eye socket. The jaws typically accommodate the teeth or form the bases for the attachment of a beak.
In reptiles, the mandible is made up of five bones. In the evolution of mammals, four of these bones were reduced in size and incorporated into the ear. In their reduced form, they are known as the malleus and incus; along with the more ancient stapes, they are the ossicles. This adaptation is advantageous, not only because a one-bone jaw gives a greater bite strength, but also because the malleus and incus improve hearing.
Inverebrates
In arthropods, the jaws are chitinous and oppose laterally, and may consist of mandibles, chelicerae, or, loosely, pedipalps.
One of the most striking examples of speciation involving jaw morphology is that of the African cichlids, a group that has undergone a rapid and extensive radiation, mostly due to morphological changes in their complex jaw structure due to disruptive selection on feeding behavior.
References
Albertson, R. C., Streelman, J. T., and Kocher, T. D. 2003. Directional selection has shaped the oral jaws of Lake Malawi cichlid fishes. Proceedings of the National Academy of Sciences, USA. 100(9): 5252-5257.
