Jaw of a deceased Loxodonta africana juvenile individual found within the Voyager Ziwani Safari
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The jaw is any opposable articulated structure at the entrance of the
mouth In animal anatomy, the mouth, also known as the oral cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on ...
, typically used for grasping and manipulating food. 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 and is part of the body plan of humans and most animals.


Arthropods

In arthropods, the jaws are
chitin Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...
ous and oppose laterally, and may consist of '' mandibles'' or '' chelicerae''. These jaws are often composed of numerous
mouthparts Mouthparts may refer to: * The parts of a mouth ** Arthropod mouthparts *** Insect mouthparts {{disambig ...
. Their function is fundamentally for food acquisition, conveyance to the mouth, and/or initial processing (''mastication'' or ''chewing''). Many mouthparts and associate structures (such as pedipalps) are modified legs.


Vertebrates

In most vertebrates, the jaws are bony or cartilaginous and oppose vertically, comprising an ''upper jaw'' and a ''lower jaw''. The vertebrate jaw is derived from the most anterior two pharyngeal arches supporting the gills, and usually bears numerous teeth.


Fish

The vertebrate jaw probably originally evolved in the
Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
period and appeared in the Placoderm fish which further diversified in the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
. The two most anterior pharyngeal arches are thought to have become the jaw itself and the hyoid arch, respectively. The hyoid system suspends the jaw from the braincase of the skull, permitting great mobility of the jaws. While there is no fossil evidence directly to support this theory, it makes sense in light of the numbers of pharyngeal arches that are visible in extant jawed vertebrates (the
Gnathostomes Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living ...
), which have seven arches, and primitive jawless vertebrates (the Agnatha), which have nine. The original selective advantage offered by the jaw may not be related to feeding, but rather to increased respiration efficiency. The jaws were used in the
buccal pump Buccal pumping is "breathing with one's cheeks": a method of ventilation used in respiration in which the animal moves the floor of its mouth in a rhythmic manner that is externally apparent.Brainerd, E. L. (1999). New perspectives on the evolution ...
(observable in modern fish and
amphibians Amphibians are four-limbed and ectothermic 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, arbore ...
) that pumps water across the gills of fish or air into the lungs in the case of amphibians. Over evolutionary time the more familiar use of jaws (to humans), in feeding, was selected for and became a very important function in vertebrates. Many teleost fish have substantially modified jaws for suction feeding and
jaw protrusion Most bony fishes have two sets of jaws made mainly of bone. The primary oral jaws open and close the mouth, and a second set of pharyngeal jaws are positioned at the back of the throat. The oral jaws are used to capture and manipulate prey by b ...
, resulting in highly complex jaws with dozens of bones involved.


Amphibians, reptiles, and birds

The jaw in tetrapods is substantially simplified compared to fish. Most of the upper jaw bones ( premaxilla, maxilla, jugal, quadratojugal, and
quadrate Quadrate may refer to: * Quadrate bone * Quadrate (heraldry) * Quadrate lobe of liver * Quadrate tubercle The quadrate tubercle is a small tubercle found upon the upper part of the femur. It serves as a point of insertion of the quadratus femori ...
) have been fused to the braincase, while the lower jaw bones ( dentary, splenial, angular, surangular, and articular) have been fused together into a unit called the mandible. The jaw articulates via a hinge joint between the quadrate and articular. The jaws of tetrapods exhibit varying degrees of mobility between jaw bones. Some species have jaw bones completely fused, while others may have joints allowing for mobility of the dentary, quadrate, or maxilla. The
snake skull A snake skeleton consists primarily of the skull, vertebrae, and ribs, with only vestigial remnants of the limbs. Skull The skull of a snake is a very complex structure, with numerous joints to allow the snake to swallow prey far larger than it ...
shows the greatest degree of cranial kinesis, which allows the snake to swallow large prey items.


Mammals

In mammals the jaws are made up of the mandible (lower jaw) and the maxilla (upper jaw). In the ape there is a reinforcement to the lower jaw bone called the simian shelf. In the evolution of the mammalian jaw, two of the bones of the jaw structure (the articular bone of the lower jaw, and
quadrate Quadrate may refer to: * Quadrate bone * Quadrate (heraldry) * Quadrate lobe of liver * Quadrate tubercle The quadrate tubercle is a small tubercle found upon the upper part of the femur. It serves as a point of insertion of the quadratus femori ...
) were reduced in size and incorporated into the ear, while many others have been fused together. As a result, mammals show little or no cranial kinesis, and the mandible is attached to the temporal bone by the temporomandibular joints. Temporomandibular joint dysfunction is a common disorder of these joints, characterized by pain, clicking and limitation of mandibular movement. Especially in the therian mammal, the premaxilla that constituted the anterior tip of the upper jaw in reptiles has reduced in size; and most of the mesenchyme at the ancestral upper jaw tip has become a protruded mammalian nose.


Sea urchins

Sea urchin Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...
s possess unique jaws which display five-part symmetry, termed the ''
Aristotle's lantern Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...
''. Each unit of the jaw holds a single, perpetually growing tooth composed of crystalline calcium carbonate.


See also

*
Muscles of mastication There are four classical muscles of mastication. During mastication, three muscles of mastication (''musculi masticatorii'') are responsible for adduction of the jaw, and one (the lateral pterygoid) helps to abduct it. All four move the jaw late ...
* Otofacial syndrome * Predentary *
Prognathism Prognathism, also called Habsburg jaw or Habsburgs' jaw primarily in the context of its prevalence amongst members of the House of Habsburg, is a positional relationship of the mandible or maxilla to the skeletal base where either of the jaws pr ...
* Rostral bone


References


External links

* {{Authority control Human head and neck Facial features