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The jaw is any opposable articulated structure at the entrance of the
mouth In animal anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science which deals with the structural organization ...

mouth
, 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 A body plan, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to many members of a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ) ...
of humans and most animals.


Arthropods

In
arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,Reference showing that Euarthropoda is a phylum: ...
s, the jaws are
chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. The second most abundant polysaccharide in nature (behind only cellulose), it ...

chitin
ous and oppose laterally, and may consist of ''
mandibles 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 tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movable ...
'' or ''
chelicerae The chelicerae () are the mouthparts of the Chelicerata, an arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form ...

chelicerae
''. These jaws are often composed of numerous mouthparts. 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 Pedipalps (commonly shortened to palps or palpi) are the second pair of appendages of Chelicerata, chelicerates – a group of arthropods including spiders, scorpions, horseshoe crabs, and sea spiders. The pedipalps are lateral to the chelicerae (" ...
) are modified legs.


Vertebrates

In most
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...
s, the jaws are
bony
bony
or
cartilaginous Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue Elastic is a word often used to describe or identify certain types of elastomer An elastomer is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-m ...

cartilaginous
and oppose vertically, comprising an ''upper jaw'' and a ''lower jaw''. The vertebrate jaw is derived from the most anterior two
pharyngeal arch The pharyngeal arches, also known as visceral arches'','' are structures seen in the embryonic development An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism that consists of mor ...
es supporting the gills, and usually bears numerous
teeth A tooth (plural teeth) is a hard, calcified Calcification is the accumulation of calcium salts in a Tissue (biology), body tissue. It normally occurs in the formation of bone, but calcium can be deposited abnormally in soft tissue,Miller, J. ...

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 The Ordovician ( ) is a geologic period A geological period is one of the several subdivisions of geologic time enabling cross-refere ...
period and appeared in the Placoderm
fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

fish
which further diversified in the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the H ...
. The two most anterior
pharyngeal arch The pharyngeal arches, also known as visceral arches'','' are structures seen in the embryonic development An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism that consists of mor ...
es 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
Gnathostomes
), which have seven arches, and primitive jawless vertebrates (the
Agnatha Agnatha (, Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refe ...
), 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 Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respira ...
(observable in modern fish and
amphibians Amphibians are ectotherm File:Junonia lemonias DSF by Kadavoor.JPG, ''Junonia lemonias'' is basking under the sun. An ectotherm (from the Ancient Greek, Greek ἐκτός (''ektós'') "outside" and θερμός (''thermós'') "hot") is an ...

amphibians
) 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 Teleostei (Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximat ...
fish have substantially modified jaws for
suction feeding Aquatic feeding mechanisms face a special difficulty as compared to feeding on land, because the density of water is about the same as that of the prey, so the prey tends to be pushed away when the mouth is closed. This problem was first identifie ...
and
jaw protrusion The jaw is any opposable articulated structure at the entrance of the mouth In animal anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organisms and their parts. Ana ...
, resulting in highly complex jaws with dozens of bones involved.


Amphibians, reptiles, and birds

The jaw in
tetrapod Tetrapods (; ) are four-limbed animals constituting the superclass Tetrapoda (). It includes extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a sp ...
s is substantially simplified compared to fish. Most of the upper jaw bones (
premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of theria, therian mamma ...
,
maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The tw ...

maxilla
,
jugal The jugal is a skull The skull is a bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer ...
,
quadratojugal The quadratojugal is a skull The skull is a bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of g ...
, and
quadrate Quadrate may refer to: * Quadrate bone * Quadrate (heraldry) * Quadrate lobe of liver * Quadrate tubercle of femur See also *Quadrat (disambiguation) {{disambig In mammals the jaws are made up of the
mandible In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science which deals with the structural organization of li ...

mandible
(lower jaw) and the
maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The tw ...

maxilla
(upper jaw). In the
ape Apes (Hominoidea ) are a branch A branch ( or , ) or tree branch (sometimes referred to in botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientis ...

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 The articular bone is part of the lower jaw of most vertebrates Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Anima ...
bone of the lower jaw, and
quadrate Quadrate may refer to: * Quadrate bone * Quadrate (heraldry) * Quadrate lobe of liver * Quadrate tubercle of femur See also *Quadrat (disambiguation) {{disambig