The jaws are a pair of opposable articulated structures at the entrance of the 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 of humans and most animals.
Arthropods
In
, the jaws are
and oppose laterally, and may consist of
mandibles or
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) are modified legs.
Vertebrates
In most
, the jaws are
bone or
cartilage and oppose vertically, comprising an
upper jaw and a
lower jaw. The vertebrate jaw is derived from the most anterior two
supporting the gills, and usually bears numerous
teeth.
Fish
The vertebrate jaw probably originally evolved in the
Silurian period and appeared in the
Placodermi fish which further diversified in the
Devonian. The two most anterior
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
Gnathostomata), 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.
Amphibians, reptiles, and birds
The jaw in
is substantially simplified compared to fish. Most of the upper jaw bones (
premaxilla,
maxilla,
Jugal bone,
quadratojugal, and
Quadrate bone) have been fused to the braincase, while the lower jaw bones (
dentary,
splenial,
Angular bone,
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
cranial kinesis. Some species have jaw bones completely fused, while others may have joints allowing for mobility of the dentary, quadrate, or maxilla. The
snake skull 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 bone) 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
theria, 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
possess unique jaws which display five-part symmetry, termed the
Aristotle's lantern. Each unit of the jaw holds a single, perpetually growing
tooth composed of crystalline calcium carbonate.
See also
External links
