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. |thumb|left|250px
Fish have two sets of jaws: the oral jaws that capture prey and the pharyngeal jaws that advance into the mouth and move prey from the oral jaws to the
esophagus for swallowing.|thumb The vertebrate jaw probably originally evolved in the
Silurian period and appeared in the
Placoderm fish which further diversified in the
Devonian. 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), 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 (observable in modern fish and
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 fish have substantially modified jaws for
suction feeding and
jaw protrusion, 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) 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 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) 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==