The quadrate bone is a
skull bone in most
tetrapods
A tetrapod (; from Ancient Greek τετρα- ''(tetra-)'' 'four' and πούς ''(poús)'' 'foot') is any four- limbed vertebrate animal of the clade Tetrapoda (). Tetrapods include all extant and extinct amphibians and amniotes, with the lat ...
, including
amphibians
Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...
,
sauropsids (
reptiles,
birds), and early
synapsids
Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant rept ...
.
In most tetrapods, the quadrate bone connects to the
quadratojugal and
squamosal bones in the skull, and forms upper part of the jaw joint. The lower jaw articulates at the
articular bone, located at the rear end of the lower jaw. The quadrate bone forms the lower jaw articulation in all classes except mammals.
Evolutionarily, it is derived from the hindmost part of the primitive cartilaginous upper jaw.
Function in reptiles
In certain extinct reptiles, the variation and stability of the morphology of the quadrate bone has helped paleontologists in the species-level taxonomy and identification of mosasaur squamates and
spinosaurine dinosaurs.
In some lizards and dinosaurs, the quadrate is articulated at both ends and movable. In snakes, the quadrate bone has become elongated and very mobile, and contributes greatly to their ability to swallow very large prey items.
Function in mammals
In
mammals, the
articular and quadrate bones have migrated to the
middle ear and are known as the
malleus and
incus
The ''incus'' (: incudes) or anvil in the ear is one of three small bones (ossicles) in the middle ear. The incus receives vibrations from the malleus, to which it is connected laterally, and transmits these to the stapes medially. The incus i ...
.
Along with the
stapes, which is
homologous to some reptilian and amphibian
columellae, these are known as the
ossicles and are a defining characteristic of mammals.
Development
In pig embryos, the mandible ossifies on the side of
Meckel's cartilage, while the posterior part of that cartilage is ossified into the incus. In later development, this portion detaches from the rest of the cartilage and migrates into the middle ear.
[Scott 2000, Paragraph starting with "The original jaw bones changed also. .."]
References
External links
*
Skull bones
{{Vertebrate anatomy-stub