skull showing the location of major dermal bones of the upper skull, including the quadrate bone (q).]]The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids (reptiles, birds), and early synapsids.
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
. Courtesy of the Peabody Museum of Natural History, Division of Vertebrate Zoology, Yale University.]]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
[DeBraga, M. and Carroll, R.L., 1993. The origin of mosasaurs as a model of macroevolutionary patterns and processes. In Evolutionary biology (pp. 245-322). Springer US.] and
Spinosauridae dinosaurs.
[Hendrickx, C., Mateus O., & Buffetaut E. (2016). Morphofunctional Analysis of the Quadrate of Spinosauridae (Dinosauria: Theropoda) and the Presence of Spinosaurus and a Second Spinosaurine Taxon in the Cenomanian of North Africa.. PLoS ONE. 11, e0144695., 01, Number 1: Public Library of Science]
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.
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. ... "]
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