Thesquamosal is askull bone found in mostreptiles,amphibians, andbirds. In fishes, it is also called the pterotic bone.[1]
In most tetrapods, the squamosal andquadratojugal bones form the cheek series of the skull.[2] The bone forms an ancestral component of the dermal roof and is typically thin compared to other skull bones.[3]
The squamosal bone liesventral to the temporal series and otic notch, and is bordered anteriorly by thepostorbital. Posteriorly, the squamosal articulates with thequadrate andpterygoid bones. The squamosal is bordered anteroventrally by thejugal and ventrally by thequadratojugal.[4]
In reptiles, thequadrate andarticular bones of the skull articulate to form the jaw joint. The squamosal bone lies anterior to the quadrate bone.[3]
In non-mammaliansynapsids, the jaw is composed of four bony elements and referred to as a quadro-articular jaw because the joint is between thearticular andquadrate bones. Intherapsids (advanced synapsids including mammal), the jaw is simplified into an articulation between thedentary and thesquamous part of thetemporal bone, and hence referred to as a dentary-squamosal jaw.
In many mammals, including humans, the squamosal fuses with theperiotic bone and theauditory bulla to form thetemporal bone, then referred to as thesquama temporalis.
In mammals, thequadrate bone evolves to form theincus, one of theossicles of the mammalian ear.[5] Similarly, thearticular bone evolves to form themalleus. The squamosal bone migrates and lengthens to become a new point of articulation with the lower jaw (at thedentary bone).[3]
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