Squamosal bone

Sq: squamosal

The squamosal is a bone of the head of higher vertebrates. It is the principal component of the cheek region in the skull, lying below the temporal series and otic notch and bounded anteriorly by the postorbital. Posteriorly, the squamosal articulates with the posterior elements of the palatal complex, namely the quadrate and pterygoid. The squamosal is bordered anteroventrally by the jugal and ventrally by the quadratojugal.^[1]

In many mammals, including humans, it fuses with the periotic bone and the auditory bulla to form the temporal bone, then referred to as the squama temporalis.

Evolution

In synapsids (mammal-like reptiles) the jaw is composed of four bony elements and referred to as a Quadro-articular jaw because the joint is between the articular and quadrate bones. In therapsids (advanced synapsids including mammal ancestors) the jaw is simplified into an articulation between the dentary and the squamous part of the temporal bone, and hence referred to as a dentary-squamosal jaw. In therapsids, the other two bones have moved into the ear to become the malleus and incus.^[2]

References

1. Roemer, A. S. (1956). Osteology of the Reptiles. University of Chicago Press. p. 772.
2. Carr, Steven M. (2005). "Quadroarticular vs Dentary-Squamosal jaw". Memorial University of Newfoundland. Retrieved August 2012.