Squamosal bone

The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. This bone is an important part of the skull structure in many animals.

In most four-legged animals, the squamosal and quadratojugal bones form the cheek series of the skull. The squamosal is usually thin compared to other skull bones and is part of the dermal roof of the skull.

The squamosal bone lies ventral to the temporal series and otic notch. It is bordered in front by the postorbital bone and connects at the back with the quadrate and pterygoid bones. On its front lower side, it is bordered by the jugal bone, and on its lower side, it meets the quadratojugal bone.

See also: Evolution of mammalian auditory ossicles

Function in reptiles

In reptiles, the quadrate and articular bones of the skull connect to make the jaw joint. The squamosal bone is located right in front of the quadrate bone.

Anatomy in synapsids

In non-mammalian synapsids, the jaw is made of four bones and is called a quadro-articular jaw because it connects at the articular and quadrate bones. In therapsids (advanced synapsids, including mammals), the jaw simplifies to connect between the dentary and the squamous part of the temporal bone, called a dentary-squamosal jaw.

In many mammals, including humans, the squamosal joins with the periotic bone and the auditory bulla to form the temporal bone, called the squama temporalis. In mammals, the quadrate bone changes to become the incus, one of the tiny bones in the mammalian ear, called the ossicles of the mammalian ear. The articular bone changes to become the malleus. The squamosal bone moves and grows to connect with the lower jaw at the dentary bone.