What Is the Squamosal?

The squamosal is a cranial bone. It makes up part of the cheek region in many animals. In other animals, such as humans, some other primates and many mammals, it fuses with other bones to make up part of the temporal bone, which is found on both sides of the head toward the base of the skull.

In some animals, such as some reptiles, birds and some extinct animals, the squamosal doesn't fuse with other bones and remains singular. In these animals, the squamous bone is found behind the postorbital bone. In some extinct animals, it's found in the same relation to the postorbital bone and lays underneath the otic notch, which is a depression in the cranium located behind the eye socket. The squamosal sits above the quadratojugal bone at the bottom back of the skull and above the jugal bone, which sits at the bottom toward the front of the skull. It sits below the pterygoid bone, which sits at the top of the skull, and above the quadrate bone at the rear bottom of the skull.

Humans have no squamosal bone as such. Instead, there is a squamosal region that's one of the four parts of the temporal bone. The other three are the petrous portion, the mastoid portion and the tympanic part. The human head has two temporal bones, which are located at the sides and base of the skull. The squamous part of the temporal bone, called the squama temporalis, connects with the parietal bone. There are two parietal bones in the human skull that connect to form the top of the head.

The squamosal suture connects the squama temporalis with the lower portion of the parietal bone. It continues toward the lower back of the human skull where it connects to the parietomastoid suture. The parietomastoid suture joins the mastoid process of the temporal bone to the mastoid angle, which is a part of the parietal bone.

Some mammals have a separate bony structure called the auditory bulla, which houses the structures of the middle and inner ear. In humans and some other primates, this structure is called the petrous auditory bulla and is made up of the mastoid portion and petrous portion of the temporal bone. The mammalian squamosal bone fuses with the auditory bulla, or petrous auditory bulla in the cases of humans and some other primates, and the periotic bone to form the temporal bone. The periotic bone itself is made up of three fused bones called the epiotic, pro-otic and opisthotic bones.