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The squamosal is a skull bone found in most reptiles,
The squamosal is a skull bone found in most reptiles, amphibians
Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...
, and birds
Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
. In fishes, it is also called the pterotic bone.
In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral component of the dermal roof and is typically thin compared to other skull bones.
The squamosal bone lies ventral to the temporal series and otic notch, and is bordered anteriorly by the postorbital. Posteriorly, the squamosal articulates with the quadrate and pterygoid bones. The squamosal is bordered anteroventrally by the jugal and ventrally by the quadratojugal.
Function in reptiles
In reptiles, the quadrate and articular bones of the skull articulate to form the jaw joint. The squamosal bone lies anterior to the quadrate bone.Anatomy in synapsids
Non-mammalian
synapsids
Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant rept ...
In non-mammalian synapsids, 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 mammalians), the jaw is simplified into an articulation between the dentary and the squamous
Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...
part of the temporal bone
The temporal bone is a paired bone situated at the sides and base of the skull, lateral to the temporal lobe of the cerebral cortex.
The temporal bones are overlaid by the sides of the head known as the temples where four of the cranial bone ...
, and hence referred to as a dentary-squamosal jaw.
Mammals
In many mammals, including humans, the squamosal fuses with the periotic bone and theauditory bulla
The tympanic part of the temporal bone is a curved plate of bone lying below the squamous part of the temporal bone, in front of the mastoid process, and surrounding the external part of the ear canal.
It originates as a separate bone (tympanic b ...
to form the temporal bone
The temporal bone is a paired bone situated at the sides and base of the skull, lateral to the temporal lobe of the cerebral cortex.
The temporal bones are overlaid by the sides of the head known as the temples where four of the cranial bone ...
, then referred to as the squama temporalis
The squamous part of temporal bone, or temporal squama, forms the front and upper part of the temporal bone, and is scale-like, thin, and translucent.
Surfaces
Its outer surface is smooth and convex; it affords attachment to the temporal mus ...
.
In mammals, the quadrate bone evolves to form the incus
The ''incus'' (: incudes) or anvil in the ear is one of three small bones (ossicles) in the middle ear. The incus receives vibrations from the malleus, to which it is connected laterally, and transmits these to the stapes medially. The incus i ...
, one of the ossicles of the mammalian ear. Similarly, the articular bone evolves to form the malleus. The squamosal bone migrates and lengthens to become a new point of articulation with the lower jaw (at the dentary bone).References
Skull bones {{Vertebrate anatomy-stub