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COMPOSITE SKULL BONES
|The human fetal skull not only begins with more bony elements than the adult skull, it begins with the same bony elements as are found in more primitive vertebrates. The fossil record indicates that many of the bones of the human skull are composite bones which developed from the fusion of multiple ancestral bones. Embryology supports this: many bones in the adult are composite bones which develop from the fusion of multiple smaller elements in embryos. The observation that human bones can form from multiple ossification centers is an old one: Galen described separate ossification centers for the sternum (7) and mandible (2). (Bagnall, 1984)|
THE OCCIPITAL BONE
In the rear portion of the adult mammalian skull (where the skull meets the vertebral column) is one solid bone called the occipital.
|The primitive vertebrates did not have and occipital bone, but rather a number of small, separate bones in the region. During the evolution of mammals, these bones fused to form the one solid occipital bone.|
|The occipital region of the primitive mammals Morganucodon is represented to the right.|
|In human fetuses, the occipital bone begins as a number of separate ossification centers that later fuse.|
|The images above depict the formation of the upper portion of the occipital bone in human fetuses. In the 3rd month, the basioccipital forms beneath this, followed by the paired exoccipitals in the 4th month.|
|It is not that uncommon that some of these ossification centers fail to fuse with the others, forming what have been called Wormian and Inca bones of the adult skull.|
THE TEMPORAL BONE
Cartilaginous fish possess a number of cartilaginous regions of the braincase, including an otic capsule (Romer, p. 192). In bony fish, the area around the otic capsule is ossified with bones such as the prootic, epiotic (ophistotic), sphenotic. These separate bones fuse to compose the petrosal component of the temporal bone in higher vertebrates (Kardong, 2002, p. 235). Bony fish also possess squamosal and angular bones which will form the squamous portion of the temporal bone in mammals (with the angular forming the support for the tympanum). Bony fish also possess the stapes, quadrate, and articular bones which form the auditory ossicles housed inside the temporal bone in mammals.
Not only is the temporal bone formed from the fusion of multiple smaller ancestral bones, these separate components are derived from all three of the regions of the skull. Thus, the mammalian temporal bone is a very complex structure with its squamosal and tympanic bulla (derived from the angular bone) originating from the ancestral dermatocranium, the petrosal from originating from the chondrocranium, and its styloid process and auditory ossicles originating from the splanchnocranium (Kardong, 2002; Carroll, p. 93).
The sphenoid bone evolved from the fusion of many separate elements. In bony fish, the sphenoid region is composed of sphenethmoid, orbitosphenoid, basisphenoid, and pleurosphenoid components (the pleurosphenoid is absent in mammals) (Kardong, 2002, p. 235). In mammals, the pterygoid was reduced and the orbitosphenoid center of ossification fills in area of medial orbit which was previously open, forming the optic foramen (Carroll, p. 403). In early mammals, the basisphenoid fused to the pterygoid and parasphenoid, (Carroll), the ectopterygoid expanded superiorly to make a lateral wall which contacted dermal bones (Romer, p. 251), and the parasphenoid was greatly reduced (Kemp, 1982, p. 305).
|In most vertebrates, premaxillary bones hold the incisors. Human embryos possess premaxillary bones but they fuse to the maxillary bones during development.|
|The vomer begins as a pair of bones which fuse.|
There are several bones which exist as a pair of bones in more primitive vertebrates and in human embryos but form a solid fused bone in adult humans. This is true of the mandible (above), the frontal, and the vomer bones (below).