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COMPOSITE SKULL BONES

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.

OCCIPITAL OCCIPITAL 2
     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.
series
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.
fetus fetus 2
drawing
     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.
cynodont skull pelycosaur
      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.
wormian 1 wormian 2
     Other bones in the human adult skull also form from multiple centers of ossification such as the temporal and sphenoid bones (the separate parts of these bones still bear the names of the individual ancestral bones).  In adults, the frontal bone is a single bone while in our ancestors and human children, it is paired.  The vomer also begins as a pair of bones which fuse.  Most mammals have premaxillary bones for their incisors which is separate from the maxillary bones which hold the rest of the upper teeth.  Adult humans do not have a separate premaxillary bone but human fetuses do.
frontal premaxillary