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OTHER ASPECTS OF THE SKELETAL SYSTEM

LOWER JAW AND AUDITORY OSSICLES

     In reptiles, the upper and lower jaws articulate at a jaw joint composed of the quadrate and articular bones.  During the evolution of cynodonts, a process of the dentary bone expanded to contact the squamosal part of the temporal bone.  This was modified to produce a second jaw joint.  Over millions of years of evolution in which cynodonts possessed two functioning jaw joints, the dentary/squamosal joint (the mammalian jaw joint) became more and more prominent while the quadrate/articular jaw joint (the reptilian jaw joint) became further reduced.  In the most advanced cynodonts, the quadrate and articular had become tiny bones, which the first mammals would incorporate into the middle ear where they function in hearing.   (Kemp, 1982, p. 193)

Human Fetus:

fetal middle ear
fetal middle ear
fetal middle ear
In the evolution of mammals, the maxillary and palatine bones fused together in the roof of the mouth to separate the oral and nasal cavities and form the secondary palate.  In human embryonic development, the oral and nasal cavities communicate (as in the pig embryo in the following photos) until the secondary palate forms to separate them. 
embryonic palate

VERTEBRAE

     By comparing the vertebrae of modern animals and their fossil relatives, a history of the development of the vertebral column presents itself as a gradual progression. The first “vertebrae” consisted only of cartilaginous neural arches around the spinal chord, on top of the notochord. The notochord (not the vertebral column) was the primary structural support for vertebrates for the first couple hundred million years of their existence.  Later, alternating elements around the notochord, called pleurocentra and intercentra, developed.  Although intercentra were the more important of the two in some sarcopterygians and amphibians, the pleurocentra became the dominant part of the vertebral “body” in anthracosaurs and reptiles.  The pleurocentra constricted the notochord until it was no longer continuous and the vertebral column became the major support.  The reptilian vertebral column was much more advanced and adapted for supporting an animal’s weight on land.   Amniote (and thus human) vertebrae today are composed of neural arches fused to a pleurocentrum (simply called the centrum).  The intercentra make little, if any, contribution in modern amniotes.  (The intercentra may be the source of small processes on ribs, each called a capitulum, and the intercentrum of the atlas is described below.)  Please notice also that the neural arches develop processes that articulate with neighboring vertebrae over time.

evolution of vertebrae
In advanced modern vertebrates, the vertebrae which are composed of single bones later in life begin as separated regions of ossification homologous to the ancestral neural arches and pleurocentra. Intercentra do not make any contributions to any vertebrae except for the atlas. Embryonic vertebral ossification centers form around an unconstricted notochord, as found in more primitive vertebrates.

     The first ossification centers of the vertebral column to form are the arches of cervical vertebrae, and bodies of thoracic and lumbar vertebrae.  Next, arches of the vertebrae other than cervical form in addition to the centra of sacral vertebrae.  Finally, additional ossification centers of cervical vertebrae form (Kjaer, 1993).  Vertebrae develop from separate ossification centers for the lateral neural arches and the centra (Bagnall, 1984).  Vertebrae centra develop from 2 lateral chondrification centers.  A hemivertebra results if only one side forms; a butterfly vertebra if the two fail to fuse (Sonel, 2001).  The atlas forms from one ossification center in each lateral mass and one in the anterior arch. (Bagnall, 1984).  Vertebra C7 has separate centers for ribs. The axis has seven ossification centers, two of which develop from the dens.  The axis has seven ossification centers, two of which develop from the dens.  (Bagnall, 1984) 

     In mammals, the cervical ribs (which are present as distinct ribs in amphibians, reptiles, and mammal ancestors) fuse to the centra during development, forming a transverse foramen for the vertebral artery.

Platypus

platypus neck vertebrae
cervical vertebrae
     Rib C7 usually fuses with transverse process but it can remain as a separate rib.  The observed incidence of this cervical rib in the general population has varied from .3 to 1%  (McNally, 1990).  Cervical ribs are universally present in mid-second trimester fetuses.  (Seller, 1998)