359-299 million years ago


vertebrae evolution

vertebrae evolving body which excludes notochord

Ancestral reptiles strengthened the vertebral column and modified the first two vertebrae to form and atlas/axis complex.

The reptilian vertebral column was much more advanced and adapted for supporting an animal's weight on land. Early amniotes possessed small intercentra and retained the suture uniting the neural arch and pleurocentrum (Carroll). 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 contributes to the atlas).

atlas and axis

In amniotes, the size of the clavicle and cleithrum was decreased and the shoulder became more agile. The humerus became more narrow and developed a supinator process (Carroll).