134 STRUCTURE OF THE VERTEBRATES 



in the mammal-like reptiles which are unknown in the mammals: 

 (1) the post-frontals and (2) the post-orbitals. The posterior 

 jaw elements have been transferred into the skull as ear ossicles. 



Evolution of the Skull. The evolution of the mammal skull 

 is intimately correlated with the shifts of the dermal bones out- 

 lined above. In addition to the chance mutation of the bones, 

 there are ontogenetic influences which depend upon the modifica- 

 tion of other structures. The development of the brain dictates 

 the size of the calvarium, or brain case. Pathological conditions 

 in the human demonstrate that the skull takes the size of a 



Dentary 



'Articular, 



Turtle (Living Anapsid Reptile) 



Seymouria (Extinct Anapsid) 



Fig. 71. Skulls of Anapsid Reptiles. Side View. (A) Turtle, (B) Seymouria. 

 Observe the lack of teeth and the greater depth of the turtle skull. 



degenerate brain, and an increase in the amount of cerebral 

 fluids causes a hypertrophy of the calvarium. In other words, 

 although bone is the hardest tissue of the body, developmentally 

 it is the most easily modified. A second influence is the shift in 

 the muscles of the head and face. There is evidence that changed 

 size and position of muscles directly influence the shape and 

 position of bony elements. It is equally true that the reverse is 

 true, and that bone changes cause the increase or decrease in the 

 length of muscles. 



The definitive vertebrate skull was laid down in the Stego- 

 cephalia. The brain case was very small, with a huge roof of 

 dermal bones. The muscles of the jaw, neck and face were at- 

 tached to the under side of the roof, and the skull was shallow 



