OUR FACE FROM FISH TO MAN 



The next stage in the long ascent is found among 

 the extinct mammal-like reptiles of the Karroo 

 system of rocks in South Africa. Among these the 

 lowest (Fig. 20A) are nearly as reptilian as lizards, 

 while the highest (Fig. 20B) almost reach the 

 mammalian grade of organization. The bony mask 

 skull advances in various details toward the 

 mammalian type especially in the modelling of 

 the lower jaw, in the further development of a 

 temporal fossa, or muscle opening, and of a cheek 

 arch essentially of mammalian type. 



THE ONE-PIECE JAW REPLACES THE COMPLEX TYPE 



In later members of the series leading toward 

 the mammals the dentary bone increased in size 

 until it so far dominated over the elements behind 

 it that finally they were crowded out entirely and 

 the lower jaw of the adult thus came to consist 

 solely of the two dentary bones (one on each side) 

 connected at the front end, or symphysis. This 

 result was fraught with momentous consequences 

 for the further evolution of the bony face toward 

 the human and other mammalian types. 



Meanwhile the dentary bone (Fig. 21) by reason 



of its enlargement came eventually to press against 



36 



