• Selenodont Artiodactyla;
  • jaw geometry;
  • premolar loss;
  • model

The jaw oflater selenodont artiodactyls is significantly longer, relative to jaw width and tooth size, than in the earliest members of this group. Although this change has a number of potentially beneficial effects, there is at least one adverse effect. A longer jaw reduces the width-to-length ratio, which eventually limits the length of the cheek tooth row at its anterior end. Buttressing the skull against torsional forces is best accomplished by tracts of bone that join the anterior and posterior divisions of the skull and that bridge the weak zone at the orbital region. As the jaw lengthens, some of the anterior premolars necessarily come to lie in front of the most anterior of these buttressing tracts. Bite force at these teeth cannot be transferred in an optimal manner from the anterior to the posterior divisions of the skull, torsion is less well resisted, and one or more anterior premolars are lost, even though there is more than enough space because of the presence of a long diastema.