The efficiency of the mouth's preparation of food for digestion depends not only on the shape and size of the teeth, but on the way they are used. Different mammals have very different ways of chewing. A great deal can be learned about chewing from examining the occlusion of upper and lower teeth when the jaws are placed together, noticing the shape of the glenoid fossa and articular process of the lower jaw, and actually working the jaws together to see how the teeth function. Even more can be discovered by carefully examining the teeth themselves. Teeth are worn away as they function, forming distinctive "wear facets" where they meet during chewing. These facets often appear striated as a result of teeth occluding repeatedly in the same pattern.
The teeth of mammals with fairly generalized (i.e., not highly specialized) dentition have three main features. Cusps serve to puncture food; crests that connect cusps shear; and basins crush or grind food. Generally incisors and canines are simple teeth used to capture or pick up food, while the cheek teeth process it for swallowing. Anterior cheek teeth (the first premolars) are usually relatively simple puncturing teeth, intermediate cheek teeth shear, and posterior cheek teeth add crushing and grinding. The way the teeth are used -- the sequence of engagement and pattern of motion of the jaw -- is influenced by the nature of the food and of the teeth themselves. A typical sequence, for example, might begin with moving the lower jaw upwards and backwards to engage the anterior teeth to bite into the food item, followed by a upward and forward movement to shear and grind the food.
Many mammals, including humans, have a fairly flexible (but complicated!) articulation and move their jaws in a combination of the motions described above when they chew. Others, such as guinea pigs and capybaras, have glenoid fossas shaped like a slot, running antero-posteriorally. They slide their lower jaws forward along the slot, grinding the vegetation that makes up their diet on well-developed crests (lophs). An opposite extreme is seen, for example, in some mustelid carnivores. The glenoid fossa of wolverines has anterior and posterior lips that lock around the articular process of the lower jaw, such that in some specimens the two parts of the skull cannot be separated. These animals are mostly restricted to an up-and-down chewing motion, useful for shearing meat and crushing bones.
Different styles of chewing require very different arrangements of the muscles that control jaw movement. It is very useful to try to imagine the demands of different chewing styles on these muscles, and the resulting influence of the muscles on the morphology of other parts of the skull.
Phil Myers (author).
Gingerich, P. D. 1984. Mammalian diversity and structure. Pp 1-16 in P. D. Gingerich and C. E. Badgley, eds. Mammals. Notes for a Short Course. University of Tennessee Dept. of Geological Sciences Studies in Geology #8. iv+234 pp.