The relative positions of the jaw joint and the tooth row in mammals

Abstract

The mammalian jaw joint almost always lies above the tooth row. A model is presented in which the moment arm ofthe resultant of jaw muscle force is held constant for a series of hypothetical jaw joints. Some of these joints lie above whileothers lie below the tooth row. The model initially considers the distance from the jaw joint to the last molar (J-M). This span islong when the joint is either high above or far below the tooth row. One can observe in the model that for animals with aposteriorly oriented muscle vector, J-M is shortest (i.e., perpendicular to the arrow representing the jaw muscle force) when thejaw joint is below the level of the tooth row. A short span might be expected in an animal if excess bone and its metabolic costare to be minimized. Yet the joint is usually above the level of the tooth row in mammals. Therefore, the distance from the jointto molar J-M is almost always longer than expected in the large number of animals with a posteriorly oriented muscle vector.Another measurement of some importance is the distance from the jaw joint to the origin, on the skull, of the head of the arrowrepresenting the muscle force (J-S). This distance is shorter when the joint is above and longer when the joint is below the toothrow. Minimizing the sum of J-M and J-S requires that the jaw joint be located at a point above the level of the tooth row. A highposition for the joint corresponds to the actual architecture of mammalian skulls, and permits an economical structure without