Origin of the mammalian feeding complex: models and mechanisms

Abstract

This paper proposes a simple, unorthodox model for use in the study of vertebrate jaw mechanics. Central to the new “bifulcral model” is the assumption that the bite point may be regarded as a distinct and independent “occlusal fulcrum” equal in status to the jaw articulation or “joint fulcrum” of more traditional biomechanical models. The bifulcral model allows all mechanical forces acting on the feeding system to be evaluated in terms of their purely rotational and purely translational components defined relative to the occlusal and joint fulcra. The major benefit of this analytical approach is that it permits a new and substantially different perspective on the functional consequences of morpho-geometric organization in feeding systems. The bifulcral model clearly establishes the dynamic relationships among muscle alignment, bite point and the resultant patterns of mechanical stress at the craniomandibular joint (CMJ). It also reveals potentially important modes of competitive interaction between otherwise seemingly synergistic jaw muscles. Further, the bifulcral model encourages preliminary investigations into the possible contribution of intramuscular dynamics to the overall operational plasticity of the feeding machinery.Specific application of the new model to structural and functional problems concerning the origin of the mammalian feeding complex lead to the following tentative conclusions. (1) Contrary to current opinion, the CMJ of most cynodont therapsids probably did not experience positive vertical loads (= compressive) when the cheek teeth were utilized for mastication. Instead, net CMJ loads were either neutral or somewhat negative (= tensile). (2) The development of a pronounced coronoid process in cynodonts was more directly related to promoting the differentiation of the masseter complex than to either improving the mechanical advantage or prehensile capacity of the temporalis muscle. (3) Differential motor activity within the complex temporalis musculature of cynodonts could have resulted in “derived lines of action” markedly different from the reconstructed lines of action employed in previous analyses of feeding mechanics in these reptiles. Such derived lines of action may have been optimal configurations for the integrated activity of the temporalis and masseter musculature. (4) Selection in cynodonts favored the evolution of a superficial masseter rather than the elaboration of the preexisting and geometrically similar pterygoideus musculature. This occurred because the masseter held the greater potential for improving bite force, motor control and facilitating the reduction of the postdentary bones of the mandible while still preserving the basic spatial economy of the cranial region. (5) The rearward growth of the condylar process of the dentary in cynodonts promoted the reduction of the postdentary unit primarily by shifting the point of load application between the dentary and postdentary units, thereby reducing bending stresses in the postdentary unit. (6) The enlarged squamosal sulcus of cynodonts was occupied mainly by a hypertrophied depressor mandibulae muscle; an auditory tube was also present in the sulcus deep to the muscle. (7) The depressor mandibulae played a major role in CMJ stabilization in therapsids; this was possibly its only function in later cynodonts. The peculiar downturned retroarticular process of the mandible in therapsids appears to have been related to this same function.