MECHANICS AND THE EVOLUTION OF THE SYNAPSID JAW.

Abstract

Over the past decade or more, adductor jaw musculature has been reconstructed for a number of fossil vertebrate taxa. From these reconstructions, enlightening studies of jaw mechanics have been developed, including interpretations in totally extinct groups. In many such studies, the possible adaptive role of specific arrangements of adductor jaw musculature has been proposed based on their special mechanical properties. However, little attention has been given to the details of the mechanical transformations within evolving jaw systems. This paper concerns alterations in the mechanical operation of the jaw from primitive reptiles to early mammals. Changes in the external adductor musculature dominate the evolution of synapsid jaw musculature. In order to construct a mathematical model of these changes, we have made assumptions concerning the line of muscle action, area of muscle attachment, and general operation of the jaw system based mainly on the reconstructions and conclusions of Barghusen (1968, 1972) in a series of synapsid morphological stages. Olson (1961, p. 209) has divided early tetrapod jaw mechanics into two basic systems: (1) the kinetic inertial system, and (2) the static pressure system. Based on this analysis, he defined differences in feeding behavior among fossil types. That is, those animals possessing the kinetic inertial system depend upon large muscular forces imparted to the lower jaw when it is widely open. Once motion is initiated by these forces, the velocity and mass of the jaw is effective in feeding. Little muscular force, according to Olson, is imparted to the lower jaw when it is closed or nearly closed. In contrast, the static pressure system depends upon muscular forces applied to the jaw when it is in occlusal or near-occlusal position. Since the synapsid reptiles discussed in this paper appear to have a significant amount of their adductor jaw musculature arranged so that the closing force at the tooth row is maximized when the jaws are closed, the groups we are concerned with fall into Olson's static pressure system. However, it is probable that the functional and adaptive significance of the arrangement of adductor musculature in the groups considered here involves more than maximization of forces when the jaw is closed. Ostrom (1964) concluded that the development of a coronoid process (and/or depression of the jaw articulation) results in an increase in the length of the moment arm of the external jaw musculature. Thus, an increase in adductive force (torque) is achieved when the force applied to the coronoid process has a posterodorsal line of action and acts on the lower jaw at some angle less than 90 degrees. The evolutionary origin and further development of a coronoid process plays a dominant role in the evolution of the external adductor jaw musculature and its mechanical capabilities in the synapsid lineages examined here. We are interested, therefore, in defining the possible selection pressures responsible for the development and evolution of the synapsid coronoid process.