Ornithopod Jaw Mechanics

Abstract

How animals move their jaws, whether for ingestion and/or mastication, has been of long-standing interest in vertebrate biology and is presently a dominant research programme in and of itself. Analyses of jaw systems in modern vertebrates, such as those by Nobiling (1977) on sharks, Anker (1974) and Liem (1980) on teleost fishes, Throckmorton (1976), Gorniak et al. (1982), Rieppel and Labhardt (1979), and Smith (1982) on reptiles, Zweers (1974, 1982) and Bühler (1981) on birds, and Greaves (1978), Hiiemae (1978), Crompton et al. (1977), and DeVree and Gans (1976) on mammals form one aspect of the mechanics and dynamics of vertebrate jaw motion. Another focus on the evolution of vertebrate jaw systems is furnished by studies of a variety of fossil groups: Barghusen (1973), DeMar and Barghusen (1972), and McGowan (1973) on reptiles, and Greaves (1972), Kay and Hiiemae (1974b), Rensberger (1978), and Krause (1982) on mammals. Similarly, the present study is based on fossils in order to understand the mechanics of jaw motion in the diverse Mesozoic herbivores known as ornithopods. Most commonly, particularly in work on mammals, vertebrate jaw mechanisms are modeled as class III levers. The class III lever model considers that the mandibular condyle acts as a fulcrum and masticatory muscles apply force between this position and the point of resistance, the bite point. The simplest mechanism in which the lever model is used is the hinge-like motion (rotation) of the mandibular rami at the jaw joints to close the jaws.