The assessment of jaw movement direction from dental microwear

In order to resolve several controversies about unusual jaw movements such as thegosis and orthal retraction, information about direction of jaw movement is essential. Ryan (1979a,b) proposed that asymmetry of microwear striations might provide such data. In vivo occurrences of asymmetric striations on chimpanzee molars are presented and analyzed. Results suggest that either thegosis contacts commonly occur on chimpanzee molars or that Ryan's model, as developed in vitro, does not accurately apply to naturally occurring molar microwear.