Herbivory in terrestrial vertebrates dates back to at least Early Permian time. Yet herbivorous tetrapods did not greatly diversify until middle Permian and into Triassic time. All of these early herbivores are characterized by isognathy and bilateral occlusion. In the Triassic Period, these herbivores can be differentiated by several trophic-related parameters: simple dentitions (procolophonids, aetosaurs), lack of chewing teeth (dicynodonts), and complex tooth morphology (trilophosaurs, rhynchosaurs, tritylodontoids). In all cases where there is a significant degree of oral processing, masticatory movement is confined to either orthal or palinal motion of the lower jaw.

The mechanics of chewing among herbivores changed...