Vertebrate herbivory in the Mesozoic; Jaws, plants, and evolutionary metrics
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 markedly, beginning in the Late Triassic and extending through the Cretaceous. These animals include sphenodontids, multituberculates, prosauropods, sauropods, segnosaurs, ornithopods, stegosaurs, ankylosaurs, and pachycephalosaurs. Although retaining the primitive isognathous jaw construction and consequent bilateral occlusion, ornithopods were the first group of herbivores to develop a transverse chewing stroke (through slight rotation of the lower jaws [heterodontosaurids] or upper jaws [pleurokinesis: hypsilophodontids, iguanodontids, hadrosaurids]).
Measures of diversity, speciation rates, extinction rates, and net profit or loss are used to map shifts in terrestrial floras and theropsid and sauropsid herbivore clades in the Mesozoic. Two linkages appear to exist, one from the end of Triassic and into Jurassic time, and another from the middle to the end of the Cretaceous Period. Each involves both floral restructuring and herbivore community modification.