Understanding the feeding mechanisms and diet of nonavian dinosaurs is fundamental to understanding the paleobiology of these taxa and their role in Mesozoic terrestrial ecosystems. Various methods, including biomechanical analysis and 3D computer modeling, have been used to generate detailed functional hypotheses, but in the absence of either direct observations of dinosaur feeding behavior, or close living functional analogues, testing these hypotheses is problematic. Microscopic scratches that form on teeth in vivo during feeding are known to record the relative motion of the tooth rows to each other during feeding and to capture evidence of tooth-food interactions. Analysis of this dental microwear provides a powerful tool for testing hypotheses of jaw mechanics, diet, and trophic niche; yet, quantitative analysis of microwear in dinosaurs has not been attempted. Here, we show that analysis of tooth microwear orientation provides direct evidence for the relative motions of jaws during feeding in hadrosaurid ornithopods, the dominant terrestrial herbivores of the Late Cretaceous. Statistical testing demonstrates that Edmontosaurus teeth preserve 4 distinct sets of scratches in different orientations. In terms of jaw mechanics, these data indicate an isognathic, near-vertical posterodorsal power stroke during feeding; nearvertical jaw opening; and propalinal movements in near anterior and near posterior directions. Our analysis supports the presence of a pleurokinetic hinge, and the straightness and parallelism of scratches indicate a tightly controlled occlusion. The dominance of scratched microwear fabrics suggests that Edmontosaurus was a grazer rather than a browser.Cretaceous ͉ Ornithopoda ͉ tooth ͉ trophic ecology ͉ Vertebrata R econstructing the feeding mechanisms and details of trophic ecology of extinct animals based on functional morphology is fraught with difficulty (1). In vertebrates, tooth form provides only a general guide to diet: the same tooth form can serve more than one function, and that function can vary with specific feeding behavior. Further complications arise because functional optimization of tooth form can be constrained by the need to process fallback foods during times of resource scarcity (2), and animals with an apparently specialized feeding apparatus can have generalist diets (3). These problems are especially acute in groups like herbivorous, nonavian dinosaurs, where most species have generalized homodont dentitions and lack close living analogues.Among herbivorous dinosaurs, feeding of hadrosaurids has attracted particular attention. They were the dominant herbivorous vertebrates in many Late Cretaceous ecosystems, in terms of both species richness and abundance, and they achieved a near-global distribution (4, 5). This success is frequently attributed to the complex jaw mechanisms possessed by these taxa, which would have given them a level of masticatory prowess equal to that of many extant mammals (6). Current models of feeding mechanisms in hadrosaurid dinosaurs are bas...

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