Ornithopod Feeding Mechanisms: Their Bearing on the Evolution of Herbivory

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Abstract
Medium- to large-sized mammalian herbivores of the present day are both abundant and diverse. One factor which undoubtedly contributes to this dominant position is the development of sophisticated methods of grinding tough plant material: complex teeth, an anisognathic jaw frame, and complex adductor muscles. Extant herbivorous reptiles are, by contrast, for the most part small-sized and neither abundant nore diverse; they are unable to effectively grind food because their teeth are simple, the jaw frame is isognathic, and their adductor muscles are simple. Ornithopod dinosaurs were dominant medium- to large-sized reptilian herbivores of the late Mesozoic, despite the presence of contemporary mammals. Detailed analysis of their jaw mechanisms has demonstrated that ornithopods were able to combine an isognathic jaw frame and relatively simple adductor muscles with complex tooth batteries and either maxillary or mandibular rotation to produce alternative modes of transverse food grinding. The ability to grind plant fibers in a manner analogous to that used by present day mammals was an important factor contributing to the rise and diversification of advanced ornithopods culminating with the hadrosaurids of the late Cretaceous.