Diversity, Functional Morphology, and Evolution of the Jaw Apparatus in Ornithischian Dinosaurs
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Ornithischian dinosaurs have considerable morphological diversity in jaw structure throughout the clade implying great diversity in craniomandibular musculoskeletal function. This study explores evolutionary comparative osteological and inferred muscular anatomy as well as mechanical advantage of jaw apparatuses in genera throughout Ornithischia. Craniomandibular anatomy is described in detail highlighting functional adaptations for feeding in each subclade and a qualitative test of existing and newly proposed jaw mechanisms for various taxa. Functional characters examined include: symphyseal predentary-dentary joint (including its mobility), tooth row, coronoid process, craniomandibular joint, and general craniomandibular shape. This study also uses criteria from previous jaw muscle studies, as well as more in-depth case-by-case analyses, to reconstruct and measure vector angles of jaw adductor musculature in a large diversity of ornithischian taxa spanning all subclades. Mandibular mechanical advantages among genera within ornithischian subclades as well as between these subclades were calculated using 2D lever arm methods. Such lever arm mechanics estimate relative adductor muscle force for one side of the mandible independently, indicating the effect of jaw shape and muscle angle difference on relative bite forces throughout the jaw. Notable trends include a transition from a more evenly distributed bite force throughout the jaw in basal ornithopods and marginocephalians to a strong caudal bite force in the derived hadrosaurids and ceratopsids convergently. A relatively low bite force is also shown among thyreophorans. Perturbation analyses, constructing hypothetical jaw morphologies with coronoid processes removed as well as the jaw joint raised to the level of the tooth row, were also performed to explore effects of jaw morphologies on the mandibular mechanical advantages for each taxon. In all taxa, both the coronoid process and lowered jaw joint increase moment arm length therefore increasing mechanical advantage of the jaw apparatus. More basal taxa tend to show more use of a lowered jaw joint while the derived hadrosaurids and ceratopsids, especially, show much more use of an elevated coronoid process, as is expected. Evolutionary trends in musculoskeletal anatomy and mandibular mechanical advantages across ornithischian taxa show that these dinosaurs evolved more complex feeding apparatuses within different clades as well as morphological convergences between clades.