Comparative anatomy and the masticatory system of basal Ceratopsia (Ornithischia, Dinosauria)

The Ceratopsia is one of the dominant herbivorous dinosaur taxa in Cretaceous terrestrial ecosystems of Asia and western North America. Ceratopsians achieved an extreme adaptation in history of vertebrate herbivory, namely the vertical shear of dental batteries. Understanding of the effectiveness of the masticatory apparatus may help to explain how ceratopsians evolved to become a dominant group of herbivorous dinosaurs. However, the evolutionary transformation of jaw structure within the Ceratopsia has not been fully understood due to the lack of knowledge of basal ceratopsian skulls. Basal ceratopsian mandibles and dentition of well-preserved specimens discovered recently were described, and jaw mechanics was examined in greater detail than has been possible previously. Basal ceratopsians are grouped into basalmost ceratopsians, psittacosaurids, and basal neoceratopsians. Detailed observation of the mandibular elements and overall morphology of basal ceratopsians provides a greater understanding of morphological variation within the basal Ceratopsia. Dental morphology of basal ceratopsians is more variable than that of ceratopsids, the derived forms. Three-dimensional analysis of mandibular lever shows that the largest bite force in basal ceratopsian masticatory system was exerted in the caudal part the tooth row at the caudal extremity of the zone with near-maximum input force, but not at the caudal end of the tooth row. The largest bite force was smaller than the maximum input force, indicating the limit in the basal ceratopsian masticatory system. In contrast, caudal extension of the tooth row in ceratopsids resulted in greater leverage, with bite force even exceeding the maximum input force. In addition, mandibular adductor muscles were much larger in ceratopsids than basal ceratopsians, producing a much larger bite force. Although the masticatory system of basal ceratopsians was not as effective as that of ceratopsids, abundant occurrence of basal ceratopsians indicate successful adaptation to the ecosystem to which they belonged. This study shows the improvement in the masticatory system throughout the evolution of the Ceratopsia.