Mandibular Anatomy in Basal Ceratopsia

摘要

NUMEROUS DISCOVERIES OF new basal ceratopsian taxa with superb skulls in the past 10 years now make it possible to document the evolution of ceratopsian jaw structure. Further preparation of the specimens after the initial descriptions, especially involving the detachment and removal of mandibles from the skulls, has also revealed new information that was not accessible in initial studies. In this study, the anatomy of mandibles in basal ceratopsians has been examined. Some basal ceratopsians have ventrally convex ventral mandibular margins and labially concave tooth rows unlike ceratopsids, which implies diversity in the early evolution of the Ceratopsia. Within basal Ceratopsia, three groups can be recognized based on the morphology of the mandible: basalmost ceratopsians including Chaoyangsaurus and Yinlong, psittacosaurids, and basal neoceratopsians. These groups conform to recent cladistic analyses. Structures of the jaw joint differ among the three groups. The portion of the mandible caudal to the coron oid process is relatively longer in basalmost ceratopsians and psittacosaurids than in basal neoceratopsians. This morphological difference reflects a smaller contribution of surangular and angular to the lateral surface of the mandible in the latter. Transverse thickening of the mandible in basal neoceratopsians is associated with medial displacement of the tooth row, indicating different feeding adaptations within basal Ceratopsia.