Nasal soft-tissue anatomy of Triceratops and other horned dinosaurs

Although ceratopsid dinosaurs possess a characteristically hypertrophied narial region, soft-tissue anatomy associated with such a skeletal structure and their biological significance remain poorly understood. The present study provides the first comprehensive hypothesis on the soft-tissue anatomy in the ceratopsid rostrum based on the Extant Phylogenetic Bracket approach. Several cranial specimens of Triceratops including the computed tomography-scan data of an isolated premaxilla as well as those of phylogenetically diverse ceratopsids were examined and compared to the anatomical features of the rostrum in extant reptiles. The resulting hypothesis includes the narial neurovascular pathways and locations of the nasal gland and nasolacrimal duct. Particularly, the narial innervation pattern in ceratopsids is inferred to be unique among reptiles and is suggested to have evolved in response to the enlargement of the naris. In addition, respiratory turbinates are inferred to have been present in ceratopsids for the first time based on an osteological correlate and would likely have served for cephalic thermal regulation as in extant birds and mammals. Acquisition of such a structure might have mitigated a thermal problem associated with the large size of the ceratopsid head.