The anatomy of a troodontid tooth from the Late Cretaceous in Jiayin, Heilongjiang Province, China

The anatomical characteristics of troodontid teeth have remained largely unexplored. This study employs high-resolution computed tomography data to reconstruct and describe the morphology of a new troodontid tooth from the Upper Cretaceous Yuliangzi Formation, located in the Sunwu-Jiayin Basin of Jiayin, Heilongjiang, Northeast China. The reconstruction reveals previously unrecognized anatomical features and presents the first three-dimensional visualization of the pulp cavity of troodontid teeth. Notably, the ratio of enamel thickness at the base of the crown to the crown height is 1.9%. The enamel thickness on the labial side exceeds that on the lingual side in the distal denticles, a phenomenon observed in theropod teeth for the first time. The pulp cavity is distinctly narrow in the labiolingual dimension, tapering sharply towards the apex, and exhibiting a distal curvature, and displays nine distinct projections along basal half of its distal margin. Additionally, the presence of a wear facet supports the hypothesis of tooth-to-tooth occlusion in troodontids, similar to that observed in tyrannosaurids. Comparative morphological analysis identifies the specimen as a Troodon-morphotype tooth, thereby providing further evidence for faunal exchange between North America and Asia via Cretaceous Beringia. Moreover, the only theropods that have been definitively identified from the Yuliangzi Formation are tyrannosaurids and troodontids. In addition, the comparison of various dinosaur faunal assemblages within Maastrichtian deposits in China indicates that all of them contain tyrannosaurids.