Triceratops with a Kink: Co-ossification of Five Distal Caudal Vertebrae from the Hell Creek Formation of North Dakota
CANOY ILLIES, Matthew. FOWLER, Denver W; Badlands Dinosaur Museum, Dickinson, ND
Paleopathologies present an unusual but direct source of information on dinosaur ecology, as they are often the immediate result of feeding or locomotory behaviors, social interaction, or predation. Here we present a series of five pathologically co-ossified caudal vertebrae from the Hell Creek Formation of North Dakota, which are part of an associated skeleton, NDGS 1715, of Triceratops (Dinosauria: Ceratopsidae). Comparisons to published ceratopsid tails demonstrate that the co-ossified mass probably represents caudal vertebrae 26-30, positioned approximately three-quarters of the way along the tail. The pathology takes the form of complete co-ossification of centrum faces, with the centra swollen by additional bone growth on the lateral and ventral margins. The co-ossified mass shows a gentle right lateral curvature through caudal centra 26-29. The posteriormost centrum, 30, continues the right lateral curvature, but at a higher angle, exhibiting a ~30° deflection at the contact with the face of the fourth centrum. The posterior face of centrum 30 is either damaged or very strongly distorted by the pathology as it is oriented at approximately 80° relative to the anterior face of the centrum 26. No caudal vertebrae more posterior than number 30 were found associated with the specimen. Thus, either the tail was bent, exhibiting a prominent 100° kink at approximately three quarters of its length, or it was truncated, immediately at the posterior end of the co-ossified mass. Similar pathologically co-ossified caudal vertebrae have been figured for other ceratopsids (TMP 1989.097.001 and TMP 98.93.77), possibly indicating a similar causal mechanism. Etiology of the pathology is however, difficult to determine, with possible causes including lateral crushing by conspecifics (trampling), collision with conspecifics or trees, truncation via predation, or disease.