A Case of Expansile Lesion in a Caudal Vertebra of Medusaceratops lokii

This study presents the first documented case of chronic osteomyelitis with a draining sinus tract in the centrosaurine ceratopsid Medusaceratops lokii , based on a pathological seventh caudal vertebra from the Upper Cretaceous Judith River Formation of Montana. Integrated macroscopic and computed tomography analyses reveal pathognomonic features of chronic osteomyelitis secondary to trauma, including extensive hyperostosis with ‘cauliflower-shaped’ bone overgrowths, moth-eaten osteolysis, sclerosis, a definitive sinus tract, and occult fractures associated with the destruction of bilateral transverse processes. The development of a substantial involucrum confirms prolonged post-traumatic survival, attesting to robust immunological resilience in ceratopsid dinosaurs. Statistical analysis of 68 reported dinosaurian infection cases further reveals a highly significant correlation between ecological niche and trauma mechanism, with infections secondary to stress fractures found exclusively in herbivorous dinosaurs. Stress fractures across Ceratopsia, Sauropoda, and Theropoda exhibit markedly different infectious outcomes between ecological guilds, a disparity attributable to the combined effects of obligate quadrupedal weight-bearing, gregarious behaviour, and differential survival capacity.