A potentially fatal cranial pathology in a specimen of Tarchia

Skulls of the Mongolian ankylosaurids Shamosaurus, Tarchia, and Saichania were scanned for information about their internal anatomy. Computed tomography (CT) imaging of the Tarchia skull revealed substantial internal anatomical differences from known Campanian North American taxa, particularly in the morphology of the airway. In addition, unexpected anomalies were detected within the airway and sinuses. The anomalies include multiple bilaterally distributed, variably sized hyperdense (mineralized) concretions within the airway and sinuses, the largest of which, positioned in the right nasal cavity medial to the supraorbitals, has an asymmetric ovoid shape that tapers caudally and which is partially encased within a hemispherical trabeculated osseous proliferation (sinus exostosis). Immediately adjacent to the exostosis is a subcircular transosseous defect in the prefrontal region of the skull roof that is partially filled with trabeculated ossified material with similar architectural features as the larger exostosis. Irregularities along the internal and external surfaces of the cranial vault may be associated. The radiologic features of the hemicircumferential exostosis suggest a chronic reactive osteoproliferation, possibly in response to an ongoing inflammatory reaction to primary sinus infection or, in combination with the unilateral transosseous defect, traumatically introduced infection with potentially fatal consequences. This report underscores the value of CT scanning of fossil vertebrate specimens, which in this case revealed large internal lesions of the skull that, at the time the scan was performed, were otherwise indiscernible.