AN IDIOPATHIC EROSIVE LESION ON THE SURANGULAR OF ALLOSAURUS FRAGILIS WITH COMPARISONS TO COMMON TYRANNOSAURID PATHOLOGIES

Pathologies and feeding traces have been thoroughly described in theropod dinosaurs. In tyrannosauridae, unusual pathologies in the form of circular, smooth-edged erosive lesions are commonly present on the surangular and posterior dentary. While these lesions are of indeterminate origin, they have not been observed or described in other theropod clades. Here we describe a right surangular of Allosaurus fragilis from the Cleveland-Lloyd Dinosaur Quarry at Jurassic National Monument possessing pathologies with similar features to those found in tyrannosauridae. The surangular (UWO-CLDQ-S-17-040) has a maximum length of 365 mm and width of 80 mm, suggesting a juvenile or subadult individual. The lesion is a small aperture measuring 6.4 mm in diameter, nestled in bone 5.1 mm thick, and is located 125 mm from the anterior end, and 19 mm from the ventral side. The bone tissue surrounding the lesion has a mildly coarse texture and undulating surface, indicating active osteological remodeling at the time of death. Previous studies of paleopathologies in theropod dinosaurs have provided insight into paleoecological interactions and behavioral interpretations. Pathologies are commonly categorized based on origin, such as infectious, traumatic, traumatic-infectious, developmental, and idiopathic. The lesions commonly described in the mandibles of tyrannosauridae have a debated etiology; they may be infectious having resulted from Trichomonas gallinae-like protozoans, or traumatic-infectious caused by intraspecific combat and subsequent infection. In either origin, both ecological and behavioral interactions play key roles. Infections due to protozoa and other pathogenic microbes may indicate transmissions in highly dense populations, such as those for A. fragilis in the Cleveland-Lloyd Quarry. Alternatively, if the pathology is of a traumatic-infectious origin, this may suggest insight into intraspecific combat among members of a population in the Late Jurassic. While the exact nature of these pathologies is currently unresolved, future analysis in the pathologic bone microstructure and further comparison with other tyrannosaurid and allosaurid specimens may assist in identifying their origin.