Dental anomalies in Pleistocene African hippopotamuses from Olduvai Bed II

Hippopotamuses are key palaeoenvironmental indicators in African Pleistocene ecosystems due to their ecological dependence on permanent water bodies and their frequent representation in the fossil record. This study examines dental anomalies in Hippopotamus cf. gorgops from several localities in Bed II of Olduvai Gorge (Tanzania), dated to ca. 1.7-1.3 Ma. A total of 107 dental elements (incisors, canines, premolars, and molars) were analyzed to document these anomalies and evaluate their behavioral and environmental implications. Dental anomalies are concentrated exclusively in the anterior dentition, affecting 58.9% of the examined incisors and canines (n = 33 of 56), while premolars and molars show no pathological alterations. Several upper canines display subluxation and malocclusion associated with abnormal growth and atypical wear patterns, consistent with traumatic displacement during life. Biometric comparisons with extant Hippopotamus amphibius suggest that these specimens likely correspond to males, supporting an interpretation linked to aggressive intraspecific interactions similar to those observed in extant common hippopotamuses. In addition, various teeth exhibit enamel hypoplasia. Recurrent linear enamel hypoplasia (LEH) indicates repeated episodes of physiological stress during tooth development. The spacing of LEH bands in one well-preserved canine suggests a higher frequency of stress episodes than previously reported for European Early Pleistocene hippopotamuses (Hippopotamus antiquus). These defects likely reflect seasonal environmental fluctuations affecting water availability or vegetation productivity/phenology in the Olduvai Basin during this part of the Early Pleistocene. Overall, the results provide new insights into the behavior of H. cf. gorgops and support the presence of recurrent environmental stress in Olduvai Bed II ecosystems during the Oldowan-Acheulean transition.