Multiple skeletal and dental pathologies in a late Miocene mesotheriid (Mammalia, Notoungulata) from the Altiplano of Bolivia: Palaeoecological inferences

Abstract

We report a partial adult skeleton of the notoungulate mesotheriid Plesiotypotherium achirense, showing multiple skeletal and dental pathologies, from the late Miocene locality of Achiri, Bolivian Altiplano. The material shows bilateral absence of hypselodont permanent molars, further associated with exostosis on various limb bones. The main pathology relates with the loss of two first molars on both sides of the jaw, perhaps due to long-lasting periodontal diseases. Other craniomandibular afflictions, likely related to the latter pathology, are an overgrowth of two first upper molars on both sides and the anomalous development of masticatory muscle insertions (m. masseter, pterygoideus medialis, temporalis, and temporalis pars profundis). A hypothetical pathological sequence of the masticatory apparatus was reconstructed according to distinctive wear of ever-growing crowns of two first upper molars and the unequal lower alveoli reabsorption. Additional pathologies are exostoses observed on several articular surfaces of limb bones, from the scapula to distal phalanges. They likely decreased the range of movements during locomotion of this individual. Given its multiple pathologies, we might have expected this abnormal individual to be a potential easy prey. However, the long-time survival of this animal suggests a low predatory pressure, a hypothesis that is consistent with the virtual absence of carnivorous vertebrates in the Achiri fossil assemblage and the composition of coeval mammalian guilds in the Bolivian Altiplano and/or a herd behaviour for Plesiotypotherium achirense, as seen in many modern grazing herbivore ungulates.