UNVEILING THE DIGGING ADAPTATIONS IN PROSCALOPIDAE HUMERUS BY MEANS OF COMPARATIVE 3D FINITE ELEMENT ANALYSIS

The extinct North American family Proscalopidae (Oligocene–Miocene) possess one of the most highly modified humeri among mammals. It has been hypothesized in the past that their functional adaptation has no extant homolog. The closest functional morphology appears to be that observable in Chrysochloridae and Talpidae, two extant families of fossorial mammals. In fact proscalopids present an extremely expanded lateral epicondyle and a distally placed teres tubercle. It has been proposed in the past that proscalopid locomotion involved both head lifting and humeral digging as happens in chrysochlorids. These two clades also share similar anatomy of the cervical vertebrae and skull suggesting functional analogies in the head lifting during digging. We tested here the mechanical performance of the humerus of the exquisitely preserved Mesoscalops montanensis (early Miocene of Wyoming) as compared to those of the extant highly specialized digger talpid Talpa romana, to the less specialized talpid Urotrichus talpoides, and to the chrysochlorid Chrysochloris stuhlmanni. We achieved this by performing 3D Finite Element Analysis starting from CT scans of humeri. Geometries were imported in Comsol Multiphysics and a 3d elasticity problem was solved. In particular, for Talpa romana and Urotrichus we placed anatomical constraints in correspondence of the humeral head and clavicular articular facet. As for Mesoscalops and Chrysochloris, instead, we used just the humeral head, as the clavicle articulates only with the scapula and not with the humerus. In all the four genera, the loads were placed in correspondence of insertions of main muscles involved in digging. For Mesoscalops these muscles are the M. teres major and M. infraspinatus, while for Talpa and Urotrichus we choose the M. teres major and M. pectoralis pars sternalis and for Chrysochloris the M latissimus dorsi and the M. triceps. We found that the stress experienced by Mesoscalops is similar to that of Chrysochloris in terms of total elastic energy. The distribution of stresses is similar in the two taxa while Talpa experiences the lowest stress and Urotrichus the highest. These results suggest that the mode of digging in Mesoscalops was more similar to that observed in extant Chrysochlorida