UNRAVELING THE MYSTERY OF DIGGING ADAPTATION OF MESOSCALOPS MONTANENSIS BY MEANS OF GEOMETRIC MORPHOMETRICS AND FINITE ELEMENT ANALYSIS

The extinct Talpoidea clade Proscalopidae show one of the most modified humeral morphologies among mammals. It has been suggested in the past that Mesoscalops montanensis, one of the best-known proscalopids from the Early Miocene of Wyoming (USA), had a unique mode of digging in comparison to both extant and extinct talpids. Differently from other burrowing mammals the digging movement was achieved by using a different set of main muscles (in particular the M. Teres major and M. Infraspinatus). It has been also suggested that the functionally closest extant group is the clade Chrysochloridae from Central-Southern Africa. In fact, African moles show a scratch-digging burrowing movement involving mainly the M. Latissimus dorsi and M. Triceps. In contrast, Talpidae show a typical humeral-rotating movement accomplished by the intensive action of M. Teres major and M. Pectoralis pars sternalis. Here we combined 2D shape analysis and 3D structural Finite Element Analysis in order to unveil the particular functional adaptation of M. montanensis. 3D models of humeri were built for 18 taxa using CT scans including M. montanensis and the whole morphological diversity of Talpidae and Chrysochloridae: Talpa romana, T. tyrrhenica, Mogera wogura, M. tokudae, M. imaizumii, M. insularis, Parascalops brewerii, Proscapanus sansaniensis, Urotrichus talpoides, Desmana moschata, D. pontica, Galemys pyrenaicus, Asthenoscapter meini, Chrysochloris asiatica, C. sthulmanni, Eremitalpa granti and Calcochloris obtusirostris. Geometries were imported in Comsol Multiphysics and a 3d elasticity problem was solved. In particular, three different boundary conditions were set on the humerus depending on the different musculoskeletal configurations occurring in Talpidae, Chrysochloridae and M. montanensis. For Talpidae we placed anatomical constraints in correspondence of the humeral head and the clavicular articular facet, while for M. montanensis and Chrysochloridae instead, we just used the humeral head, as in these taxa the clavicle articulates only with the scapula and not with the humerus. The loads were placed in correspondence of insertions of the three above-mentioned muscular configurations. All models were scaled to the same size and the same force reaction was set in correspondence of the ulnohumeral joint. Von Mises stress was calculated over the whole volume and compared with 2D Geometric Morphometrics data taken on the same models. We found that the structural behaviour of M. montanensis is close to that of C. sthulmanni and C. asiatica as previously hypothesized on the basis of qualitative observations. The Talpinae clade experienced the least structural stress while Desmaninae, Uropsilinae and Urotrichinae showed the most stressed morphologies. We also found that, on the phylogeny, the phenotypic evolutionary rate of Von Mises stress underwent a neat deceleration in correspondence of the Talpinae clade.