Geometric morphometric analysis of potential hybridisation between cave bears (Ursus spelaeus) and brown bears (Ursus arctos) in the late Pleistocene.

The brown bear (Ursus arctos) is one of the closest living relatives to the extinct Late Pleistocene cave bear (Ursus spelaeus). According to a preliminary analysis late Pleistocene bear skull fragments found in caves in Belgium show intermediate phenotypes between U. arctos and U. spelaeus. To analyse a potential hybridisation between these two species a 3D geometric morphometric analysis was performed. In total eight skull fragments were analysed: three cranial fragments and five mandible fragments. The fragments were digitized using a CT scan and then a 3D model was created using segmentation. Landmarks were chosen corresponding to existing studies on U. spelaeus and an existing dataset was used to compare the potential hybrids to existing landmark data of U. spelaeus and U. arctos. The existing dataset was combined with the potential hybrid data and a generalized Procrustes superimposition was used to project the landmarks onto tangent space by orthogonal projection. A linear regression was performed on the Procrustes coordinates onto log centroid size to remove the effect of allometry. A principal components analysis was then performed using the regression residuals. Depending on the landmarks chosen and the principal components displayed, some specimen showed to be in an intermediate space between the two reference groups. Some specimen were plotted as belonging to U. arctos although they possess the dental formula of U. spelaeus. These results could point to a potential hybridisation between U. arctos and U. spelaeus in the Late Pleistocene.