Morphological evolution of the cave bear (Ursus spelaeus) mandibular molars: coordinated size and shape changes through the Scladina Cave chronostratigraphy

Out of all extinct megafaunal mammals of the Quaternary, the cave bear Ursus spelaeus is one of the best represented in the fossil record. This species has been found to exhibit skeletal morphological adaptations when exploiting varied environmental niches, be that spatially or temporally. Here, we employ geometric morphometrics and phenotypic trajectory analysis to explore temporal morphological changes across the entire lower molar tooth row from the infilling of Scladina Cave, Belgium. We show that molar tooth size increases from Marine Isotope Stage (MIS) 5 – MIS 3, with cusp position varying temporally in relation to a larger talonid grinding platform in later time periods. Phenotypic trajectory analyses further show similar evolutionary shape trajectories in the first and second molars, but not in the third molar. Morphological changes related to a larger grinding platform are found in the second and third molars, with the divergent morphological change of M3 suggesting that this tooth is less constrained and appears much more responsive to environmental changes. The need to cope with harder fibrous plant material present in the palaeoenvironment potentially constrained morphological evolution of the cave bear until its disappearance throughout Europe.