Mandibular morphology of Middle Pleistocene cave bears (Ursus deningeri) from Santa Isabel de Ranero-Bear sector

The cave of Santa Isabel (301 m above sea level) is located in the Ranero neighborhood (Karrantza, Biscay, Spain) and currently has two openings and approximately 500 m of development. During the 1970's a speleological group from Catalonia (G.E. Badalona) explored the cave system and found over a hundred fossils which were given to the paleontologist F. Villalta. During the spring of 1985 the Grupo Espeológico Esparta further explored the cave finding new fossil remains, including a complete skull. Due to the fact that the site was at risk from visitors to the cave, an excavation season was performed in 1991 to gather all the fossils present at the site (>2,000 remains), followed by a second visit in 1994 in order to sample the site for ESR dating which has yielded an age of c. 300 ka for the bear sector (the most important of the 4 fossiliferous sectors in the cave). In order to quantitatively compare the anatomy of the mandibles of the Santa Isabel de Ranero (SIR) bears with other Middle Pleistocene bears as well as cave, brown and American black bears, both traditional and 3D geometric morphometric analyses were applied. The SIR mandibles were scanned using a CT or a surface scanner and the surface models subsequently landmarked. Comparative bear mandibles were digitised with a Microscribe. The landmarks were chosen based on a compromise between functional morphology and sample size. A principal component analysis using a complete set of landmarks indicates that the SIR mandible has very similar PC1 scores, but slightly lower PC2 scores than the cave bears. This implies that the SIR mandible had a relatively longer mandibular corpus and the distance between the fulcrum (mandibular joint) and the tooth row was smaller than U. spelaeus. An analysis using only landmarks on the mandibular body in order to include more specimens showed more overlap between the species in morphospace. Middle Pleistocene cave bears overlap strongly with cave and brown bears, but not with black bears. The allometric analysis shows that the SIR mandibles show an intermediate position between U. arctos and U. spelaeus. Our traditional morphometric assessement is consistent with the previous results. The results show that middle Pleistocene cave bears are morphologically closer to cave bears than to brown or black bears. Our results are consistent with the Santa Isabel de Ranero sample being U. deningeri as previously proposed.