Cranial and mandibular morphology of Middle Pleistocene cave bears (Ursus deningeri): implications for diet and evolution

Deninger’s bears (Ursus deningeri) have been studied less frequently than Ursus spelaeus s.l. Our objective is to present, for the first time, an analysis of the skull shape of U. deningeri. Bear crania and mandibles were digitised with a Microscribe or CT-scanned and the surface models subsequently landmarked. The landmarks were chosen based on a compromise between functional morphology and sample size. Results show that U. deningeri and U. spelaeus mandibles display very similar morphologies and allometric trajectories, both to each other and to Ailuropoda melanoleuca. It is inferred that masticatory adaptations to a herbivorous diet were already present in the Middle Pleistocene. U. deningeri displays a cranial morphology that is similar to that of U. spelaeus when comparing all species, but U. deningeri has a relatively narrower and dorsoventrally lower zygomatic arch than U. spelaeus, although the masticatory signal is less strong in the skull. We observe intraspecific differences between different populations of U. deningeri, which could parallel the genetic diversity found in U. spelaeus. The intraspecific differences found within U. deningeri may be temporal and/or geographical in nature and could be related to the evolution of the Late Pleistocene cave bear, but this hypothesis remains to be tested.

丁氏熊（Ursus deningeri）的研究频次远低于洞熊复合群（Ursus spelaeus s.l.）。本研究的目标是首次开展丁氏熊的头骨形态分析。研究人员通过三维数字化仪（Microscribe）或计算机断层扫描（CT）对熊类颅骨与下颌骨进行数字化处理，并对生成的表面模型进行解剖标志点（landmark）标注。解剖标志点的选取兼顾了功能形态学与样本量的平衡。结果显示，丁氏熊与洞熊复合群的下颌骨在形态与异速生长轨迹上均极为相似，二者之间以及与大熊猫（Ailuropoda melanoleuca）的对比均呈现这一特征。据此推断，中更新世时期已出现适应植食性饮食的咀嚼适应性特征。整体而言，丁氏熊的颅骨形态与洞熊复合群相近，但其颧弓（zygomatic arch）相对更窄且背腹向高度更低，不过颅骨所反映的咀嚼相关信号相对较弱。本研究还观察到丁氏熊不同种群间存在种内差异，这一现象或与洞熊复合群中发现的遗传多样性相平行。丁氏熊内部的种内差异可能具有时间或地理属性，且可能与晚更新世洞熊的演化历程相关，但该假说仍有待验证。