Department of Paleontology, Institute of Organismic Biology, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany.
J Morphol. 2024 Sep;285(9):e21761. doi: 10.1002/jmor.21761.
Essential for sustaining a high metabolic rate is the efficient fragmentation of food, which is determined by molar morphology and the movement of the jaw. The latter is related to the jaw morphology and the arrangement of the masticatory muscles. Soricid jaw apparatuses are unique among mammals, as the articulation facet on the condylar process is separated into a dorsal and a ventral part, which has often been linked to more differentiated jaw motions. Soricidae also possess a remarkably elongated angular process. However, the precise function of the unique morphology of soricid jaw apparatuses has not been fully understood yet. By digitally reconstructing the masticatory musculature via the diffusible iodine-based contrast-enhanced computed tomography technique, we show how the unique jaw morphology is reflected in the spatial organization as well as the inner architecture and respective fascicle orientations of the muscles. From the lines of action of the m. masseter and the m. pterygoideus internus, both muscles inserting on the lateral and medial side of the angular process, respectively, we infer that the angular process is substantial for roll and yaw rotations of the mandible. The m. masseter is subdivided into four and the m. pterygoideus internus into five subunits, each exhibiting a slightly different line of action and torque. This enables Soricidae to adjust and adapt these rotational movements according to the properties of the ingested food, allowing for more efficient fragmentation. Additionally, those guided rotational motions allow for precise occlusion despite tooth wear. The temporalis is the largest of the adductor muscles and is mainly responsible for exerting the bite force. Overall, the unique jaw bone morphology in conjunction with the complex muscle arrangement may contribute towards a more efficient energy gain and the maintenance of a high metabolic rate, which is crucial for small-bodied mammals such as shrews.
维持高代谢率的关键是食物的有效破碎，这取决于摩尔形态和颌骨的运动。后者与颌骨形态和咀嚼肌的排列有关。食虫动物的颌骨结构在哺乳动物中是独一无二的，因为髁突的关节面分为背侧和腹侧两部分，这通常与更分化的颌骨运动有关。食虫动物还拥有非常长的角状突。然而，食虫动物独特的颌骨形态的精确功能尚未完全理解。通过使用基于弥散碘的对比增强计算机断层扫描技术对咀嚼肌进行数字重建，我们展示了独特的颌骨形态如何反映在肌肉的空间组织以及内部结构和各自的纤维束方向上。从咀嚼肌和翼内肌的作用线可以推断，角状突对于下颌的滚动和偏航旋转非常重要，这两块肌肉分别附着在角状突的外侧和内侧。咀嚼肌分为四部分，翼内肌分为五部分，每个部分的作用线和扭矩都略有不同。这使得食虫动物能够根据摄入食物的特性调整和适应这些旋转运动，从而提高食物的破碎效率。此外，这些引导旋转运动可以在牙齿磨损的情况下实现精确的咬合。颞肌是最大的咀嚼肌，主要负责施加咬合力。总的来说，独特的颌骨形态与复杂的肌肉排列可能有助于提高能量获取效率和维持高代谢率，这对于像鼩鼱这样的小体型哺乳动物至关重要。
