Shape analysis of the preorbital bar in caviomorph rodents,Journal of Morphology

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Shape analysis of the preorbital bar in caviomorph rodents
Journal of Morphology ( IF 1.5 ) Pub Date : 2023-10-04 , DOI: 10.1002/jmor.21646
Federico Becerra ₁ , Aldo I Vassallo ₁

Affiliation

The highly specialised masticatory apparatus of rodents raises interesting questions about how their skull withstands the intensive and sustained forces produced by biting on hard items. In these mammals, major systematics were explored for a long time based on the adductor muscles’ architecture and the related bony structures. The infraorbital foramen stands out, where a hypertrophied head of the zygomaticomandibular muscle passes through—in hystricomorphous rodents—as a direct consequence of the lateral and posterior shift of the preorbital bar. Interestingly, this bar moved laterally and backwards—enlarging the foramen—but it never disappeared throughout evolution, even showing morphological convergence among rodents. Previous research proposed this bar as behaving mechanically similar to the postorbital bar in ungulates, i.e., a safety structure against torsion stress while chewing. We analysed its morphology by mathematically modelling it under bending and torsion scenarios (linearly and elliptically shaped, respectively), and as for biting load propagation (catenary curve). Although the preorbital bar primarily seems to be shaped for withstanding torsional stress (as the postorbital bar in ungulates) and as an escaping point for force propagation, these forces are not a consequence of chewing and grinding foods, but preventing the zygomatic arch from failing when the powerful laterally-displaced jaw adductor muscles are pulling the dentary upwards at biting.

中文翻译：

豚鼠眼眶前杆的形状分析

啮齿动物高度专业化的咀嚼器官提出了有趣的问题，即它们的头骨如何承受咬硬物体时产生的密集和持续的力量。在这些哺乳动物中，基于内收肌的结构和相关的骨骼结构，人们对主要的系统学进行了长期的探索。眶下孔非常突出，在啮齿类动物中，肥大的颧下颌肌头部穿过该孔，这是眶前杆横向和后移的直接结果。有趣的是，这个杆横向和向后移动——扩大了孔——但它在整个进化过程中从未消失，甚至在啮齿类动物中表现出形态趋同。先前的研究提出，该杆的机械性能类似于有蹄类动物的眶后杆，即咀嚼时抵抗扭转应力的安全结构。我们通过在弯曲和扭转场景（分别为线性和椭圆形）以及咬合载荷传播（悬链线曲线）下对其进行数学建模来分析其形态。虽然眶前杆的形状主要是为了承受扭转应力（如有蹄类动物的眶后杆）并作为力传播的逃逸点，但这些力不是咀嚼和磨碎食物的结果，而是防止颧弓在咀嚼和磨碎食物时失效。咬合时，强大的横向移位的下颌内收肌将齿骨向上拉。

更新日期：2023-10-04

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