铲齿象类(amebelodontids)摄食行为相关的下颌形态学分化及力学适应

Mandibular functional morphology diverse and mechanical adaptations of amebelodontids in relation to their feeding behaviors

铲齿象类(amebelodontids)是一类下颌特化为铲状形态的化石长鼻类,对于其下颌的功能形态学的认识在不断提升之中.早期的观点认为铲齿象各属种均生活在近水区域,用宽扁的下颌和下门齿摄食水草;近期通过研究牙齿微磨痕认为,铲齿象不同属种的摄食行为有差异,其中铲门齿象属(Amebelodon)摄食方式非常灵活多样,铲齿象属(Platybelodon)则特化为以下颌和下门齿切割植物的摄食方式.虽然摄食方式是一个典型的力学问题,但尚无从力学角度对铲齿象类摄食方式的研究和论证.本研究通过建立铲齿象属和铲门齿象属的平面下颌有限元力学模型,分析了在"挖掘"和"切割"两种工况下,铲齿象属和铲门齿象属两个模型在力学响应方面的差异.结果证明,铲门齿象下颌模型的力学性能在总体上优于铲齿象模型.这表现在,在相同主动力(肌肉收缩力)的条件下,铲门齿象模型可以对抗较大的远端外力矩载荷;在相同的外力载荷下,尤其是在"挖掘"工况的外力载荷下,铲门齿象模型的最大von Mises等效应力往往要小于铲齿象模型,这符合铲门齿象在演化中的优势积累;同时,铲门齿象模型在"挖掘"和"切割"两种工况下,各项力学响应的指标均无明显差异,支持铲门齿象摄食方式非常灵活多样的论点.另一方面,铲齿象模型在"切割"工况下的力学响应具有一些突出的优势.首先,在相同主动力条件下,铲齿象模型在远端的"切割"力输出要远大于铲门齿象模型;其次,在相同的"切割"外力载荷下,铲齿象模型的总体应变能要小于铲门齿象模型;第三,在"切割"工况下,铲齿象模型的最大von Mises等效应力比在"挖掘"外力载荷下有了很大的改善,与铲门齿象模型在同样大的"切割"外力载荷下的最大von Mises等效应力相差无几.以上结果支持铲齿象属特化的切割植物的摄食方式.形态学在力学方面的优劣是两种铲齿象类具有不同的摄食行为的直接证据,因此本研究比前人的研究更加深入了一步,也为研究长颌象类史前广泛的适应辐射提供了新的方法学参考.

Amebelodontids were a group of fossil proboscideans characterized by highly derived,shovel-like mandibles.For a long time,their mandibular functional morphology remained an interesting issue.In an early view,all amebelodontids were thought to have lived near inland waters,and scooped up aquatic plants using their shovel-like lower incisors(mandibular tusks).Recent studies mainly based on dental microwear,however,revealed apparent partition of feeding ecology during the evolution of this group.One direction was represented by Amebelodon and allies,which displayed rather flexible feeding behaviors;while Platybelodon and its immediate relatives took a different evolutionary direction—Its mandible and mandibular tusks were specialized for cutting vegetations.Mechanical factors are of obvious importance in feeding behaviors.However,biomechanical studies have rarely been applied for understanding fossil proboscidean paleobiology.In the present work,we constructed planar 2D models to undertake finite elements analysis(FEA)of Amebelodon and Platybelodon mandibles,respectively.The Amebelodon 2D model was generated based on the lateral photo of the type mandible of Amebelodon fricki,from the Early Hemphillian of the North America(~6–7 Ma);whereas the Platybelodon2D model was generated following the lateral photo of Platybelodon grangeri mandible from the Linxia Basin of northern China(~12–13 Ma).The masseter and temporalis forces were exerted on the two models as the active forces.The direction of the two active forces was reconstructed based on each morphology of the posterior and anterior borders of the mandibular ramus,where the masseter and temporalis muscles were attached.The two forces were set as being the same magnitude,due to the lacking of experimental data.Two working conditions,"digging"(vertical external loads on the tusk tip)and"cutting"(horizontal external loads on the tusk tip),were undertaken to investigate their mechanical responses.The results indicate that Amebelodon mandible model(AMM)is generally more mechanically advantageous than the Platybelodon model(PMM):under the same active forces,AMM shows greater capability to withstand a larger external moment;and under the same external loads,the maximal von Mises equivalent stresses of AMM in two conditions are usually smaller than those of PMM.Amebelodon probably acquired these advantages in being of a more derived evolutionary grade than Platybelodon.Furthermore,in"digging"and"cutting"modes,the mechanical responses of AMM display insignificant differences,which supports the flexible feeding behaviors in Amebelodon;whereas,the mechanical responses of PMM exhibit several special advantages in the"cutting"mode.Firstly,under the same active forces,the exporting of"cutting"force at the tip of the mandibular tusks in PMM is remarkably higher than that in AMM.Secondly,in the"cutting"mode,under the same external loads,the whole strain energy of PMM is smaller than that of AMM.Thirdly,in the"cutting"mode,the maximal von Mises equivalent stresses of PMM are significantly reduced compared to the"digging"mode,which are close to the von Mises equivalent stresses of AMM in the same conditions.These results support the specialized feeding behavior of Platybelodon,cutting vegetation using its extraordinary mandible.Additional convergence tests depended on the element count and sensitivity tests for the active forces were also performed to make sure the stability and validity of the 2D models.This study provides the direct evidence of disparity in functional morphology among amebelodontids in relation to their feeding behaviors.However,further optimized models(e.g.,3D models,more precise reconstruction of active forces)should be applied for accomplishing more rigorous studies to reveal the co-evolution and adaptive radiation of feeding behaviors and ecomorphology of various fossil longitrostrine proboscideans.