Comparative cephalopod shell strength and the role of septum morphology on stress distribution.
Comparative cephalopod shell strength and the role of septum morphology on stress distribution.
复制标题
比较头足动物的壳强度和隔膜形态在应力分布中的作用。
作者：
Lemanis R;Zachow S;Hoffmann R
菊石中复杂折叠的隔膜的演化一直是一个有争议的话题。对这些折叠的隔膜功能的解释可以分为生理假说和机械假说，机械功能往往得到广泛的支持。头足类壳的复杂性使得直接测试这些结构的机械性能变得困难，而不过度简化隔膜的形态或提取一个小的子域。然而，现代有限元分析的能力现在允许对从计算机断层扫描数据中获得的完整的、经验的贝壳模型进行直接的力学假设测试。在这里，我们首次使用经验模型比较了现存的蓬氏鹦鹉螺壳、螺旋藻和灭绝的菊石Cadocera sp.的壳的能力。能够承受静水压力和点载荷。结果表明，静水压力在最终隔膜上施加的应力最大，其余壳体的压缩最小。在静水压力下，螺旋藻的应力最小，而庞贝的应力最大。Cadocera sp.显示沿隔鞍部与壳壁附着处的高应力发展。当点力沿着缝合线而不是无支撑的腔壁定向时，点载荷产生的应力会减少。Cadocera sp.显示与所有其他模型相比，点载荷之间的应力降幅最大。隔沟的幅度越大，静水压力产生的应力就越大；然而，幅度越大，沿缝合方向的点载荷的应力大小就越小。在我们的模型中，缝合的复杂性并不意味着对静水压力的抵抗力更大，但它似乎确实增加了对点载荷的抵抗力，例如来自捕食者的抵抗力。这一结果允许在隔形态的基础上讨论古生态重建。我们进一步认为，隔膜强度指数中用于表征隔膜形态的比率以及用于计算珍珠层抗拉强度的比率可能是不够的。更好地了解头足珍珠层的材料性质，可以通过有限元分析来估计有壳头足类的最大深度极限。
The evolution of complexly folded septa in ammonoids has long been a controversial topic. Explanations of the function of these folded septa can be divided into physiological and mechanical hypotheses with the mechanical functions tending to find widespread support. The complexity of the cephalopod shell has made it difficult to directly test the mechanical properties of these structures without oversimplification of the septal morphology or extraction of a small sub-domain. However, the power of modern finite element analysis now permits direct testing of mechanical hypothesis on complete, empirical models of the shells taken from computed tomographic data. Here we compare, for the first time using empirical models, the capability of the shells of extant Nautilus pompilius, Spirula spirula, and the extinct ammonite Cadoceras sp. to withstand hydrostatic pressure and point loads. Results show hydrostatic pressure imparts highest stress on the final septum with the rest of the shell showing minimal compression. S. spirula shows the lowest stress under hydrostatic pressure while N. pompilius shows the highest stress. Cadoceras sp. shows the development of high stress along the attachment of the septal saddles with the shell wall. Stress due to point loads decreases when the point force is directed along the suture as opposed to the unsupported chamber wall. Cadoceras sp. shows the greatest decrease in stress between the point loads compared to all other models. Greater amplitude of septal flutes corresponds with greater stress due to hydrostatic pressure; however, greater amplitude decreases the stress magnitude of point loads directed along the suture. In our models, sutural complexity does not predict greater resistance to hydrostatic pressure but it does seem to increase resistance to point loads, such as would be from predators. This result permits discussion of palaeoecological reconstructions on the basis of septal morphology. We further suggest that the ratio used to characterize septal morphology in the septal strength index and in calculations of tensile strength of nacre are likely insufficient. A better understanding of the material properties of cephalopod nacre may allow the estimation of maximum depth limits of shelled cephalopods through finite element analysis.