The shells of ammonoid cephalopods are among the most recognizable fossils, whose fractally folded, internal walls (septa) have inspired many hypotheses on their adaptive value. The enduring explanation for their iterative evolution is that they strengthen the shell against pressure at increasing water depths. The fossil record does not definitively support this idea and much of the theoretical mechanical work behind it has suffered from inaccurate testing geometries and conflicting results. By using a different set of mathematical methods compared with previous studies, I generate a system of finite-element models that explore how different parameters affect the shell's response to water pressure. Increasing the number of initial folds of the septa ultimately has little to no effect on the resulting stress in the shell wall or the septum itself. The introduction of higher-order folds does reduce the tensile stress in the shell wall; however, this is coupled with a higher rate of increase of tensile stress in the septum itself. These results reveal that the increase in complexity should not be expected to have a significant effect on the shell's strength and suggests that the evolution of ammonitic septa does not reflect a persistent trend towards deeper-water habitats.

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菊石隔膜不是深水适应：重新评估一个世纪前的想法

菊石头足类动物的外壳是最知名的化石之一，其分形折叠的内壁（隔膜）激发了许多关于其适应性价值的假设。对它们反复演化的持久解释是，它们在水深增加时增强了外壳抵抗压力。化石记录并没有明确支持这一想法，其背后的许多理论机械工作都受到不准确的测试几何和相互矛盾的结果的影响。与之前的研究相比，通过使用一组不同的数学方法，我生成了一个有限元模型系统，探索不同的参数如何影响壳对水压的响应。增加隔膜的初始折叠次数最终对壳壁或隔膜本身产生的应力几乎没有影响。高阶褶皱的引入确实降低了壳壁的拉应力；然而，这与隔膜本身的拉伸应力的更高速率增加相结合。这些结果表明，复杂性的增加不应预期对壳的强度产生显着影响，并表明氨质隔膜的演变并未反映向深水栖息地的持续趋势。