Fractal geometry and multi-scale mechanics of cephalopod shells

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The complex, fractal structures in the shells of ammonoid cephalopods, the septa, have been and the object of speculation for over 100 years. Reconstructing the function that drove the iterative evolution of these structures is a difficult task as there are no modern animals with analogous structures. This means the potential information about biotic interactions or physiology that could be reflected by the septa are likewise unknown, so the insight they might afford about ammonoid evolution and extinction is lost. The most widespread functional explanation, connecting the evolution of complexity to deeper water habitats, has recently been dismissed on a structural basis. However, previous work has suggested another possible mechanical function: resistance to point loads. This project will aim to explore this potential function through a combination of theoretical modeling, computational mechanics, 3D printing, and dynamic mechanical analysis. The viscoelastic properties and strengths of different cephalopod shells will be characterized as a function of humidity as the properties of mineralized tissues have been shown to vary with changes in relative moisture content. This information will inform both physical and virtual mechanical tests to model strength and buckling of different representative septal morphologies at varying morphological complexities. Further tests will expand on previous models by introducing shell complexity and model how morphological variations in the shell-septa system affect strength and buckling. This work will ultimately aim to address the main remaining mechanical hypothesis about septal function, namely whether the septa could reflect a form of escalation. Instead of being driven by habitat migration, the evolution of complex septa may, in fact, reflect increasing pressure from pelagic predators. The iterative evolution of septal complexity has the potential to offer key insights into ammonoid evolutionary trends as long as the driving force for their evolution can be determined.