Analyzing the mechanical consequences of septal complexity in ammonites

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Ammonoids are an extinct group of cephalopods that span 350 million years and were distributed across the world's oceans. The ecology of these animals has been used to reconstruct palaeoenvironment, macroevolutionary trends, and interpret geochemical data. However, much of their ecology is not well understood, specifically habitat depth, which is often connected to the complex morphology of the highly curved and multi-lobate septa. Hypotheses argue that the increased complexity of these structures helped to increase the resistance of different parts of the shell to implosion due to increasing hydrostatic pressure, thus allowing forms with more complex sutures to inhabit deeper waters. Evidence of this mechanical function of the septa is limited to theoretical mechanical models, the results of which have directly contradicted each other. One study argued that septal complexity weakens the shell against implosion while another argued septal complexity strengthens the shell against implosion. This project will combine nano-mechanical material testing, high-resolution computed tomography and finite element analysis to systematically study empirical, mechanical models of ammonoid shells to determine whether the complex morphology of the septa strengthen the shell against external loads. The mechanical properties of the shells of cephalopods will be tested using nanoindentation under controlled humidity and temperatures. This data will be combined with nano-CT derived 3D models in order to perform finite element analyses to test the pre-existing hypotheses of septal function. This work will help determine whether the complex septal morphology unique to ammonoids reflects aspects of their ecology. Understanding the functions of the septa will influence our interpretation of: ammonoid paleobiology, evolutionary trends, the persistence of ammonoids through mass extinctions, and provide realistic values for potential habitat depth limits, which will affect the interpretation of derived geochemical data from ammonite shells.