EXPLORING THE COVARIATION OF MORPHOLOGY AND MECHANICS OF AMMONITE SHELLS

Ammonites are an iconic fossil group whose long persistence through time, global distribution, and high disparity underlie their study in the fields of stratigraphy, ecology, evolutionary dynamics, extinction selectivity, and constructional morphology. The most striking feature of these shells is the complex, near fractal pattern of their septa that set them apart from all other cephalopods. These complex structures present us with a functional puzzle. The common, textbook explanation is that highly convoluted septa are adaptations to increase the shells resistance to ambient water pressure thereby allowing forms with more complex septa to inhabit deeper water depths. This interpretation has been challenged multiple times and the theoretical mechanical work behind the classical interpretation often lacks important information and accurate geometries. Our finite element analyses done with tomographic models have shown that past theoretical models failed to accurately predict the location of maximum stresses in the shells under hydrostatic pressure. These models also showed high septal curvature increased the stress across the septum rather than decreasing it. However, tomographic models are limited by the reality of covariation of different morphological traits that all influence the mechanical response of the shell making isolating the contribution of a single morphological element very difficult. Here we use simple, cylindrical shell models of varying thicknesses with septal morphologies created using minimum surface calculations whose boundary conditions are suture lines to create finite element models. By varying septal complexity, shell thickness, and septal spacing, we can explore the mechanical effect each morphological parameter has on the shell’s resistance to water pressure. Understanding the influence of different morphological parameters allows us to contextualize morphological changes during evolution and better interpret ammonite ecology.