Analyzing the mechanical consequences of septal complexity in ammonites
Final Report Abstract
The iterative evolution of the highly folded ammonitic septum is a long-standing paleontological mystery oft explained by strengthening the shell against ambient water pressure required by deeper water habitats. This project has investigated this idea through comparative computational mechanics and confirmed the project’s working hypothesis: the increase in complexity does not have a significant effect on the shell’s strength under pressure and suggests that the evolution of ammonitic septa does not reflect a persistent trend towards deeper-water habitats. The first step towards achieving this result was material characterization of the cephalopod shell. Nanoindentation and dynamic mechanical analysis (DMA) collected during this project revealed a significant range of properties of different cephalopod shells under varying relative humidities and full water immersion. Nanoindentation measurement showed a surprising lack of dependency on relative humidity in the shells of Nautilus pompilius and Argonauta argo. DMA measurements on beams cut from both of the shells revealed a potentially unique response of A. argo. The shell of A. argo shows a dramatic decrease in the measured elastic modulus when fully immersed in water with a linear dependency on frequency, whereas the N. pompilius shell shows no significant change in properties under full immersion. The nanoindentation results from N. pompilius were used as input material properties for finite element simulations of the theoretical ammonite shells. In order to directly address the potential adaptive value of septal morphology I first decomposed the important morphological parameters that have the potential to affect shell strength under simulated hydrostatic loading. Then a method was developed to create a series of theoretical models whose exact morphological parameters, septal complexity, shell thickness, and septal spacing, could be directly controlled without affecting the other parameters. Simulation results showed that the number of primary folds of the septum have little to no effect on the stress in the shell under equivalent loads. Higher order folds did show some systemic effect in the shells response to hydrostatic pressure but the effect was relatively small and the general decrease observed in the outer shell wall was offset by an increase in stress on the septum itself. An additional and unexpected observation from these simulations was the relatively high displacement observed in the septum and the gradual decrease of this displacement magnitude with increasing septal complexity. This observation suggests a potential buckling of the septa under external loading. The investigation of this potential buckling was coupled with the investigation of the effect of septal complexity on resistance to point loads. Using the same models and adding several models created from computed tomographic data I was able to show that the increase in septal complexity works to delay structural instability and increase the force necessary to buckle the shell. I suggest a potential anti-predatory function for the ammonitic septum that reframes the evolution of septal complexity in ammonoids as a feature of escalation rather than of habitat depth. Future work can now focus on the interplay between more complex shell shapes and septal complexity as well as explore the possible link between predator evolution and septal morphology.
Publications
- Crystal growth kinetics as an architectural constraint on the evolution of molluscan shells. Proceedings of the National Academy of Sciences 116(41): 20388-20397
Schoeppler, V., Lemanis, R., Reich, E., Pusztai, T., Granasy, L., Zlotnikov, I.
(See online at https://doi.org/10.1073/pnas.1907229116) - 2020. The role of mural mechanics on cephalopod palaeoecology. Journal of the Royal Society Interface 17(164): 20200009
Lemanis, R., Stier, D., Zlotnikov, I., Zaslansky, P., Fuchs, D.
(See online at https://doi.org/10.1098/rsif.2020.0009) - The ammonite septum is not an adaptation to deep water: re-evaluating a centuries-old idea. Proceedings of the Royal Society B: Biological Sciences 287: 20201919
Lemanis, R.
(See online at https://doi.org/10.1098/rspb.2020.1919) - Virtual modeling of the ammonoid conch to study its hydrostatic properties. Acta Palaeontologica Polonica 65: 1-14
Morón-Alfonso, D.A., Peterman, D.J., Cichowolski, M., Hoffmann, R., Lemanis, R.
(See online at https://doi.org/10.4202/app.00776.2020) - A reply to a comment on Lemanis (2020); The ammonite septum is not an adaptation to deep water. Proceedings of the Royal Society B: Biological Sciences 288: 20210068
Lemanis, R.
(See online at https://doi.org/10.1098/rspb.2021.0068)
