Complexity of septal surfaces and suture lines in ammonoids - implications for the hydrostatic apparatus and palaeoecology using modern CT-techniques
Final Report Abstract
The goal of this project is to use computed tomography to create 3D models directly from the original shells and use these models to explore ammonoid palaeobiology, specifically buoyancy and the potential functions of complex septa. Comparisons with tradition methods of buoyancy and hydrostatic calculations demonstrate the inaccuracy of volume calculations and the biases in hydrostatic calculations. The volume of the Cadoceras shell reconstructed from mathematical approximation shows a persistent underestimation of shell volume. Traditional hydrostatic analysis identified a progressive bias in the calculation of the center of gravity in which overall hydrostatic stability is overestimated for shorter body chamber forms. Furthermore, a methodology for the calculation of buoyancy using tomographic data is created and used to show that a hypothetical ammonite could remain in the water column despite a potential, overall negative buoyancy. Ontogenetic changes in the ratio of chamber surface area to volume are calculated based on tomographic data of extant Nautilus, Allonautilus, Spirula, the Paleozoic ammonoid Arnsbergites, and the Mesozoic ammonoids Amauroceras, Cadoceras, and Kosmoceras. Previous work has suggested that septal complexity increased the relative surface area of the chambers thereby allowing rapid fluid diffusion out of the chambers. Our results reject this hypothesis over most of ontogeny; however, an increase in the relative surface area of the chambers is shown for all tested ammonoids that preserved their early ontogeny. This might reflect adaptations towards an increase in growth in early ontogeny that are coupled with a shift in early ammonoid evolution towards smaller eggs and higher fecundity. The mechanical function of ammonitic septa is investigated through comparative finite element analysis on the shells of Nautilus, Spirula, and Cadoceras. The hypothesis that increases in septal complexity should increase shell resistance to hydrostatic pressure is not supported as Spirula, which has the simplest septal morphology, shows the highest resistance to hydrostatic pressure. The use of septal amplitude as a proxy for palaeobathymetry is rejected as increasing septal amplitude is shown to increase the stress due to hydrostatic pressure as opposed to reducing it. Septal amplitude is shown to decrease stress due to point loads indicating a potential anti-predatory function of septal complexity which agrees with the increase in septal curvature that would help compensate for lost shell material. Reliance on simplified geometric models in studies of ammonoid palaeobiology seem to lead to oversimplified to outright incorrect conclusions and should be used with caution.
Publications
- (2014): Non-invasive imaging methods applied to neo- and paleontological cephalopod research. Biogeosciences, 11: 2721-2739
Hoffmann, R., Schultz, J.A., Schellhorn, R., Rybacki, E., Keupp, H., Gerden, S.R., Lemanis, R. & Zachow, S.
(See online at https://doi.org/10.5194/bg-11-2721-2014) - (2015): A new approach using highresolution computed tomography to test the buoyant properties of chambered cephalopod shells. Paleobiology, 41(2): 313-329
Lemanis, R., Zachow, S., Fusseis, F. & Hoffmann, R.
(See online at https://doi.org/10.1017/pab.2014.17) - (2015): Non-invasive imaging techniques combined with morphometry: a case study from Spirula. Swiss Journal of Palaeontology 134: 207-216
Hoffmann, R., Reinhoff, D. & Lemanis, R.
(See online at https://doi.org/10.1007/s13358-015-0083-0) - 2015. Chapter 16 - Ammonoid buoyancy. – In: Ammonoid Paleobiology: From anatomy to ecology. Klug, C., Korn, D., De Baets, K., Kruta, I. & Mapes, R.H. (eds.). Topics in Geobiology 43, 613-648
Hoffmann, R., Lemanis, R., Naglik, C. & Klug, C.
- 2015. GPU-accelerated denoising of large tomographic data sets with low SNR. ZIB-Report 15-14 (December 2015), 1-65
Lamas-Rodríguez, J., Ehlke, M., Hoffmann, R., Zachow, S.
- (2016): Comparative cephalopod shell strength and the role of septum morphology on stress distribution. PeerJ 4:e2434
Lemanis, R., Zachow, S., Hoffmann, R.
(See online at https://doi.org/10.7717/peerj.2434) - (2016): The evolution and development of cephalopod chambers and their shape. PlosOne. 11(3): e151404
Lemanis, R., Korn, D., Zachow, S., Rybacki, E. & Hoffmann, R.
(See online at https://doi.org/10.1371/journal.pone.0151404)
