PRELIMINARY INVESTIGATIONS INTO THE ECOLOGY AND FUNCTIONAL MORPHOLOGY OF THE ANNULATED ORTHOCONIC CEPHALOPOD SPYROCERAS

Orthoconic cephalopods were significant components of marine communities throughout the Paleozoic Era. In that time, many orthoconic cephalopods convergently evolved prominent transverse annulations on their external shell sculpture. The possible functions of this conch structure have not been the focus of much investigation, and the ecology of orthocone nautiloids is generally understudied. Here, we investigate the general ecology of the pseudorthocerid genus Spyroceras, from the Middle Devonian Hamilton Group of New York State, and explore the potential role(s) of shell annulations. Spyroceras is common in the higher-energy (silty sand) facies of the Hamilton Group and frequently co-occurs with a diverse fauna of bivalves, brachiopods, bryozoans, and trilobites. The ecological breadth and consistency of occurrence and associations, along with facies relationships, will be investigated using field observations and data from the Paleobiology Database. Annulations (ribs) have been suggested to strengthen the shell against crushing predation, and recent research has also shown the significant potential for external conch sculpture to influence movement performance in water. We evaluate the latter of these hypotheses through physical and numerical simulations. In order to quantify the morphology of Spyroceras’s conch over ontogeny, rib height and spacing was measured versus shell diameter along the lengths of dozens of specimens. Diameter measurements were corrected for compaction and used to calculate shell apex angles. Physical and digital 3D models are then constructed using these parameters, and unsculptured orthocone models are also prepared for comparison. Numerical simulations will be used to characterize the flow fields around the models in various orientations, and future work may incorporate experiments on physical models placed in a water tunnel. As the annulated sculpture of Spyroceras is present in other orthocone taxa, our findings will be applicable to other paleoregions and ectocochleate cephalopod taxa.