CHARACTERIZING THE RELATIONSHIP BETWEEN HYDRODYNAMIC EFFICIENCY AND SHELL ORIENTATION IN EXTINCT AMMONOIDS

The swimming biomechanics of ammonoids have long been a topic of research focus due to their broader ecological role in ancient marine ecosystems. While the drag coefficient experienced by these animals in motion has been the primary focus of the previous literature, a shell's orientation relative to the direction of motion is a frequently overlooked topic. As a response to a propulsion force generated by the internal body of an ammonite, the shell advances in a forward motion. At the same time, the exertion of the propulsion force causes the shell orientation to rotate forward. After initiating a forward motion, the propulsion force decreases over the course of what we call a "cruising period" and the resistance from the incoming flow grows relative to the decreasing propulsion force. This results in the shell orientation reverting back to its neutral orientation. In this numerical investigation, we orient a 5-cm planispiral sepertincone in incremental angles from -90 to 90 degrees from its neutral orientation relative to the stream direction and observe the energy loss along the stream and across the shell using a computational fluid dynamics model. We compare the resulting energy loss for each orientation against the distribution of volume and surface area posterior to anterior and the change in the projected frontal area. The comparison revealed that the shell orientation relative to the stream direction determines the hydrodynamic efficiency of a shell. We find that the optimal orientation occurs close to the animal's neutral orientation. In contrast, the least efficient orientations occur both when the aperture is parallel to the flow and when it is perpendicular to the flow below the rest of the shell. Finally, we discuss the potential implications the identified characteristics may have for ammonoids in sustaining locomotion and their broader implication on ammonoids' ecological role.