BREAKING DOWN THE SPIRAL: EVALUATING HYDRODYNAMIC EFFECTS OF ISOLATED WESTERMANN MORPHOSPACE PARAMETERS IN AMMONOIDS

The trophic level and ecological role ammonoids may have occupied throughout their 300 million year history is a mystery subject to continual research. Ammonoid shell shapes express tremendous variation through time, space, phylogeny, and ontogeny; yet the potential ecological impacts of these shell shapes remain open to many interpretations. Experimental and computational approaches have attempted to quantify the hydrodynamic costs - or benefits - of shell shape, with success distinguishing first-order parameters. Our recent computational fluid dynamics (CFD) studies have supported the trends found in past experiments; inflated shell morphologies incur greater drag at faster swimming velocities (and/or larger size). The second- and third-order contributions of other shell shape variables have been difficult to distinguish, rank, and quantify.

We produce synthetic ammonoid shell shapes to model variation across each of two distinct shape parameters: whorl expansion, and umbilical exposure. We use ANSYS Fluent to resolve the flow fields around the shells and calculate the drag incurred by each. We compare each of these drag values against a control morphotype that is at the center point of the chosen parameter variation. Our results show that the magnitude of change in drag is sensitive to both velocity and direction of shape change (i.e., increase or decrease in umbilical exposure). This non-uniform distribution reveals a significant hierarchy of influence between morphological traits. We present new thresholds and gradients that may bracket the potential swimming efficiency of animals that occupied and built these shells through time. At a large scale, these results can present testable hypotheses for the structure, change, and consequences in pre-and-post extinction ecosystems of the Paleozoic and Mesozoic.