Meeting Abstract
Ammonoid cephalopods are an extinct group that are notable for their 300 million year history dominated by recurrent cycles of diversity boom and bust. Throughout these biodiversity cycles they repeatedly evolve distinct coiling shell shapes. These coiled ammonoids have shown very pronounced morphological shifts during their recovery following bust periods. Paleoecological research into these enigmatic animals, which have no strong modern analogue, has long featured discussions of how these changing shell morphologies influenced swimming ability. Experimental and computational approaches have attempted to quantify the hydrodynamic costs, or benefits, of shell shape, with success distinguishing most relevant first-order parameters. We advance this work by using computational fluid dynamics to investigate the hydrodynamics of theoretical ammonoid morphotypes. We use 3D modeling to create synthetic ammonoid shells that model variation in two key morphological parameters: Whorl expansion (the rate at which an individual coil increases in diameter) and umbilical exposure (the amount of central coiling exposed to flow). We use Ansys FLUENT to resolve the flow fields around each shell and the drag they incur. These drag values are then compared against a control morphotype that represents the centerpoint of each parameter variation. Our results show that the magnitude of change in drag is non-linearly sensitive to both the direction and magnitude of change within a parameter. We also recover a distinct hierarchy of effect between morphological parameters. We present new gradients of these animals swimming potential that, at larger scales, can provide the groundwork for testable hypotheses on the structure of paleozoic and mesozoic marine systems through time.
