A STUDY OF DIPLOPORAN ECHINODERM FEEDING STRATEGIES USING COMPUTATIONAL FLUID DYNAMICS

Computational fluid dynamics (CFD) is a powerful tool to analyze feeding strategies and other life functions of aquatic organisms, both extant and extinct. Paleontological CFD simulations require three-dimensional (3D) models of the fossil morphology for input into a digital domain. However, the reconstruction of Paleozoic stemmed echinoderms is difficult due to a lack of extant analogs. This is further compounded by poor preservation of the more delicate parts of their morphology (e.g. feeding appendages and attachment structures). These two factors have hindered paleoecological understanding of behaviors, in particular feeding strategies.

Here, we present the first CFD work on understanding the feeding behaviors of diploporan echinoderms. We digitally reconstructed 3D models of two diploporan taxa from 2D images using Rhinoceros 3D modeling software, and then imported them into Ansys Fluent CFD software. The genera Holocystites and Eumorphocystis were selected for this study because of the morphological variability of their feeding appendages: Holocystites bears relatively large feeding appendages, while Eumorphocystis bears triserial feeding appendages (unique among diploporans). Reconstruction of feeding appendages in Holocystites was done using a rare example of preserved feeding appendages in a closely related organism. CFD results indicate that both animals were likely utilizing strategies of suspension feeding still observed in modern marine invertebrates. The relatively large feeding appendages of Holocystites and the high-velocity flow patterns around them suggest it was feeding through inertial impaction, where the inertia of nutrient particles causes them to impact feeding appendages. Flow patterns around Eumorphocystis suggest it was likely feeding through direct interception; this strategy was aided by ascending vortices directing fluid up the stem and increased residence time of nutrient particles around the mouth, strategies similar to those observed in CFD studies of encrinid crinoids.