CFD MODELING OF THECAL SHAPE IN BLASTOIDS (ECHINODERMATA)

Blastoids are an extinct class of Paleozoic echinoderms that were important members of Paleozoic Crinoid Macroevolutionary Faunas. Blastoids were locally very abundant and diverse, primarily the transition from camerate-dominated to cladid-dominated communities. It is little appreciated that thecal geometries that range from spherical to cone-shaped and ambulacra that vary significantly in length strongly affect water flow around the theca and produce significantly different brachiolar filtration fan morphologies that influence feeding. One goal of this research was to determine differences in hydrodynamic efficiency (HE), the impact of the theca on water flow; and feeding effectiveness, which is the inverse of HE with the development of zones of slow water eddying, turbulence, etc. We constructed 3D models with theca, stem, and brachioles and used the models as input into computational fluid dynamics simulations visualized as contour plots of water velocities along the oral-aboral plane of the specimen. Simulations were run at 0.5, 2, 10, 25 and 50 cm/s. Hydrodynamic efficiency (HE) was calculated as the average current velocity normalized as a percent of the initial velocity. This allowed us to calculate average velocity values throughout the plane and delineate variation across the range of water velocities simulated for each thecal shape. Globoblastus has a spherical theca and showed little change in HE (85% of initial velocity) across the range of velocities tested. The other taxa tested, Pentremites robustus, Pentremites pyriformis, Pentremites godoni,Codaster acutus, Deltoblastus subdelta, Deltoblastus elongatus, Hyperoblastus sp., Monoschizoblastus rofei, and Nucleocrinus sp., all showed significant differences in HE at different velocities. The greatest variance in HE occurred in currents from 0.5 cm/s to 10 cm/s and ranged from 17% (P. robustus) to 1.2% (Globoblastus sp.). All thecal configurations had HE values of ~85% at current velocities > 25 cm/s with the exception of Nucleocrinus sp. which had an HE value of 92% suggesting that all the thecal shapes had similar gross hydrodynamic properties at these velocities. These data suggest that blastoid thecal shapes were optimized for feeding efficiency at current velocities from 2-10 cm/s producing new insight into their paleoecology.