HYDRODYNAMIC MODELING OF BRACHIOLAR FILTRATION SYSTEMS IN BLASTOIDS (ECHINODERMATA)

Blastoids are pelmatozoan echinoderms and major components of shallow and deep-water Paleozoic marine ecosystems. Blastoids are interpreted as passive suspension feeders with a 3D brachiolar filtration fan similar to modern stemmed crinoids. Although blastoids are rarely preserved as complete specimens, we can construct 3D models with theca, stem, and brachioles and use the models as input into fluid flow simulations to analyze the dynamics of water flow over various thecal and filtration fan morphologies. We have modeled Globoblastus, spherical theca, Hyperoblastus, pyriform theca with long protruding basalia, Deltoblastus, godoniform theca with invaginated basalia, and Timoroblastus, stemless theca adapted for sitting on sea floor. Stemmed blastoids were modeled in horizontal currents flowing adorally at velocities from 2cm/s to 50cm/s and in vertical water flow with very low velocity (<0.5cm/s) simulating plankton rain. The results were compared to in situ observations of modern stemmed crinoids in similar current regimes off Roatan. Water flow changes from laminar to turbulent leeward of the point of maximum thecal circumference producing lower velocity eddies adjacent to the oral area and the adoral parts of the ambulacra. The volume and configuration of turbulent flow change with current velocity and thecal shape. Although the filtration fans were stable under all modeled currents, the fans seem optimized for lower velocity horizontal currents. In higher currents, the eddies above the oral surface are poorly structured. Low velocity vertical water flow over blastoid models on a vertical stem produce a plausible filtration fan for feeding on plankton rain or marine snow. Based on observations of modern Democrinus, blastoids likely could dynamically shift orientations of the filtration fans to accommodate changing current velocities and directions. Hydrodynamic modeling of blastoids in groups demonstrates that spacing (perpendicular to current direction) equal to ½ the width of the filtration fan is the minimum distance required to eliminate current interference between individuals. Individuals aligned in the direction of water flow have an effect on the water velocities of blastoids downstream. Modern stemmed crinoids often occur in regularly spaced groups with similar spacing.