EVALUATING ATRYPIDE BRACHIOPOD FEEDING USING 3D PRINTED MODELS

Brachiopods of the order Atrypida were among the most common and diverse Siluro-Devonian organisms. A previous study has associated globose, biconvex atrypides with higher-energy and cleaner carbonate systems, and shield-shaped, plano-convex atrypides with low-energy, muddy systems. In quiet-water, the rate of available food particles can be low. Suspension-feeders living in this setting would benefit from cost-efficient mechanisms to increase particle capture; shape can be one such mechanism. Our study aims to evaluate the effects of the two end-member atrypide shapes on feeding performance through biomechanical experiments. Ambient flow through models of fossil organisms can be used as a proxy for flow through a once-living animal, even if that animal actively pumped, as ambient flow represents the most energy efficient path for food particles or waste removal. Thus, accurate models can help evaluate hypotheses on life orientations and feeding.

We used 3D scanning and printing, to construct two highly accurate gaping models of the end-member shapes, including simulated mantle tissue and a lophophore. The valves were printed using a translucent PLA. A solid support for the lophophore was also printed using PLA, making use of an additional water-soluble PVA support material to enable printing of the delicate structure, which was then lined with false eyelashes to simulate tentacles. We used a recirculating flume tank set at a velocity of 0.2 m/s and dye streams to visualize ambient flow within the models in multiple orientations to flow. Throughout all trials the shield-shape outperformed the globose shape. This was demonstrated by a strong, steady ambient flow that crossed the lophophore and exited at an elevated position on the commissure, with the flow resembling that of a chimney.

Our results suggest that shield-shaped atrypides may have benefited from enhanced ambient flow, while globose atrypides may have relied more on active pumping. This result agrees with other observed distributions of morphology along environmental gradients, where calm water taxa, such as shield-shaped atrypides, often have body shapes that maximize ambient flow. A morphology that enhances ambient flow across the lophophore increases food particle capture with little additional energetic cost.