LIFE POSITION IN AN EXTINCT ECHINODERM CLASS: A BIOMECHANICAL APPROACH

Appearing during the Ordovician radiation, blastoids became a prominent constituent of the Paleozoic Evolutionary Fauna and persisted until the Middle Permian mass extinction. Stalked members of these sessile, passive suspension-feeding echinoderms relied on ambient water currents for food and are traditionally interpreted with an upward-facing filtration fan of arm-like brachioles oriented perpendicular to the substrate (the ‘collecting bowl' position). Alternatively, the filtration fan may have been oriented parallel to the substrate with the brachioles concavely arrayed toward the flow, the stalk bent approximately 90° and the oral pole facing either up-current or down-current as in some Recent stalked crinoids. To explore the hydrodynamic behavior of these orientations, water flow was examined around a life-sized model of Pentremites pyriformis from the Mississippian Chester Series of Illinois at current velocities typical of shallow carbonate shelf or reef settings (2-10 cm/s). Experimental observations in a flow tank suggest that the ‘collecting bowl' position is unlikely because of long water residence times (60-120 s) around the oral pole, which delays waste removal from the spiracles and anus. In contrast, the oral pole facing up-current experienced shorter residence times; turbulent wakes emerged downstream of the theca, however, with the resulting backwash of re-circulated water fouling upstream feeding and respiratory structures. Dynamic pressure generated from the incoming flow could also potentially force waste into the mouth, spiracles, and anus. The oral pole facing down-current experienced short residence times, but without significant backwash, and thus appears to be the most likely orientation. While the application of living taxa in inferring the functional morphology and life habits of fossil taxa is often complicated (e.g., Nautilus as an uncritical model for ammonoids), at least some Recent crinoids may be posturally analogous to blastoids despite several important morphologic and ecologic differences.