Hydrodynamic advantages in the free-living spiriferinide brachiopod Pachycyrtella omanensis: Functional insight into adaptation to high energy flow environment

It has been suggested that immobile benthic animals with no attachment and cementation have adapted to quiet bottom environments. Since the free-living Lower Permian spiriferinide brachiopod Pachycyrtella omanenesis inhabited a sandy bottom with high energy flow condition, water flow experiments have been performed to show possible hydrodynamic advantages of shell morphology in postural stability and generation of feeding flows. Threshold velocity of removal from the bottom indicates that the vertical posture with the commissure plane perpendicular to the bottom was the most instable, though it is supposed to have been the original life posture. On the other hand, the passive flows inside the model in vertical posture exhibited vortex movement with constant degree of inflows and outflows, conferring advantage in effective filtration of food particles using a spiral feeding organ. The intensity and movement of passive flows for feeding could have been adjusted through changes of valve opening angle. As the shoreface habitat was affected by oscillatory flows, a small-sized animal could have undergone high risk of burial, while an increase in size would have led to easier removal from the sandy bottom. To avoid both physical risks, Pachycyrtella developed a thick shell with a high rate of growth, specifically increasing the weight of umbo without altering the morpho-function to generate passive feeding flows.