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

Immobile benthic organisms lacking attachment or cementation mechanisms are considered to be best adapted to quiet bottom environments. Since the free-living Lower Permian spiriferinide brachiopod Pachycyrtella omanensis inhabited a sandy substrate with high-energy water flow, flume experiments were performed to show the possible hydrodynamic advantages of shell morphology in postural stability and generation of feeding flows. Modelling indicates that a vertical position, with the commissure plane perpendicular to the seabed, was the most unstable, although it is considered to have been its original life position. On the other hand, the passive flow inside the model in vertical position exhibited vortex movement with constant degree of inhalent flow and exhalent flow, conferring advantages on the effective filtration of food particles using a spiral lophophore. The intensity and movement of the passive flow for feeding could have been adjusted through changes in the angle of opening of the valves. As the shoreface habitat was affected by oscillatory flows, a small-sized animal could have undergone a 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 the ventral umbo without altering its morphofunction to generate passive feeding flows. Biomechanics, functional morphology, opportunistic species, Pachycyrtella, Spiriferinida, suspension feeder.