Hydrodynamic behavior of brachiopod shells: Experimental estimates and field observations

摘要

The effects of unidirectional currents on the behavior of empty shells of the articulate brachiopod Terebratalia transversa were studied both in a laboratory flume and in a tidal-channel environment. Shells were tested on a hard, high-friction surface in the flume and on unconsolidated, well-sorted sands both in the flume and in the field. The velocity of the current competent to move terebratuloid shells is controlled by the starting orientation of the shell and is not strongly related to size or shape within the range of shells tested. Most shells assumed a preferred final orientation, both in the laboratory and field, with the anterior-posterior axis parallel to the current, ventral valve up, and pedicle opening facing up-current. Brachiopod shells that achieved hydrodynamically stable positions tended to be transported mainly by tumbling, with the anterior-posterior axis parallel to the current direction. Observations of shells on natural, current-swept sand substrates indicate that scour marks are formed around the up-current side of the brachiopod, with the scoured sediment deposited as a wedge down current from the shell. If scour continues, the brachiopod may slide down into the up-current depression, assuming variable inclinations. If the shell slides sufficiently far down into the depression, the solenoidal vortex that generates the scour may weaken or fail. Sediment carried in the bedload then buries the shell.