Comparative hydrodynamic stability of brachiopod shells on current-scoured arenaceous substrates

Selected brachiopod specimens were placed on a well-sorted medium grained sand (0.5 mm) in a recirculating flume and subjected to a constant unidirectional current of 26–27 cm/sec. The specimens were placed in each of six feasible life-orientations. In three orientations, the valves were reclining with either the lateral, anterior or posterior profile upcurrent. In three orientations, the commissural plane was perpendicular to the substrate with either the pedicle or brachial valve upcurrent, or a lateral margin upcurrent. Destabilization indices were calculated for each specimen in each orientation based on the elapsed time until the specimen reoriented. Nonstrophic biconvex brachiopods are most stable with the valves reclining on the substrate. Among orientations with the valves erect, the pedicle valve upcurrent and the brachial valve upcurrent are the most and least stable orientations, respectively. Shell stability increased among nonstrophic specimens that were more equi-biconvex. Biconvex strophic brachiopods are also more stable if the valves were equi-biconvex, cither compressed (biplanate) or inflated, provided that the specimen was propped up on its hingeline with a lateral margin upcurrent. Catacline interareas afforded greater stability than procline, apsacline or orthocline interareas when the valves were perpendicular to the substrate. Accentuated plication reduced shell stability. Alate biconvex forms were often more stable if the valves were vertical to the substrate rather than reclining. Alate biconvex geometries are more stable than nonstrophic and strophic biconvex forms if the commissural plane is perpendicular to the substrate and parallel to the current. Broad catacline interareas increased stability in all orientations. Plano-convex and dorsi-biconvex alate forms are usually less stable than equi-biconvex or ventri-biconvex alate geometries. Concavo-convex geometries are stable in all orientations except with the valves nearly vertical to the substrate and parallel to the current. Spines greatly retard sediment-scour and maintain concavo-convex specimens in orientations with the valves elevated above the substrates. D Brachiopoda, shell shape, hydrodynamic stability.