Taphonomic and paleoecologic implications of flow-induced forces on concavo-convex articulate brachiopods: an experimental approach

The Orientation of benthic marine organisms may be disturbed by flow-induced forces (i.e. drag and lift) caused by wave and current activity. Drag and lift are partly a function of organism size and shape. Consequently, morphology may affect stability (defined as resistance to reorientation, flipping, or entrainment) both during the life of an organism and after its death. An understanding of drag-and-lift effects is therefore essential to the interpretation of paleoecology and biostratinomic processes. An experimental method for quantifying the relative effects of flow-induced forces is described. These forces are measured during flume experiments using transducers and plaster replicas of fossils. As an illustration of the method's potential for taphonomic research, results from experiments investigating the effects of concavo-convex morphologies of articulate brachiopods are presented. Concave-up and convex-up orientations are commonly used to infer paleohydraulic conditions. Two geniculate brachiopods (Rafinesquina alternata and Leptaena richrnondensis) and three flattened forms (a second morphotype of Rafinesquina altemata, Strophodonta demissa , and Tropidoleptus carinatus) were tested in convex-up and concave-up postures and in three azimuthal orientations (hingeline oriented upstream, hingeline downstream, and hingeline parallel to flow). Concave-up orientations consistently exhibit higher drag than convex-up orientations, and this supports the common observation that valved fossils are typically found convex up in paleoenvironments dominated by traction transport. The presence of geniculation significantly increases drag. Lift is relatively insignificant for all models in most orientations. □ Taphonomy, paleoecology, brachiopods, flow-induced forces, transport.