TAPHONOMIC AND PALEOECOLOGICAL IMPLICATIONS OF FLOW-INDUCED FORCES ONCONCAVO-CONVEX ARTICULATE BRACHIOPODS - AN EXPERIMENTAL APPROACH

The orientation of benthic marine organisms may be disturbed by flow-i nduced forces (i.e. drag and lift) caused by wave and current activity . Drag and lift are partly a function of organism size shape. Conseque ntly, morphology may affect stability (defined as resistance to reorie ntation, flipping, or entrainment) both during the life of an organism and after its death. An understanding of drag-and-lift effects is the refore essential to the interpretation of paleoecology and biostratino mic processes. An experimental method for quantifying the relative eff ects of flow-induced forces is described. These forces are measured du ring flume experiments using transducers and plaster replicas of fossi ls. As an illustration of the method's potential for taphonomic resear ch, results from experiments investigating the effects of concavo-conv ex morphologies of articulate brachiopods are presented. Concave-up an d convex-up orientations are commonly used to infer paleohydraulic con ditions. Two geniculate brachiopods (Rafinesquina alternata and Leptae na richmondensis) and three flattened forms (a second morphotype of Ra finesquina alternata, Strophodonta demissa, and Tropidoleptus carinatu s) were tested in convex-up and concave-up postures and in three azimu thal orientations (hingeline oriented upstream, hingeline downstream, and hingeline parallel to now). Concave-up orientations consistently e xhibit higher drag than convex-up orientations, and this supports the common observation that valved fossils are typically found convex up i n paleoenvironments dominated by traction transport. The presence of g eniculation significantly increases drag. Lift is relatively insignifi cant for all models in most orientations.