RELATIVE HYDRODYNAMIC DISPERSAL POTENTIALS OF SOFT-SHELLED TURTLE ELEMENTS - IMPLICATIONS FOR INTERPRETING SKELETAL SORTING IN ASSEMBLAGES OF NONMAMMALIAN TERRESTRIAL VERTEBRATES

Current velocities required to transport skeletal elements from a mode rn soft-shelled turtle (i.e., competent velocities) were measured in f lume tests to allow the inference of their relative hydrodynamic dispe rsal potentials. The competent velocities of many turtle elements vary considerably when the elements are placed in different initial orient ations with respect to current direction. In addition, competent veloc ities of some elements are highly variable even in the context of a si ngle orientation. Thus, in contrast to data from previous experiments on mammalian bones, competent velocities of turtle elements correlate poorly with bone density. Bone shape, however appears to exert a stron g influence on the hydrodynamic dispersal of many turtle elements. Tur tle bones vary considerably in their height above the substrate; conse quently, for turtle species of intermediate size, the dispersal of man y flat elements may be delayed by interactions with the layer of reduc ed flow velocity near the stream bed, while the dispersal of taller el ements will not be so hindered. These results were applied to an assem blage of soft-shelled turtles from the Upper Cretaceous (Campanian) of Montana, in order to assess the degree of hydrodynamic sorting at the site. The relative dispersal potentials of many of the turtle bones t ested are very different from those observed in previous flume tests o f homologous elements in large and small mammals. These differences, a t Least in part, probably result because the boundary layer interactio ns of elements were likely much less variable within each of the large and small mammal species examined in earlier studies than they were a mong the intermediate-sized and diversely-shaped turtle elements teste d here. Uncritical application of mammalian hydrodynamic sorting patte rns, thus, may generate misleading interpretations of the relative adv ances of skeletal elements in non-mammalian assemblages.