THE EFFECTS OF BONE SHAPE AND DENSITY ON FLUVIAL TRANSPORT

Terrestrial vertebrate fossil or archaeological assemblages are frequently interpreted based on the presumed effects of bone shape and density on fluvial transport. The suggestion is that smaller less dense bones are transported farther and faster than larger and denser bones. To test these assumptions, 24 different adult sheep (Ovis aries) bones were molded and cast hundreds of times with three different densities (1.12-1.31, 1.32-1.50, and >1.50 g/cm³). Atomized steel was included in the casting resin so recovery was facilitated with a metal detector. Casts were seeded in Levelock Creek, AK (1272 casts), Big Beef Creek, WA (368 casts), and in three localities in the East Fork Sevier River, UT (1149, 291, and 606 casts). Empirical trials were observed annually for one to four years and the location, orientation, transport distance, and facies associations were recorded for all recovered casts. A total of 3686 casts were placed, and 520 have been recovered yielding a 14.1% recovery rate. While small sample sizes are ubiquitous in the recovered cast assemblages, no correlation exists between shape and transport distance for bone casts with the same density. Denser casts of the same shape moved less than lower density casts however there was substantial overlap in transport distances for casts of variable density with the same shape. These results suggest that shape has less influence on transport than density, and while density has an effect on transport it is complicated by additional variables. Observations suggest that interactions with the bed primarily control transport of consistent-density casts.