Deposition Bias in the Late Miocene McKay Reservoir

Presenter: Kelsey Stilson, Geology

Fossils found in association have the potential to tell us about the ecology of a specific time and place in geologic history, suggest 9something about the changing prehistoric world, and give clues about the effects of present-day anthropogenic climate and habitat change. However, this is no simple process of counting up the number of bones found. A community is rarely, if ever, fossilized in the same proportions as it lived. This study suggests ways to measure the preservation bias in a system and extrapolate the composition of original fauna. My site is McKay Reservoir in north-central Oregon, where fossils were deposited by fluvial transport in the late Miocene (5.5 million years ago). Current approaches to depositional bias in such systems use the shape and evidence of postmortem damage to the bones in an assemblage to estimate the degree of sorting due to river flow. Voorhies diagrams, which use bone type (e.g. radius or metapodial) to determine the degree of sorting, indicate the McKay bones were transported at high velocity and were deposited relatively close to where they entered the water. Another method suggests that the bones were not exposed to air for long (little to no weathering) and rolled a medium distance along the stream bed (moderate abrasion). These results show that the bones came from nearby, perhaps as little as a mile from the site of desiccation to the site of final burial, and would therefore be relatively accurate indicators of ancient local species distribution.