A critical evaluation of our understanding of bone transport and deposition in fluvial channels

Abstract

Forensic scientists, archaeologists, and paleontologists are interested in understanding fluvial bone transport respectively to find human remains, determine if human behavioral information persists in skeletal assemblages, or to estimate the temporal and spatial resolution of fossil assemblages. This dissertation reviews what we think we understand about fluvial bone transport, then tests the hypotheses that: 1. Elongate and concave bones adopt preferred orientations relative to flow, 2. There is a relationship between bone shape and relative transportability, and 3. There is a relationship between bone density and relative transportability. Generally two research techniques prevail, 1. Flume observations, and 2. Fluvial seeding trials. Flume data are often poorly reported, have small sample sizes, and the conditions within the flume are usually incompletely reported. Fluvial seeding trial results are characterized by a series of well documented anecdotes, largely due to specimen loss causing small sample sizes. The results from these techniques are contradictory so research to clarify what conclusions are valid is needed. Three techniques were utilized to address these problems, river surveys, river seeding trials, and river seeding trials using bone casts. No correlation between elongate bone orientation (N=157) and flow direction was observed, though concave bones (N=89) oriented ~70% concave down, while flat bones lay flat against the bed. Similarly, there was no association between bone shape and transportability. Denser bones were less transportable than less dense bones however there was substantial overlap in transportability between dense and less dense bone casts. These results suggest our understanding of bone transport is simplistic and incomplete. This is probably because most research has utilized flumes which provide unrealistically uniform conditions, so flume results are a poor analog for the heterogeneous natural environment. Moreover, bones are constantly changing density which is a variable previous authors have more or less assumed was constant. This simplifying assumption is violated so frequently that this assumption has led the scientific community to assume bone transport behaviors that are not frequently observed in natural systems. Ultimately the analytical tools based on this incomplete understanding of bone transport should be discontinued or validated to avoid spurious conclusions when interpreting skeletal assemblages.