Taphonomy and time resolution of bone assemblages in a contemporary fluvial system: The East Fork River, Wyoming

Abstract

Experiments that recorded the dispersal of 142 bones within a meandering, 2030 m-long reach of the East Fork River, Wyoming over a 13-year period provide a basis for interpreting distribution patterns and time averaging in fossiliferous channel deposits. Results show that light and porous bones, (e.g., vertebrae, patellae, and phalanges) were transported farther than heavy bones (e.g., limb bones and mandibles). Dispersal patterns of bones from individual experimental sets representing point sources demonstrate that bones became sorted by size and shape within 1 to 2 years and that sorting patterns varied according to initial channel position. The combined distribution of bones from all the experimental sets, however, was unsorted and generally random, suggesting that unsorted fluvial bone assemblages reflect multiple bone sources and differences in the time at which bones enter the channel. Estimates of time-averaging of potential and observed natural bone assemblages in the East Fork River and the South Platte River range from 101-1O4 years. The upper limit for this estimate is controlled by both the age of fossiliferous floodplain deposits that border the rivers and by the ability of the rivers to rework these floodplain deposits. The lower limit reflects either the scarcity of bones in the floodplain sediments or the inability of the rivers to rework these older bones; in this case channel bone assemblages should represent only remains from deaths in the channel or remains that were transported into the channel from adjoining land surfaces, resulting in short intervals of time-averaging (101-102 years). The East Fork study further suggests that sandstone geometry, paleosol development, and the sedimentary context of fossil occurrences can be used to evaluate time-averaging in ancient fossiliferous channel deposits. Fossil bone assemblages that are present exclusively in ribbon-shaped channel deposits associated with weakly developed paleosols and unfossiliferous floodplain deposits should represent shorter time intervals than similar fossil assemblages associated with sheet sandstones and moderately developed paleosols with abundant fossils.