VERTEBRATE TAPHONOMY IN DISTRIBUTIVE FLUVIAL SYSTEMS

Approximately 87% of facies across all modern sedimentary basins are deposited as part of Distributive Fluvial Systems (DFS), a class of fan-shaped landforms including alluvial fans, fluvial fans, and megafans. There is evidence that DFS are also common in the rock record. The prevalence of DFS in the terrestrial rock record has implications for the vertebrate fossil record because as DFS deposit they prograde, producing an autochthonously generated secular series of environmental changes at any one location. According to the simplest DFS model, environments will shift from poorly-drained overbank deposits with many small channels lower in section to well-drained overbank deposits with few, larger, amalgamated channels higher in section. This environmental change will affect both the distribution of organisms on the DFS and a range of taphonomic factors (e.g., transport energy, rate of burial, surficial and subsurface degradation processes).

To investigate the taphonomic consequences of the DFS model a quantitative taphonomic model was written to describe the changes in vertebrate preservation associated with the sedimentological transitions across the surface of a DFS. Using initial conditions for intermediate to large DFS (10s-100s of km length), the mean overbank transport distance of vertebrate specimens is anticipated to increase upsection in DFS settings, leading to a shift from preservation of associated specimens near to the point of carcass deposition to broadly dispersed, unassociated fossils. Similarly, the proportion of specimens surviving to burial is projected to decrease upsection and the magnitude of size bias in the assemblage will increase upsection. What limited quantitative taphonomic data are available from known or hypothesised DFS settings show some agreement with the expectations of the quantitative model. If shown to be broadly present, these results suggest that secular changes in preservation should be anticipated in vertebrate sequences, and should be accounted for when developing palaeoecological hypotheses.