Turbulence, displacement, death and worms: A day in the life of a fluvial Carboniferous bivalve

In the Pennsylvanian Rough Rock Flags and Rough Rock of northern England trace fossils attributed to the non-marine bivalve Carbonicola are found. Carbonicola, recorded by Lockeia and associated trace fossils, lived a semi-infaunal lifestyle and thus were influenced by both the sediment in which they were hosted, and the currents which supplied their nutrients and oxygen. A number of palaeocurrent indictors are commonly associated with Lockeia: (1) downstream inclination of vertical burrows; (2) palaeoflow-parallel orientation of long axes; (3) steeper scouring and higher sediment surface on the upstream side; and (4) diffuse lamination downstream of the trace or more widespread downstream erosion. Semi-infaunal, Carbonicola bivalves were partly exposed to the prevailing flow and acted as bed defects; flow separation and acceleration enhanced flow turbulence around the bivalve leading to erosion and the development of a variably developed fan-shaped zone of scour immediately downstream. Disturbance and destabilization of sediment in this way may have affected bivalves immediately downstream, plausibly explaining the relatively regular spacing pattern of individual or clustered Lockeia, exposed on bedding planes and revealed by nearest neighbour analyses. Bivalves that did not survive high-energy flow events were either trapped within the sediment, or transported downstream and deposited in lower energy environments within the otherwise high-energy deposits of the Rough Rock.