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

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Description: 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 and are confirmed by this study: (a) downstream inclination of vertical burrows; (b) palaeoflow-parallel orientation of long axes. Additional palaeocurrent indicators include: (c) steeper scouring and higher sediment surface on the upstream side; (d) diffuse lamination downstream of the trace, or, more widespread downstream erosion. These semi-infaunal bivalves were partly exposed to the prevailing flow and acted as bed defects, disturbing flow over an otherwise relatively smooth surface; 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 destabilisation of sediment in this way may affect bivalves immediately downstream, plausibly explaining the relatively regular spacing pattern of individual Lockeia, or clusters of 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. These are often associated with Planolites and Cochlichnus, trace fossils of scavenging worms which radiate around the imprints of dead bivalves. This assemblage of trace fossil s indicates that areas suitable for bivalve colonisation occurred in upstream areas.

License: Academic Free License (AFL) 3.0

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