Sandstones, shales, and coal of the Fentress Formation

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(Lower Pennsylvanian) of north-central Tennessee were deposited in a complex of deltaic and nearshore environments. A geographically restricted unit near the top of the Fentress consists of laminated, fine-grained sandstones with thin interbedded mudstones. It grades laterally into cleaner sandstones lacking mudstone interbeds. Exceptionally well-preserved trace fossils occur on large exposed, bedding-plane surfaces. Commonly, individual traces can be followed for considerable distances allowing direct comparison of behavioral patterns between the fossil organisms and modern counterparts.

Polinices duplicatus produces traces of different morphology in intertidal environments at Barnstable Harbor as a response to both depth of movement beneath the substrate-water interface and sediment grain size. In tidal channels, V-shaped trails result from snails moving several centimeters below the sediment surface. On high tidal flats, the trail is wider and shallower as the snail moves nearer the surface. On compacted sands, the trace has transverse markings resulting from surface probing by the snail's foot. Experimental results indicate that P. duplicatus produces more V-shaped and deeper traces in coarse sands than in muddy sands.

Traces in the Fentress Formation also exhibit differing morphology attributable to depth of movement and sediment grain size. Single traces from clay-rich parts of the Fentress sandstones change from V-shaped to bilobed and finally to longitudinal rows of tiny knobs produced, apparently, by movement at decreasing depth below the sediment surface. Traces produced in coarser sands are flat-bottomed and lack transverse markings.

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