TRACE FOSSIL SIZE CATEGORIES AND SOURCE OF FOOD FOR SOME TRACEMAKERS: OBSERVATIONS FROM THE UPPER DEVONIAN OF WEST VIRGINIA

Upper Devonian siliciclastic strata of eastern West Virginia, western Virginia, western Maryland, and central Pennsylvania consist of a series of westward prograding sedimentary units. The Foreknobs Formation is typical of these deposits and is well exposed by recent highway construction in West Virginia. Detailed logging of trace fossils and physical sedimentary structures in the Foreknobs was carried out near Elkins, WV during the summer of 2005. Horizontal and vertical feeding and dwelling traces were observed to occur in two size categories, very small (3-7 mm in maximum dimension) and small (2-5 cm in maximum dimension). In addition, Diplocraterion, Paleophycus, Phycodes, Planolites, and Teichichnus were also present in a third size category, large (10-50 cm in maximum dimension). These classes appear to be non-overlapping - traces of intermediate size are rare or absent. The occurrence of traces segregated into different size categories may be related to the behavior of co-existing juvenile and adult arthropods of the same species. However, why there should be only two or three distinct sizes or ages of individuals is unclear. This phenomenon may also be the result of the activities of unrelated tracemakers of differing sizes creating a suite of traces similar in shape but different in size. The dearth of body fossils (except inarticulate brachiopods) probably precludes a definitive answer.

An additional observation was made at this locality regarding the source of food for some of the tracemaking organisms. In particular, Phycodes were observed in the form of large, hemispherical traces surrounding or immediately adjacent to fragments of coalified or pyritized wood. We suspect that tracemakers were feeding on a "halo" of bacteria associated with the decaying wood rather than on the woody material itself.