MASS AGGREGATIONS OF THREE-DIMENSIONAL RUSOPHYCUS-LIKE MEGABURROWS IN THE CHAGRIN SHALE (FAMENNIAN) OF ASHTABULA COUNTY, OHIO
Outlines of the trace fossils may be rounded, but most are elongately suboval or subrectangular in outline. The tops of the traces typically consist of shallow depressions; the bottoms are typically weakly bilobed, marked with major striations normal to the long axis of the traces. Finer striae are sometimes found on the major striae. One end of the bottom of the fossils is typically rounded, and may have a medial invagination. Cross sections may consist of catenary layers concordant with top and bottom surfaces. Basal layers of some ichnofossils preserved in siltstone may contain bits of shaley rock, the result of mud carried into the silt. In situ measurements made of specimens protruding from the underside of siltstone ledges show some, but not strong, preferred orientation. Most beds contain traces made by generally similar sized arthropods; some beds contain burrows made by different sized organisms. Burrows may cross-cut each other. The ichnofossils resemble Rusophycus but have a shallow, rather than deep, median furrow and are marked by coarse, rather than fine, primary striae.
The Chagrin Shale lacks trilobites, but phyllocarid fossils are common to the west of the megaburrow sites. Horseshoe crab traces and body fossils have been found in coeval rocks of western Pennsylvania. Based on their size, shape, and the robust nature of their striae, it is possible that the Chagrin megaburrows were made by horseshoe crabs. Horseshoe crab traces found in the Upper Devonian of western Pennsylvania provide some evidence for a transition from typical horseshoe crab traces to Rusophycus. Whatever the maker, the megaburrows document repeated mass aggregations occurring in a coarser grained sediment in a shallow marine environment.