Large Camborygma isp. in fluvial deposits of the Lower Permian (Asselian) Dunkard Group, southeastern Ohio, U.S.A.

Abstract

The lower Washington Formation of the Lower Permian (Asselian) Dunkard Group in southeastern Ohio contains large burrows exposed in cross section in a laterally discontinuous sandstone bed. This study examined the morphology of the burrows and their associated lithofacies to interpret the environmental conditions under which they were produced as well as the behavior and identity of the trace maker. Mudstone facies below and above the burrow-bearing sandstone consist of reddish-brown to variegated paleosols containing rhizohaloes, argillans, and large-scale slickensides, as well as thinly laminated, organic-rich shales containing plant fossils. The sandstone facies is 94–147cm thick, thinly bedded to massive, and fine- to medium-grained. The lithofacies are interpreted as deposits of crevasse splays, abandoned channels, and proximal to distal floodplains. Sixty burrows consist of vertical, subvertical, J-shaped, Y-shaped, and complex networks of branching shafts and tunnels. The burrows range from 5 to 180cm in length and 0.8–3.6cm in width. The main shaft's angles are typically ~90°, although are rarely 30–70°. Branching angles of subvertical and Y-shaped burrows vary from 0 to 90°. Many burrows extend to the top of the sandstone bed, but others turn into or out of the bed. The morphology of the burrows is most similar to Camborygma, known from continental deposits of the Permian to recent. While Camborygma has been previously reported from the Permian, this is the first occurrence in the Late Paleozoic Appalachian Basin extending its geographic range. The Dunkard burrows are similar to those of modern freshwater decapods, particularly crayfish. Decapod body fossils are not known from the Dunkard Group, yet these burrows are highly suggestive of their presence. Therefore, these fossil burrows contribute to the paleoecological interpretation of the Dunkard Group, helping to fill in gaps of the terrestrial fauna not otherwise preserved in the fossil record. Decapod burrow morphologies are highly variable depending on burrow function and environment. The Dunkard burrows are similar to secondary dwellings, with few open water attachments, multiple branches, and shafts likely extending to the water table.