Tubes of modern polychaete Diopatra cuprea as current velocity indicators and as analogs for Skolithos-Monocraterion

Tidal channel hydrodynamics exert a pronounced influence on the distribution and geometry of dwelling burrows of the modern polychaete Diopatra cuprea and the sediments in which they occur. These burrows share strong distributional similarities with the trace fossils Skolithos and Monocraterion, a relationship which permits more accurate and detailed interpretations of ancient tidal deposits.

Diopatra cuprea builds lined, vertical tubes in a wide range of restricted and open-marine environments. The positions and orientations of these tubes on estuarine point bars in South Carolina reflect several aspects of back-barrier tidal processes and their resultant deposits. Tubes are concentrated on the upstream ends of point bars, which are exposed primarily to ebb currents. On the intertidal bar crest, where population density is highest, tubes are arranged in rows normal to flow. Tube apertures are oriented at 45 degrees to the row axis and flow direction. On the ebb-dominated bar apex, strongly ebb-oriented scour pits form around the detritus-reinforced tube caps.

Several characteristics of D. cuprea tubes resemble features of the trace fossils Monocraterion and Skolithos. Like Skolithos, tubes are subcylindrical, vertical, straight, and unbranched. The upper portions of high intertidal tubes are surrounded by Monocraterion-Yike funnels; these occur in thin-bedded muddy sands of the bar crest. Tubes without funnel-tops occur in clean, well-sorted sands of the bar flank. This stratigraphic position is analogous to reported Cambrian Skolithos - Monocraterion assemblages.