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STRANDED IN UPSTATE NEW YORK: CAMBRIAN SCYPHOMEDUSAE FROM THE POTSDAM SANDSTONE

¹ Amherst College, Department of Geology, Amherst, Massachusetts, USA 01002 jwhagadorn{at}amherst.edu

The Cambrian portion of the Potsdam Sandstone contains a suite of scyphomedusae impressions in fine-grained to medium-grained quartz arenites that outcrop on the periphery of the Adirondack Mountains, New York. The fossils are similar taphonomically and morphologically to coeval scyphomedusae from the Elk Mound Group of Wisconsin and were likely stranded on a sand flat. Soft-tissue preservation in such sandstones is rare, except in Ediacaran Konservat-Lagerstätten. Although subtidal facies are abundant and continental facies are present in the Potsdam, soft-bodied fossils are found only in emergent coastal facies. These units are characterized by microbial structures including domal sand buildups, sand shadows, and breached ripples and by such horizontal trace fossils as Climactichnites and Protichnites. Domal sand buildups mantle some medusa carcasses and suggest that carcasses were exposed at the sediment-water or sediment-air interface for significant intervals of time prior to burial. It is unknown if microbial binding mediated preservation of these carcasses, but evidence for rapid flow regime changes in the section suggest stranded medusae were resistant to the upper-flow regime deposition that buried them. In many Laurentian Cambrian sandstones, microbial binding is common, and metazoan bioturbation is minimal in intertidal and emergent facies. The Potsdam Sandstone, thus, exemplifies how the Ediacara-style taphonomic window persisted in emergent Cambrian settings. This preservational regime may persist because bioturbating metazoans did not fully colonize tidal flats until the Middle Ordovician, which allowed soft-bodied animal tracks and carcasses to be preserved without scavenging or disturbance.