249-3 The Widespread Distribution of Cambrian Medusae: Scyphomedusa Strandings in the Potsdam Group of Southwestern Quebec

Tuesday, 7 October 2008
George R. Brown Convention Center, Exhibit Hall E
Mario A. Lacelle, Laval, QC, Canada, James W. Hagadorn, Department of Geology, Amherst College, Amherst, MA and Pierre Groulx, Valleyfield, QC, Canada
Here we report the third konservat-lagerstätte from the Late Cambrian epicratonic arenites of Laurentia – in the Cairnside and Covey Hill Members of the Potsdam Group of Quebec. The most abundant soft-bodied fossils are scyphozoan medusae; unmineralized arthropods are extremely rare. Medusae occur by the hundreds in planar bedded fine- to medium-grained quartz arenites. Most are simple convex mounds, or convex mounds surrounded by a premortem pulsation moat. Several specimens exhibit impressions of the coronal ring or frilly ornamentation of the bell margin, or have triradial or quadriradial convex structures in their centers – these are interpreted as partially folded or undistorted gastrovascular cavities, and are most similar to semaeostomeae medusae symmetry. One specimen is mantled by a mm-thick veneer of sand characterized by dozens of radial folds – this may mantle radial canals. Many carcasses exhibit evidence of transport and reworking, occurring as agglomerated clusters in which carcasses are rarely discoidal; where medusae are less abundant, they tend to be preserved subumbrella down and exhibit discoidal or ovoid symmetry.

Fossils are similar to the roughly coeval scyphomedusae mass-stranding deposits in the Potsdam Group of New York (Keeseville Member), and in the Elk Mound Group (Mt. Simon and Wonewoc Sandstones) of Wisconsin. Together with other recurrent paleocommunity associations in these units (e.g., Climactichnites-Protichnites-Arenicolites ichnocoenosis), the Quebec occurrences demonstrate that similar paleoecologic and taphonomic conditions persisted across vast continental coastlines. The high numbers of individuals and their large scale geographic distribution also suggest that medusae are not as rare as previously thought. The low number of Phanerozoic medusae is probably attributed to taphonomic restriction rather then small population size or ecological factors. These medusae, together with soft-bodied arthropods from the deposit, may be the oldest soft-bodied animals described from Quebec.