TAPHONOMY AND CLASSIFICATION OF MEDUSAE FROM THE LATE CAMBRIAN OF WISCONSIN AND NEW YORK

In Wisconsin and New York tens of thousands of medusae occur in Late Cambrian (Paibian) medium-grained orthoquartzites of the Elk Mound and Potsdam Groups. Medusae occur in intertidal and sand flat facies which contain abundant microbial sedimentary structures.

Over 90% of medusae-bearing bed surfaces contain individuals that are nearly evenly distributed across bed surfaces and are closely associated with sedimentary structures indicating extremely shallow marine to emergent conditions. One medusae-bearing surface exhibits direct evidence of subaerial exposure and the remaining beds are intercalated between beds bearing polygonal mudcracks, raindrop imprints, and adhesion structures. The shallow to emergent conditions indicated by this facies suggest that medusae may have been deposited as a result of stranding events. External concave rings, suggesting bell pulsation escape behavior, are present in many individuals and suggest that both living and dead individuals were transported or swam into shallower settings and were stranded onshore. A small proportion (<10%; n=2436) of the medusae occur in agglomeritic clusters on beds lacking surficially produced sedimentary structures and are preserved in full relief within the beds; this suggests possible entrainment and burial of medusae by sediment-laden fluids. Pustular and stromatolic textures are associated with at least seven medusae horizons, suggesting that microbial biofilms or mats may have facilitated preservation.

At least two distinct species of scyphomedusae can be recognized. One taxon is characterized by large diameter (20-45 cm), high relief, tri- or quatra-radial gastrovascular structures, trailing oral arms that have ‘beaded' or ‘packeted' segments, and escape rings (≥35%; n=2261). The second taxon is smaller (5-15 cm diameter), has very high relief relative to its size, a more circular bell shape, ‘packeted' oral arms, and less frequent evidence of escape (~5-10%; n=175). Absence of any indication of a chitinous float suggests that medusae were not chondrophores, whereas the presence of oral arms suggests a morphology closer to that of scyphozoans, rather than hydrozoans.