UPPER CAMBRIAN MEDUSAE FROM WISCONSIN

Thousands of medusae are exposed in 9 localities of the Upper Cambrian (Dresbachian-Franconian) Wonewoc Formation in central Wisconsin.  Fossils occur in quarries in a large erosional outlier of sandstones deposited in an intertidal equatorial setting.

Medusae were stranded atop and capped by a variety of bedforms, yet sediments associated with deposition of the medusae reflect a waning flow regime, including mm-cm scale normally graded beds.  Elsewhere, shallow tidal channels and wave  ripple-dominated flat-lying to undulatory bed surfaces are characterized by Climactichnites,  Protichnites, and other traces.  Intensity of horizontally-oriented bioturbation can be high, yet medusae stranding surfaces are unbioturbated.  Microbial sedimentary structures are locally abundant, including elephant skin, sand peloids, domal sand stromatolites, and sand chips.  Such structures may underlie/overlie medusae stranding surfaces, but the role of microbes in soft-tissue preservation and bioturbation is unclear.

The best preserved medusae occur in fine- to very fine-grained sandstone; rarely, minor clay is present as matrix.  Preservation quality is inversely correlated with stranding bed grain size; size and synoptic relief of medusae carcasses is not correlated with grain size or stranding bed thickness.  Polygonal shrinkage cracks may cross-cut, occur within, and occur adjacent to carcass boundaries; syneresis cracks more commonly occur within gut centers.

Fossils are epirelief and hyporelief impressions.  Sediment contained within the gastric cavity and attached to oral arms and lappets is preserved in full relief.  Radial canals, long oral arms, and reduced tentacles associated with umbrella-shaped impressions, coupled with medusae size and lack of ring canal impressions, suggest a similarity to extant scyphomedusae.

Most medusae are subumbrella down based on abundant evidence for syndepositional or postdepositional pumping of their bells and impressions of oral arms and radial canals.  Medusae size ranges (2-71 cm diameter, n=2261) are small on individual stranding surfaces, but differ greatly between the 17 measured stranding surfaces.  Sedimentologic and taphonomic data suggest that medusae horizons are event beds deposited during major storm surges, rather than time-averaged accumulations.