Cambrian Madusae Jellyfish Fossils

from Blackberry Hill, Elk Mound Group, Wisconsin

The Cnidarians
"Fiercest Predators of the Cambrian"
Stranded on a Cambrian Shoreline

Cambrian Madusae Jellyfish FossilsNear a sleepy rural burg named Mosinee Wisconsin is a flagstone quarry that outcrops the Mount Simon Wonewoc Sandstone, Elk Mount Group, that may help fill in some pieces in the Cambrian puzzle. Among the fossils and trace fossils found in the Krakowski quarry are numerous circular impressions in layers of rippled sandstone where once was a shallow tropical seashore during the Cambrian. In the February 2002 issue of Geology, Hagadorn, Dott and Damrow publish a paper titled: "Stranded on a Late Cambrian shoreline: Medusae from central Wisconsin". The implications are of some note to paleontology since such huge jellyfish were likely one of the fiercest predators in the early Paleozoic. According to Hagadorn, the jellies from the Mosinee quarry are not just large for the Cambrian, but are the largest jellyfish in the entire fossil record. The quarry contains different layers with densely populated impressions the likely result of a mass-stranding of groups of madusae on a Paleozoic shoreline. Such strandings frequently still occur on Earth, but in the Cambrian there were no land predators. TheMadusae Jellyfish Fossil fossils exhibit features nearly identical to those observed in modern scyphozoan jellyfish strandings. Some event in the absence of shore erosion may have enabled the stranded jellyfish to be buried and thus allow these fossil imprints to be formed.

Scyphomedusae Chrysaora melanasterIt was during the Cambrian that animals with hard shells first appeared, enabling a rich fossil record to begin. Trace fossils such as these soft-bodied jellyfish are exceedingly rare. The Medusae jellyfish fossils have so far been found in seven layers in the quarry, representing some 12 vertical feet of rock and corresponding to a span of time of about one million years. Hagadorn, et. al., state that the quarry's features are "consistent with an intermittently exposed intertidal and shallow-subtidal setting that was probably located in a shallow lagoonal area with limited wind fetch . . . . within a possible sandy barrier island system on the flank of the Wisconsin dome may have further restricted the environment, and severe tropical storms provide a plausible mechanism for Medusoid stranding. Hagadorn also points out: “These jellyfish are not just large for the Cambrian, but are the largest jellyfish in the entire fossil record. What is also of interest is that they were among the largest two types of predators in the Cambrian.” The medusae were stranded atop and capped by a variety of bedforms. 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 jellyfish. Sedimentologic data support the theory that the medusae horizons are event beds deposited during major storm surges, rather than time-averaged accumulations.

Yet another layer of the quarry yields jellyfish that are smaller, but have tenacles in the familiar radial symmetry of Cnidarians, as shown in the picture. Jellyfish were some of the most ferocious preditors of the Cambrian marine environment, and may well have munched on the arthropods that left the arthropod trackways known as Diplichnites and Protichnites that are also abundantly found in the quarry.


Krukowski Quarry Jellyfish Fossils for Sale