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A Beach in Ancient Wisconsin

April 2, 2018


In a flagstone quarry near Mosinee, Wisconsin are numerous rare jellyfish impressions some as large as three feet across set in layers of rippled sandstone where there once was a tropical seashore. These huge Scyphozoan medusae jellyfish, the largest such fossils ever found, were one of the fiercest predators of their time. The quarry contains different layers with densely populated impressions - the likely result of a mass-stranding of these jellyfish by a series of tropical storms. Such strandings still occur on Earth, but 500 million years ago during the late Cambrian period there were no land predators to eat their remains. 
The medusae jellyfish fossils have so far been found in seven layers in the quarry - 12 vertical feet of rock, a time span of about one million years. I love imagining ancient landscapes, and Wisconsin's was extraordinary. It's hard to drive by the Mosinee area in Marathon County without thinking about the great island dotted shallow seas that once dominated our landscape. To stand upon these rocks today is to stand upon a perfectly preserved ancient beach.