Rare Jellyfish Fossils Unearthed in Wisconsin

More than a half-billion years ago, thousands of jellyfish were washed up in a small lagoon, stranded by a freak tide or storm and buried by sand just hours later.

Fossilized impressions of those jellyfish, some up to 3 feet in diameter, have now been discovered in a Wisconsin quarry, in what scientists say is one of the largest finds of its kind in the world.

“Preservation of a soft-bodied organism is incredibly rare, but a whole deposit of them is like finding your own vein of gold,” said James Hagadorn, a scientist at the Cal Tech in Pasadena and co-author of an article reporting the find in February’s issue of the journal Geology.

The jellyfish, which have no durable body parts, were fossilized during the ancient Cambrian period, when the world’s oceans exploded with a diversity of life. The creatures were apparently buried within hours after being stranded in a shallow lagoon about 510 million years ago.

Fossil dealer Dan Damrow, an article co-author, discovered the jellyfish about four years ago in a quarry in Mosinee, Wis., about 200 miles northwest of Milwaukee. In the quarry, beds of sandstone lie stacked horizontally in neat layers--perfect for flagstone and other commercial uses.

“These could have ended up as someone’s bathroom floor or in their side garden,” Hagadorn said.

Hagadorn said they found fossilized jellyfish in seven layers in the quarry, encased in about 12 vertical feet of rock representing a span of time of up to 1 million years.

The layers of rock also record the delicate ripples that striped the ocean bottom in what were presumably shallow coastal waters.

“It gives you a kind of aura of standing in this instant in time,” Damrow said. “You’re standing right on that beach just as it formed.”

Circular impressions mark where each jellyfish was washed ashore, probably during a storm-enhanced high tide, Hagadorn said. Each fossil typically includes a concave, circular shape that records the tiny moat excavated by the pumping action of the bell-shaped jellyfish as it attempted to swim to deeper water.

Surrounding that ring, a rim of higher rock represents the sand that washed against the dead or dying jellyfish in subsequent tides. Tiny piles in the center is likely sand ingested by the creature as it struggled, Hagadorn said.

The fossilized jellyfish appear similar in size and characteristics to their modern brethren, but the specific species cannot be pinpointed.

Beached jellyfish now fall prey to everything from birds to curious children. In the Cambrian period, however, there were few scavengers to disturb the creatures once they were grounded and buried, Hagadorn said. That, and the speed with which they were buried, accounted for their survival in the fossil record, he said.