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RARE FOSSIL FIND. Top view of a 35-centimeter jellyfish stranded on a tropical beach 495 million years ago.Hagadorn

A sandstone quarry that normally supplies flagstone for hearths and custom countertops recently served up a rare scientific find nearly half a billion years in the making: fossils of an armada of jellyfish that stud the site’s stone slabs.

At least seven thin layers in the quarry near Mosinee, Wis., contain impressions left by thousands of jellyfish stranded on what was a tropical beach about 495 million years ago. Some specimens measure more than 50 centimeters across, making them the largest jellyfish in the fossil record, says James W. Hagadorn, a paleontologist at the California Institute of Technology in Pasadena.

“These things were bigger than a dinner plate!” says George D. Stanley Jr., a paleontologist at the University of Montana in Missoula. “They’re a new example of the 99.9 percent of the fossil species we’ve yet to discover.”

These jellyfish could have been one of the top predators of their age, says Hagadorn. Although modern jellyfish can reach the size of the Wisconsin fossils, few previously described jellyfish fossils exceed 10 cm in diameter. Hagadorn and his colleagues describe the fossils in the February Geology.

“This is a spectacular find,” says Ronald K. Pickerill, a paleontologist at the University of New Brunswick in Fredericton, who previously described fossils of the only other known mass stranding of ancient jellyfish. Like their modern counterparts, the early species probably fed on microorganisms or small, soft-bodied creatures in the water, says Pickerill.

At the Wisconsin site, the sizes of sand grains and ripples preserved in the sediments suggest that the jellyfish were stranded in a shallow lagoon. In those ancient times, land that’s now in central Wisconsin was beachfront property about 10° south of the equator.

In some of the fossils, a mass of sand grains provides a hint of the animal’s stomach, which became jammed with the material as the stranded creatures tried to pump their way to freedom.

Fossils of organisms such as jellyfish are rare because soft tissues typically don’t last long after death. Hagadorn conjectures that the Wisconsin jellyfish impressions probably remained intact because at the time, there weren’t many scavengers or burrowing animals. Modern jellyfish stranded on a beach usually are quickly set upon by birds seeking an easy snack, he notes.

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