Jellyfish jackpot found on fossil beach

Some of the fossils are over 50 cm in diameter, making the jellyfish among the biggest predators of their time (Photo: James Hagadorn)

Palaeontologists have hit a jellyfish jackpot in a Wisconsin quarry, giving scientists an unusual glimpse of ancient sea life.

The gelatinous organisms seldom appear in the fossil record because they have no bones, meaning their carcasses decay easily. They also frequently fall prey to scavenging birds. But a fortuitous set of circumstances conspired to preserve the circular impressions of thousands of jellyfish on an ancient beach.

The sandstone hosting the ghostly evidence was deposited almost 500 million years ago at the edge of a shallow, tropical sea. The jellyfish may have been washed up by a storm and left there to die when the sea receded, says paleontologist James Hagadorn of the California Institute of Technology, who led the team that found them. Because there were not any birds back then, the carcasses remained stranded until they were buried by subsequent storms.

	The telltale rings form when jellyfish try to escape (Photo: James Hagadorn)

"It's very unusual to get beach surfaces preserved in this way," says Bruce Runnegar from the University of California, Los Angeles. "It provides a window to things we couldn't otherwise see."

Only one other collection of around a hundred stranded jellyfish fossils has been reported, along with a few isolated fossils. Many reports turn out to be false alarms but Hagadorn is convinced the Wisconsin fossils are genuine.

The impressions bear the telltale rings marooned jellyfish make when trying to escape by pumping their umbrella-like bodies. "These structures are dead ringers for the type of structures modern jellyfish make when they are stranded," says Hagadorn.

The Wisconsin fossils are the largest ever found, some measuring more than 50 centimetres in diameter, placing the jellyfish among the biggest predators of their time.

Journal reference: Geology (vol 30, p 147)

Betsy Mason, San Francisco