Stranded in Wisconsin: The ghosts of jellyfish past. Credit: J.W. Hagdorn

A flotilla of giant jellyfish marooned on a beach 500 million years ago has been unearthed in what is now central Wisconsin. This largest-ever find of the biggest-ever fossil jellyfish provides insights into life on Earth before animals came to land.

"It's rare to find a single jellyfish fossil; to find a lot of them is almost unheard of," says James Hagadorn of the California Institute of Technology in Pasadena, who found the thousands of "hulking huge" jellyfish1.

Gelatinous with no bones or scales, jellyfish fossilize poorly. Waves and sand destroy their bodies before they can be covered in sediment - essential for the slow process of fossilization.

Yet on the shores of an ancient sea in what is now North America, schools of creatures up to 70 cm long were somehow stranded and buried. Hagadorn suspects that they were trapped in a shallow tidal lagoon. Bacteria in the sediments would have consumed the gelatinous flesh, replacing it with minerals.

"It's a spectacular find," says palaeontologist Ronald Pickerill of the University of New Brunswick in Canada. More so, he adds, because the hapless jellies are found in several different layers of fossilized beach. "It's not just a one-off event, it happened at least six times."

There are fossilized shorelines from the same period worldwide. Hagadorn suspects that hordes of other fossilized jellies await discovery.

They dug their own graves

They must have been buried extremely quickly Ronald Pickerill, University of New Brunswick

From the size and shape of ripples on the sand of the preserved beach, it looks as if the ill-fated jellies were trapped in stormy conditions.

If modern jellyfish behaviour is anything to go by, Hagadorn suspects the creatures inadvertently preserved themselves by digging into the sand trying to escape.

Pickerill agrees: "They must have been buried extremely quickly." As the water drained away the jellies were left covered in sand.

Today stranded jellies would be eaten by birds and beach-dwelling crustaceans before they could fossilize. But during the late Cambrian period, with animals yet to colonize land, there were no terrestrial scavengers.

Large and numerous

The saucer-shaped fossils are more than a palaeontological curiosity. Their very existence provides a "unique window into geologic time", says Hagadorn.

Like many modern-day jellies the fossilised creatures were probably carnivores. Until now there was evidence of only one other similarly sized meat-eater at the time - a meter-long arthropod. And the numbers imply the jellyfish were quite common.

Large and plentiful, "they were really important in the ecosystem", Hagadorn speculates - probably towards the top of the late Cambrian food chain.