This deposit provides us a rare opportunity to study such animals

Dr James Hagadorn

The circular impressions left in 500-million-year-old sandstone - several measure up to a metre across - represent some of the largest finds of their kind anywhere in the world.

It is very unusual for jellyfish to be preserved in the fossil record; they have no bony parts and when they are stranded on a beach, they are usually eaten by predators.

These jellyfish must have been covered by sand soon after they came ashore.

"It is very rare to discover a deposit which contains an entire stranding event of jellyfish," said Dr James Hagadorn, a scientist at the California Institute of Technology and co-author of an article reporting the find in February's issue of the journal Geology.

"These jellyfish are not just large for the Cambrian, but are the largest jellyfish in the entire fossil record. What is also of interest is that they were among the largest two types of predators in the Cambrian."

During the Cambrian, Wisconsin is thought to have enjoyed a tropical environment, and was most likely covered by a shallow inland sea.

Skewed view

Dr Hagadorn and colleagues believe that the jellyfish were preserved because of a lack of erosion from seawater and wind, the lack of scavengers, and the lack of any significant sediment disturbance by other organisms burrowing into the sand after it had covered the jellyfish.

Hagadorn believes jellyfish may have been under-appreciated in previous studies of Cambrian ecosystems and that they were probably important predators in Cambrian food chains.

"We use fossils to assess the diversity and ecology of ancient communities," the geologist said. "To date, most of our information about the trophic (food chain) structure of the Cambrian - when multicellular animals burst onto the scene - is based on animals with hard parts or on exceptional deposits which contain soft-bodied organisms."

He added: "When we analyse the trophic structure of the Cambrian - who ate whom, who ate them, and so forth, or when we analyse how abundant each type of organism was in each part of the food chain - we may have been inadvertently omitting a huge amount of information about all of the soft-bodied animals that were swimming around in the water column, munching on other organisms, but which were rarely fossilized.

"This deposit provides us a rare opportunity to study such animals."