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The oldest known fossils of jellyfish have been found in rocks in Utah that are more than 500 million years old, a new study reports.

The fossils are an unusual discovery because soft-bodied creatures, such as jellyfish, rarely survive in the fossil record, unlike animals with hard shells or bones.

"The fossil record is biased against soft-bodied life forms such as jellyfish, because they leave little behind when they die," said study member Bruce Lieberman of the University of Kansas.

These jellyfish left their lasting imprint because they were deposited in fine sediment, rather than coarse sand. The film that the jellyfish left behind shows a clear picture, or "fossil snapshot," of the animals.