Fossils from soft bodied Cnidarians are extremely rare. Because they have no skeleton, the environment needs to be perfect for fossil preservation in order for anything to show up.
There was one fossil found in 2007 that showed a very high resemblance to a modern (Live Science)jellyfish----->.
This proved that these jellyfish have been around for over 540 million years.
However Cnidarians like corals and sea fans, that are made up of some calcium carbonate have much better results as fossils.