This is a fossil impression of a jellyfish. Considered to be one of the first fossilized evidence of multicellular organisms and still going strong today. Just last year they discovered a jellyfish fossil that was 500 mya old, pushing back the origins of jellyfish by over 200 mya (link here). The impressions were incredibly similar to jellies found today, implying that the origins of jellies may be earlier.In my opinion, cnidarians are interesting in their simplicity. They only posses two tissue layers, lacking the mesoderm layer which provides the cellular base for many internal organs that other organisms posses. There is some interesting research going on involving cnidarians trying to determine how mesoderm could have arisen from endoderm or ectoderm (I'm in the middle of reading a paper on it now, and promise to write it up later).