Exceptionally Preserved Jellyfishes from the Middle Cambrian. 2007. Cartwright. P. et al. PLoS ONE 2(10): e1121.
Cambrian fossil jellyfish shows similarity to the modern jellyfish, Cunina (right). Fossil photo by B. Lieberman. Cunina photo by K. Raskoff, copyright
From the press release:
Researchers describe four types of 500 million years old cnidarian fossils from Utah that preserve traits that allow them to be related to modern orders and families of jellyfish. The specimens are about 200 million years older than the oldest previously discovered jellyfish fossils.
The jellyfish left behind a film in fine sediment that resembles a picture of the animal, including a distinct bell-shape, tentacles, muscle scars and possibly even the gonads.
With the discovery of the four different types of jellyfish in the Cambrian there is now enough detail to assert that the types can be related to the modern orders and families of jellyfish. The specimens show the same complexity. That means that either the complexity of modern jellyfish developed rapidly roughly 500 million years ago, or that the group is even older and existed long before then.