Jellyfish may not have changed a lot in hundreds of millions of years. I guess that’s one measure of success, or at least an ability to survive environmental changes. There are in fact plenty of varieties of jellyfish today, but we don’t know that much about their evolutionary history because as soft-bodied animals, they don’t leave much in the way of fossils.
|Middle Cambrian cnidarian jellyfish. Black bar is 5 mm (© 2007 Cartwright et al.; under Creative Commons license)|
In 2007 scientists from the University of Kansas, the University of Utah, the Smithsonian, and the University of Sao Paulo described some Cambrian jellyfish fossils from Utah that are remarkable in their preservation. The authors were able to identify such fragile structures as tentacles and organs, suggesting that modern aspects of jellyfish were developed within a few million years of the Cambrian explosion. For most other phyla, especially the chordates, which include us, evolving modern characteristics was a long process. Here's the paper. (also source of photo, used under Creative Commons license)
The rocks that hold these fossils are about 505 million years old, which puts them in the Middle Cambrian, just before the start of the Late Cambrian.
—Richard I. Gibson