Literature

Although the number of reports of Cambrian jellyfish has greatly increased in the past few years, "there have been no previous reports of fossils possessing preserved characters diagnostic of particular medusozoan clades". New fossils from the Middle Cambrian of Utah "have very well preserved soft tissue, which the authors interpret as evidence that representatives of modern jellyfish existed by the middle Cambrian period."
How have they concluded they are "modern"? The fossils are entombed in fine-grained sediment so that fine details have been preserved. "Given the available character information, they also may comprise representatives of three separate classes of modern medusozoans: Cubozoa; Hydrozoa; and Scyphozoa. This suggests that an important aspect of modern marine pelagic ecosystems was in place shortly after the Cambrian radiation."
The authors also comment on biological complexity:

"the living cubozoan Tripedalia cystophora has sophisticated reproductive behavior that includes mate recognition and courtship, involving the indirect transfer of sperm through spermatophores. Cubozoans also have complex eyes and nervous systems. The existence of our newly described fossil material may suggest that these complex traits could have evolved within the Cnidaria by the Middle Cambrian."

In a press release the implications for rapid species diversification were described as follows:

"Lieberman said the jellyfish the group describes, found in Utah, offer insights into the puzzle of rapid species diversification and development that occurred during the Cambrian radiation, a time when most animal groups appear in the fossil record, beginning roughly 540 million years ago. [. . .]
With the discovery of the four different types of jellyfish in the Cambrian, however, the researchers said that there is 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."

The most interesting aspect is the ability to identify modern orders and families. Here is another case of sudden appearance of complex life forms followed by stasis. This is evidence against the gradualist emphasis of Darwinism and it adds weight to the question asked by Steve Meyer ("The Origin of Biological Information and the Higher Taxonomic Categories", Proceedings of the Biological Society of Washington, 117(2004): 213-239.): "Can neo-Darwinism explain the discontinuous increase in CSI that appears in the Cambrian explosion--either in the form of new genetic information or in the form of hierarchically organized systems of parts?"

Exceptionally Preserved Jellyfishes from the Middle Cambrian
Cartwright P, Halgedahl SL, Hendricks JR, Jarrard RD, Marques AC, Collins, AG, Lieberman BS.
PLoS ONE, 2007, 2(10): e1121. doi:10.1371/journal.pone.0001121

Abstract: Cnidarians represent an early diverging animal group and thus insight into their origin and diversification is key to understanding metazoan evolution. Further, cnidarian jellyfish comprise an important component of modern marine planktonic ecosystems. Here we report on exceptionally preserved cnidarian jellyfish fossils from the Middle Cambrian (~505 million years old) Marjum Formation of Utah. These are the first described Cambrian jellyfish fossils to display exquisite preservation of soft part anatomy including detailed features of structures interpreted as trailing tentacles and subumbrellar and exumbrellar surfaces. If the interpretation of these preserved characters is correct, their presence is diagnostic of modern jellyfish taxa. These new discoveries may provide insight into the scope of cnidarian diversity shortly after the Cambrian radiation, and would reinforce the notion that important taxonomic components of the modern planktonic realm were in place by the Cambrian period.

See also:

Fossil record reveals elusive jellyfish more than 500 million years old, EurekAlert, 30 October 2007.

The earliest fossil evidence of comb jellies, ARN literature blog.