If you were suddenly transported to the western Utah of half a billion years ago, treading water near the ocean's shore, you'd be stunned by the alien life-forms surrounding you.

Trilobites scuttle across the muddy bottom. A monster called Anamolacaris glides past, its mouth filled with sharp, grinding teeth and its pair of long, curving claws bristling with spines.

You might be startled by a small creature called Opabinia, an animal with paddles along both sides, a cluster of five eyes and a trunk with a claw on the end.

But scientists now know that among these odd animals were some the time traveler would immediately recognize: jellyfish.

Today jellyfish are known to have flourished in the middle Cambrian seas because of fossil discoveries in western Utah by Richard D. Jarrard, professor, and Susan Halgedahl, associate professor, at the University of Utah. Husband and wife, they undertook the field work of cracking open rocks without outside funding.

On Tuesday, the National Science Foundation announced their discovery of the world's most ancient fossils that can be definitively shown to be of jellyfish.

A scientific report detailing the find, "Exceptionally Preserved Jellyfishes from the Middle Cambrian," was published Wednesday by the journal "PLoS ONE."

"The fossils that they're finding are absolutely wonderful," said Francis H. Brown, dean of the U.'s College of Mines and Earth Sciences and distinguished professor of geology and geophysics. "The detail is exquisite. It really is. I'm really pleased with it."

The find extends the jellyfish line by 200 million years, back to 505 million years ago, during the middle Cambrian era.

Commented Patrick Herendeen, a National Science Foundation program director, "This study clearly shows what paleontologists have long suspected — that jellyfish have a history that's much older than the known fossil record."

He noted, according to a NSF press release, "Adding some 200 million years to the age of jellyfish is quite a jump. What's even more surprising is the apparent diversity of jellyfish forms present at that time."

Bruce S. Lieberman, geology professor at the University of Kansas, Lawrence, one of the paper's authors, told the Deseret Morning News that the team that examined the fossils included both paleontologists and biologists who are specialists in cnidarian life, animals like jellyfish and corals.

The biologists were saying, "Yes, you've got this material and it looks so similar to modern things in the cnidarian family," he said.