SOUTH HADLEY, Mass. (AP) _ A fossil found in the Mexican desert could force scientists to shift the search for early animal life from Australia, Russia and Africa to the American continent.

An American geologist found what scientists said could be the world's oldest animal fossil, the outline of a jellyfish-like creature that lived on the sea floor up to 600 million years ago.

Mark McMenamin of Mount Holyoke College made the discovery in March. He was hiking when he came across the fossils, etched in sandstone and shale lying exposed on the desert floor about 100 miles south of Tucson, Ariz.

The fossil shows the 2-inch-long form of a new species of Ediacaran biota, which are among the oldest creatures that can be considered animals, McMenamin said. The fossil clearly shows a central bell, like on a jellyfish, with apparent tubes radiating outward.

``If the age turns out to be anywhere near correct, animal life may have evolved earlier than we thought,'' said Stephen Rowland, a geologist at the University of Nevada at Las Vegas and a leader in the field.

Normally, scientists measure the radioactive decay of volcanic crystals to date a specimen. But the fossil did not have such crystals, and McMenamin estimated its date by comparing it with similar rock formations of known age.

Most other Ediacaran fossils are believed to be no more than 580 million years old, though some from Canada may be as old as 600 million years, scientists said.

However, scientists cautioned that the practice used to date the newfound fossil is not as precise as the radioactive decay method.

Also, there is a debate among scientists over whether Ediacaran should be regarded as animals. Some liken them more to primitive plants, and others put them in a third category entirely.

The oldest discovered fossil of any kind is a 3.5 billion-year-old bacterium.

McMenamin said his finding shows evidence that the creature carried photosynthetic microbes inside, allowing it to draw energy from the sun instead of eating other animals. The theory, while embraced by some scientists, is hard to prove because it focuses on soft tissue that does not survive in fossils.

But McMenamin believes his fossils suggest a brief, idyllic age before the evolution of animal predators, when some animals lived without the protective shell or stinging cells in some of today's animals, such as coral.

Rowland has reviewed the work with four other specialists for the journal Proceedings of the National Academy of Sciences, which is considering it for publication.