An American geologist has 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 Mt. Holyoke College made the discovery in March in the Mexican desert, about 100 miles south of Tucson. He was hiking when he came across the fossils, etched in sandstone and shale lying exposed on the desert floor.
The fossil shows the two-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.
Stephen Rowland, a geologist at the University of Nevada at Las Vegas and a leader in the field, said: "If the age turns out to be anywhere near correct, animal life may have evolved earlier than we thought."
He said the findings may help shift the search for early animal life from Australia, Russia and Africa to the Americas.
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.
The oldest discovered fossil of any kind is a 3.5 billion-year-old bacterium.