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Animals quickly colonized freshwater

Fossilized worm burrows show that marine life rapidly adapted to other ecosystems

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Earth’s early animals moved upstream not long after conquering the seas, newly discovered fossils show.

Rocks near the California-Nevada border preserve traces of tiny worms that squiggled through river mud some 530 million years ago. That’s roughly 80 million years earlier than other freshwater animal fossils, paleontologists report online May 4 in Geology, and not long after the first appearance of diverse animal forms in marine environments.

Changing levels of saltiness can make it tough to evolve from living in the ocean to living in rivers and lakes, says Mary Droser, a paleontologist at the University of California, Riverside. The new work shows that “clearly animals had crossed that physiological barrier very early on,” says Droser, who made the find with Martin Kennedy of the University of Adelaide in Australia.

The scientists stumbled across the fossils in eastern California’s Wood Canyon Formation, parts of which were deposited under a salty sea and other parts under a river. In the freshwater layers the paleontologists spotted lots of squiggly marks — traces of U-shaped burrows in which two wormlike species once lived.

Animals must have worked their way from the sea through brackish water and into freshwater by the time the rocks formed, Droser says. If so, freshwater environments were a fairly hospitable place to live early in animal history — a time well before plants colonized land about 450 million years ago, which some scientists think was a crucial stage in stabilizing river landscapes enough for animals to thrive there.

“The knee-jerk thing, since most of the world is covered by ocean, is to say that most fossils are marine, and the onus is to prove that they’re not,” Droser says. “This will open people’s eyes up.”

Other paleontologists, she says, might now start finding earlier and earlier evidence for this key freshwater step in animal history.

But not all scientists are convinced by the new report. Knowing which rocks were truly deposited in a river, as opposed to the ocean or along the coast, is difficult, says paleoecologist Molly Miller of Vanderbilt University in Nashville. A study published earlier this year in Sedimentology, for instance, argues that the Wood Canyon rocks may have lain quite close to the coast and thus been flooded with both freshwater and saltwater. "The fact that these are sandwiched between rocks deposited in marine environments raises the bar of evidence required," Miller says.

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