Marine Animals Moved Upriver Early: Fossil Worm Burrows Suggest Rapid Colonization of Freshwater
Witze, Alexandra, Science News
Earth's early animals moved upstream not long after conquering the seas, newly discovered fossils show.
Rocks in eastern California 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 salinity 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 the 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 squiggly traces of U-shaped burrows in which two wormlike species once lived. …