Tiny burrows. A dense aggregation of U-shaped fossil burrows in 520-million- to 542-million-year-old rocks (main image; one example in inset) pushes back the appearance of freshwater ecosystems at least 85 million years, a new study suggests

M. J. Kennedy and M. L. Droser, Geology, 39 (June 2011)

Come On In, the Water's Fresh

Newly discovered fossils left by creatures burrowing in the sediments of an ancient riverbed push back the beginning of freshwater ecosystems by at least 85 million years. The find hints that there was little if any delay between the development of freshwater and marine ecosystems, contrary to what many biologists and paleontologists have proposed.

For a variety of reasons, paleontologists have suggested that life arose in marine environments and only later moved into freshwater. The lack of complex communities of organisms on land meant that freshwater ecosystems didn’t have a large source of nutrients, including organic substances, needed to nourish life. Also, organisms typically expend more energy in a freshwater environment to maintain their inner chemical balance, particularly their salty body fluids, than they do in a marine setting. And, says Mary Droser, a paleontologist at the University of California, Riverside, researchers hadn’t previously found much evidence for freshwater creatures in the fossil record until much after their marine counterparts appeared.

But now, Droser and colleague Martin Kennedy report in the June issue of Geology that they have found fossil burrows in ancient sandstones that they propose were deposited as sediments in a river. The pattern of individual grains in the rocks suggest that the material was deposited by a flow that ran only in one direction—a sign, the researchers say, that the ancient sand wasn’t exposed to the back-and-forth sloshing of waves and tides. The fossil burrows exist in seven different layers within eastern California’s Wood Creek formation, which was deposited sometime between 520 million to 542 million years ago, around the same time, or maybe even slightly before, a successful group of marine arthropods known as trilobites were scurrying across ocean floors worldwide.

Most of the fossil burrows are U-shaped, with a depth of less than 5 centimeters, and the exits are spaced between 1 to 1.5 centimeters apart. Diameters of the burrows are typically about 1 millimeter, Droser says. “These were tiny little animals.” The same sorts of burrows are found elsewhere in marine sediments of that era, but there they are typically about 10 times larger—a size disparity similar to that seen between modern-day creatures that burrow in freshwater sediments and their sea-floor-dwelling kin, she adds.

Because the sediments are coarse-grained and the burrows are relatively small, no scratch marks are preserved in the walls, Droser says. Therefore, it’s difficult to say what type of creature made the burrows. But some sort of invertebrate—possibly an arthropod, maybe even a worm—is the likeliest candidate. The large concentration of burrows in some layers suggests that the creatures lived in that ecosystem and weren’t merely migrating through the area.

Previously, the oldest evidence for freshwater ecosystems, including burrows and trackways in ancient pond deposits, comes from rocks about 435 million years old. So the newly described burrows push the origin of these ecosystems back 85 million to 107 million years, the researchers say.

“This is an exciting find,” says Anthony Martin, a paleontologist at Emory University in Atlanta. “There’s no doubt that there are trace fossils in these deposits.” Furthermore, he adds, there’s no doubt that the material in these sandstones were deposited by rivers. Nevertheless, Martin suggests, the burrow-makers could have been marine creatures, especially juveniles, that simply moved upstream in a bottom-hugging mass of saltwater during an extended drought or other period of low flow in the ancient river.

The burrows add to the mounting evidence that land-based ecosystems were evolving at the same time marine life was proliferating, during an era known as the Cambrian Explosion, Droser says.

The new find may cause other paleontologists to look at ancient riverine deposits more closely, Martin says. “A lot of people may not have looked simply because they didn’t expect to find anything.”