Freshwater animals have been found at 435 million years ago, and were assumed to come later than saltwater ones. But “Come On In, the Water’s Fresh”, cries Sid Perkins at Science (17 May 2011):
Tiny burrows. A dense aggregation of U-shaped fossil burrows in 520-million- to 542-million-year-old rocks pushes back the appearance of freshwater ecosystems at least 85 million years, a new study suggests.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.
The theory had been that life started in the oceans and only later moved to fresh water because the latter lacked nutrients from organic substances, needed for life.
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.
The freshwater creatures are assumed to be worms or arthropods that lived in the area (due to the heavy concentration of burrows).
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.”
Why didn’t they? When does a slew of wrong predictions endanger a theory?
File under: ET (Earlier than Thought)