Thursday, August 2, 2012

To Get Fossil Guts, Extreme Chemistry Needed

Eyes, guts and gills rarely show up in the rock record. Bacteria are so ardent and ubiquitous that they eat away soft tissue long before sediment can turn to stone. Typically only shells, bones and teeth endure. And yet paleontologists have discovered troves of exquisite fossils of entirely soft-bodied animals, such as the four-eyed arthropod Leanchoilia (above), that lived half a billion years ago. How can that be? It turns out that the chemical makeup of the ocean half a billion years ago was very different than it is today. This period of extreme ocean chemistry stymied hungry bacteria and hindered decay -- granting humanity insight into a stage of our early ancestry that otherwise would have been lost to us forever. Take Leanchoilia. It is just one of dozens of curious creatures, without skeletons or other hard parts, that represent the immediate aftermath of the Cambrian Explosion, the rapid flowering of complex life from our single-celled ancestors. NEWS: Ancient Animal Explosion Gets Bigger With New Finds For more than a century since Leanchoilia and other soft-bodied creatures were first discovered in the famous Burgess Shale of British Columbia, scientists have wondered what made this peculiar preservation possible. The problem has been identifying conditions unique to that time period, explains Tim Lyons, a biogeochemist at the University of California, Riverside. Now, a team led by paleontologist Robert Gaines of Pomona College in Claremont, Calif., has identified a striking global pattern of seawater chemistry that could finally explain how Leanchoilia and its contemporaries eluded decay. Their new interpretation, published in Proceedings of the National Academies of Science, entails three special conditions that came together to keep hungry bacteria at bay: a dearth of oxygen, not much sulfate and a surplus of calcium carbonate. Based on this latest evidence, here is how Leanchoilia's remarkable preservation may have played out, step-by-step: 1. DEARTH OF OXYGEN The first special condition working in Leanchoilia's favor was a dearth of oxygen in and near the seabed, where a storm or underwater avalanche quickly buried the creature's lifeless body in soft, gooey mud. So, oxygen-loving bacteria -- the fervent decomposers that dominate the entire ocean and seabed today -- weren’t much of a threat. (Gaines and his colleagues knew this from previous studies.) 2. NOT MUCH SULFATE A second plus for Leanchoilia's preservation was an apparent scarcity of sulfate in the overlying seawater. This special chemistry suggests a second line of potential bacterial attackers were also starved of energy. Evidence for low sulfate turned up when Gaines’ team analyzed the sulfur chemistry of Burgess Shale-type sediments from six locations around the world. That discovery is critical, because plenty of bacteria would have been perfectly happy to use sulfate instead of oxygen to fuel their dirty work, Lyons explains. As he noted in a commentary on the new study, sulfate is a common constituent of modern seawater, and in many marine settings, sulfate-loving bacteria wreak as much rot as the oxygen-dependent varieties do. In Leanchoilia's days, though, it seems both types were struggling.
Still, even a slim supply of sulfate diffusing into the sediment could have activated enough decay to consume all traces of its body long before its muddy shroud turned to rock, Lyons says. That suggests a third line of chemical defense must have hindered hungry microbes. ANALYSIS: Freaky Fossils: Don't Smell These Tulips! 3. SURPLUS OF CALCIUM CARBONATE Soon after Leanchoilia’s haphazard burial, a thin layer of calcium carbonate formed on the seafloor, sealing the entombed creature -- and any resident bacteria -- away from the seawater above. In other words, a cement barrier cut off the near-starving, sulfate-dependent microbes from their only meager energy supply. Earlier studies had indicated that seawater packed a much higher dose of calcium carbonate during the Cambrian than it does today. But Gaines’ team was first to locate direct evidence of actual carbonate layers within fossil-laden sediments. They found the telltale layers in two new drill cores they recovered from the Chengjiang deposit, a stack of Burgess Shale-type sediments in China’s Yunnan province. To acquire those enlightening cores, Gaines lived for five weeks in an abandoned chicken coop as he oversaw the drilling operation in Yunnan. It may not have been the ideal campsite, but Gaines says "it was totally worth it" for the chance to discover that the world’s most famous fossils are the product of a triple-decker dose of extreme ocean chemistry.

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