Few animal fossils have been found in the mine—chemicals in the ancient swamp's water may have dissolved shells and bones—but other sites from more than 300 million years ago, a period known as the Carboniferous, have yielded fossils of millipedes, spiders, cockroaches and amphibians. Monster dragonflies with 2.5-foot wingspans ruled the skies. (It would be another 70 million years before the first dinosaurs.)
And then the earthquake struck, and this swampy rain forest was gone.
One of the reasons the site is so valuable to scientists is that it opens a window on the natural world just prior to a period of great, and puzzling, change. For several hundred thousand years after this rain forest was entombed, tree ferns, lycopsids and other plants competed for dominance—"a kind of vegetational chaos," says DiMichele. For some unknown reason, the tree ferns prevailed, he says, and eventually took over the world's tropical wetland forests.Two-thirds of the species found in Riola-Vermilion Grove would vanish. The mighty lycopsids virtually disappeared.
Researchers offer several possible reasons for the great makeover in plant communities around 306 million years ago: precipitous changes in global temperatures; drying in the tropics; or, perhaps, tectonic upheaval that eroded even older coal deposits, exposing carbon that then turned into carbon dioxide. Whatever the reason, earth's atmosphere suddenly acquired a lot more carbon dioxide. Determining the relationship between this ancient atmospheric change and the changes in vegetation could offer clues about how today's ecosystem will react to carbon dioxide increases caused by the burning of fossil fuels.
The Riola-Vermilion Grove team, DiMichele says, is using the fossil forest as a reference point. The researchers are analyzing the chemical makeup of earlier and later coal deposits for measures of ancient carbon dioxide, temperature, rainfall and other variables. So far, the rise in carbon dioxide seems to be fairly smooth over time, but the change in vegetation is jerkier.
Comparing fossils from before 306 million years ago and after, "you have a total regime change without much warning," says DiMichele. "We need to look much more closely at the past," he adds. "And this is our first opportunity to see it all."
Guy Gugliotta has written about cheetahs and human migrations for Smithsonian.