One of Earth's first rainforests has been found in an Illinois coal mine, where an earthquake entombed it 300 million years ago. At 10 square kilometers--roughly three times the size of New York City's Central Park--the discovery represents the largest swath of fossil forest ever seen and may offer scientists their best chance yet to understand the ecological changes that preceded a widespread extinction.
In the forest's Carboniferous heyday, Illinois was a land of tropical swamps and rainforests, sitting astride the equator and at the mercy of a major fault line. An earthquake sank this particular rainforest below sea level, and sediment quickly filled the resulting hole, sealing off the forest and ensuring its preservation. Eons later, as workers mined the coal surrounding the forest, a landscape of fossils emerged, embedded in the mine's ceiling. The landscape resembled no rainforest on Earth today, according to Bill DiMichele, the study's lead author and a paleobotanist with the Smithsonian's National Museum of Natural History in Washington, D.C. In these so-called coal forests, 2-meter-wide and 40-meter-tall trees with no crowns or branches-- think "telephone poles with leaves," says DiMichele--harbored dragonflies with 60-centimeter wingspans and were visited by giant crablike creatures called eurypterids. DiMichele and colleagues report in the May issue of Geology that plant and animal fossils were found remarkably intact; some tree stumps are still upright and rooted in the underlying coal bed. "We can reconstruct the forest more or less as if we were walking through it," says DiMichele. "It's pretty wild."
Although the rainforest may be alien, its circumstances aren't. The trees lived during a global warming period that eventually melted ice caps and reshaped coastlines. Researchers think that a massive die-off of the telephone-polelike lycopsid trees, certain fern species, and some marine creatures occurred around the same time. Paleobotanists have long wondered if the extinction resulted from the climate change, or if tectonic shifts that released CO2 into the atmosphere caused the extinction and the climate change. DiMichele and his colleagues hope to determine which came first by comparing the newly discovered forest with older pockets of Carboniferous rainforest from North America and Europe. By tracing changes in vegetation "we want to see if you can pick up signals that change is coming," says DiMichele.
Although he's certain the discovery will "offer new insight into a critical part of Earth's history," geologist Chris Cleal of the National Museum of Wales in Cardiff, U.K., urges caution in predicting what researchers will learn. It's too early to say what the forest tells us, he says. Paleobotanist Andrew Scott of the University of London is similarly reserved. "This is a glimpse of the past, but not a full picture of it," he says. But he adds, "There's no doubt this is important."