An Underground Fossil Forest Offers Clues on Climate Change

William A. DiMichele in the Springfield Coal. The dark mass is a coal seam; the lighter shale above is interrupted by a fossil tree stump

In the clammy depths of a southern Illinois coal mine lies the largest fossil forest ever discovered, at least 50 times as extensive as the previous contender. 

Scientists are exploring dripping passages by the light of headlamps, mapping out an ecosystem from 307 million years ago, just before the world’s first great forests were wiped out by global warming. This vast prehistoric landscape may shed new light on climate change today.

Dating from the Pennsylvanian period of the Carboniferous era, the forest lies entombed in a series of eight active mines. They burrow through the rich seams of the Springfield Coal, a nationally important energy resource that underlies much of Illinois and two neighboring states and has been heavily mined for decades.

Pushed downward over the ages by the crushing weight of rock layers higher up, the Springfield forest lies at varying depths, 250 to 800 feet underground. The researchers have only sampled it so far, in the vicinity of Galatia, Illinois, but they think it extends more than 100 miles in one direction; its width has not been ascertained. An earlier discovery by the same team, the Herrin Coal forest farther north in Illinois, is just two miles long.

“Effectively you’ve got a lost world,” said Howard Falcon-Lang, a paleontologist at Royal Holloway, University of London, who has explored the site. “It’s the closest thing you’ll find to time travel,” he added.

Curiously, the forest can be viewed only from below. The scientists crane their necks, illuminating the ceiling with miners’ helmet lamps. Hundreds of millions of years ago, trees and other plants grew atop thick peat that eventually compressed into coal; when that was excavated, the forest’s fossilized remains could be seen in the mine’s shale ceiling.

“It’s a botanical Pompeii, buried in a geological instant,” said William A. DiMichele, a paleobiologist and curator of fossil plants at the Smithsonian Institution in Washington and one of the forest’s discoverers. He believes it was gently entombed by floods that successively washed through a swamp.

A river as wide as the Mississippi snaked through the fossil-forest landscape; its course is still clearly visible. As the climate grew drier with rising temperatures in the late Carboniferous period, rainfall became seasonal and pounded sediment out of the soil, filling the river with silt. This suffocated the forest as the river spilled over its banks.

The flooding was incremental and gradual, hardly ruffling the fern leaves that it entombed in mud and that can be seen, down to the smallest frond, on the ceilings of the coal mines.

Huge fossilized trees still stand rooted in their original but compacted soil, surrounded by the litter of leaves that once fluttered down.

Primitive, lizardlike reptiles were then evolving in the swamps, but there are almost no animal fossils in the Springfield forest — save for the occasional cockroach wing — since such creatures easily fled the rising waters.

Such snapshots of the very distant past — tens of millions of years before the age of dinosaurs — are hard to come by. “It is extraordinarily rare to get fossil forests of any extent at all,” said Kirk Johnson, a paleobotanist at the Denver Museum of Nature and Science. 

“It’s usually just a few trees here and there. But here is an ancient geography — effectively unheard of.”

Dr. DiMichele and colleagues have explored a five-mile path, or transect, starting at the ancient riverbank and arrowing through the swamp. Just as if this were a living forest, they have stopped along the route to identify individual leaves or study fallen trunks. Moving away from the river, a dense thicket of seed ferns gives way to tree ferns and low ground cover. Farther out, tree ferns are dwarfed by forest giants called scale trees.

“It was a Dr. Seuss world,” Dr. Johnson said of the scale-tree forests: sun-washed quagmires studded with giant green stalks like asparagus spears, hundreds of feet tall. (Scale trees did not unfurl spreading crowns until the very end of their life cycle.) Dr. DiMichele has followed a fallen scale tree for 100 feet, before it disappeared behind coal not yet mined away. Six feet wide at the base, it was hardly any narrower at that great height.

Scale trees had reptilian-looking, photosynthetic bark that coal miners sometimes mistake for dinosaur remains. Tube-shaped with spongy pulp inside, the trees snapped in two when storms ravaged the swamp. Immense, cylindrical roots kept stumps firmly upright, as seen in the mines.

By coincidence, the earliest ancestor of these scale trees has just been discovered in a fossil forest in New York State. Repairs at Gilboa Dam, north of the Catskills, uncovered the floor of what may be the world’s oldest forest, scientists reported last month in the journal Nature. Dating from the Devonian Period, it is 78 million years older than the Springfield find, but the mapped remains are much smaller, covering about a third of an acre.

There were no birds in the Pennsylvanian period, so insects flourished in the oxygen-rich air. Hiking through the Springfield forest would have meant dodging millipedes six feet long and dragonflies the size of crows.

And yet the fossil leaves show much less chewing by insects than the vegetation in our modern backyards. Animals had barely evolved herbivory, the habit of eating live plants, and instead subsisted on putrefying remains in the fetid swamp.

Two million years later — a geological eye blink — came vast extinctions of plants, wiping out many of the species found in the Springfield forest. The mighty scale trees all died off. 

Their modern relatives are quillworts, just six inches high.

The reach of the Springfield forest should allow scientists to undertake ecosystem-wide analyses in a way never before possible in landscapes so ancient, and such studies may help them predict the effects of global warming today.

“With our own CO2 rises and changes in climate,” said Scott D. Elrick, a team member from the Illinois State Geological Survey, “we can look at the past here and say, ‘It’s happened before.’ ”

Today, we burn the scale trees of the Carboniferous by the billions: they have all turned to coal. Newly discovered, the Springfield forest is already crumbling to bits, as coal-mine ceilings quickly do after exposure. But with continued mining, more ceilings are being revealed every day.

“You have to dig to find fossils, going inside the anatomy of the planet,” Dr. Johnson said. “Bill DiMichele realizes he has an entire industry digging for him, creating a tunnel into an ancient world.” 

By W. BARKSDALE MAYNARD@The New York Times Science