BioEd Online

Geologists have found the remains of a huge underground rainforest hidden in a coal mine in Illinois. The fossil forest, buried by an earthquake 300 million years ago, contains giant versions of several plant types alive today.

Experts say the forest was growing on top of peaty soil when an ancient tremor plunged it about 5 metres down, allowing it to be buried and fossilized beneath further layers of more recent rock. It dates from a time when North America and Europe were joined together, at the Equator — similar forests went on to be transformed into the rich coal seams of the two continents.

The forest was discovered in 2005 by John Nelson of the Illinois State Geological Survey, who was making routine measurements in a mine in Vermilion County. He called in a team of palaeontologists to investigate the forest¹. As they drove down to 100 metres below ground, they saw the forest's remains in the glare from their miners' lamps as they looked up at the ceiling. "You actually see roots coming down; you see tree trunks lying in the ceiling," says Howard Falcon-Lang of the University of Bristol, UK, a member of the research team.

Supersize tree

The forest is not the oldest to be discovered — others are known that are up to 370 million years old — it is the sheer size of the forest that is significant. It has allowed Falcon-Lang and his colleagues to show that the distribution of plant species that made up the forests in the Carboniferous era differed from region to region, rather than being randomly mixed.

"Forests closer to the coast differed from forests slightly further inland," says Falcon-Lang. "It is a subtle point but one that could not have been made without the great size of the sampled area," says Kirk Johnson, chief curator at the Denver Museum of Nature and Science, Colorado. "Without a large area to sample, there was really no way to know."

The ancient forest bears little resemblance to modern equivalents. "The diversity of the first rainforests was bizarre," says Falcon-Lang. He and his team found the remains of tree-sized clubmosses, horsetails and ferns — plants that today grow 2 or 3 metres tall, but in the ancient forest reached heights of up to 40 metres. Also surprising is the presence of remains from mangrove-like plants. "It was always assumed that mangrove plants had evolved fairly recently," says Falcon-Lang.

The forest probably had about 50 different plant species, although Falcon-Lang says that this is a conservative estimate. We probably lumped several similar species together as one," he explains. Modern rainforests are more diverse, containing as many as 500 plant species per hectare.

Imminent collapse

This discovery also shows that the fundamental processes that guide the complexity and evolution of forests has been around for hundreds of millions of years, says Scott Hocknull, a curator at the Queensland Museum in Brisbane, Australia. "Knowing this and how it has played out so many times in history will allow ecologists to better understand the complexity of modern forest systems," he adds.

The forest's long life will now be cut short — the mine is likely to collapse in the next few years and there are no plans to preserve it for the sake of the fossils. But Falcon-Lang is philosophical about losing the forest, pointing out that if it weren't for mining, the forest would never have been discovered in the first place.