Courtesy Mark RyanLast year, news came out about the discovery of a large fossil forest dating from 300 million years ago in a coal mine located in eastern Illinois. Now, five more prehistoric forests have been identified in the same region.
Courtesy Illinois State Geological SurveyThe remains of the ancient tropical rainforests cover a tremendous area – 36 square miles – and have been under study by scientists from the Smithsonian, the UK, and the Illinois State Geological Survey. A presentation given at the British Association Science Festival held in Liverpool this week detailed some of the highlights of this incredible find.
"Theses are the largest fossil forests found anywhere in the world at any point in geological time,” said Dr Howard Falcon-Lang a paleobotanist at the University of Bristol.
The prehistoric landscapes existed within only a few million years of each other – a short span geologically speaking – and are found stacked one upon the other. Segments of the forest fossilized in their original vertical position. At places, scientists can trace the original ground cover in well-preserved fossils.
Donning cap lamps, battery packs, and rock hammers Falcon-Lang and his colleagues rode an armored vehicle 250 feet beneath the Herrin coal seam in the Riola and Vermillion Grove coal mine. Once underground, the scientists took an incredible hike through a long-gone prehistoric fossil forest, illuminated only by lights on their caps.
Courtesy Illinois State Geological Survey“We walked for miles and miles along pitch-black passages with the fossil forest just above our heads,” Falcon-Lang said. "It's kind of an odd view looking at a forest bottom-up. You can actually see upright tree stumps that are pointed vertically up above your head with the roots coming down; and adjacent to those tree stumps you see all the litter.”
Courtesy Illinois State Geological SurveyIn some cases toppled trees – complete with crowns – and over 100 feet long were measured lying stretched out in the shale across the ceiling. For paleobotanists it presents a remarkable opportunity to actually stroll through a 300 million year-old ecological system as if taking a walk in the local woods today.
The reason for this unusual preservation is thought to be due to the prehistoric rain forest growing in an estuary near the Royal Center fault in Indiana, which caused the terrain to subside below sea level making it vulnerable to incidents of flooding and abrupt drowning. Geologists suspect earthquakes along the fault are the reason for the subsidence.
The soil that once supported these rainforests was later transformed into coal. Once this coal seam was mined from underground, the base of the fossilized forest was revealed encased in a shale matrix.
These tropical rain forests originally flourished during the Pennsylvania period (known as the Upper Carboniferous in Britain), back when the US Midwest was located near the equator. Forests of giant club moss trees and tree-sized horsetails came and went over a geologically short span of time. At the same time, major shifts in climate were taking place, alternating from cooler temperatures with large planetary ice caps to periods of extreme warming.
The episodes of climatic change coincide with changes in the forest ecology. Close study of the fossil vegetation show that several times the climatic stress pushed the rain forests into extinction, making way for skimpier fern growths to replace them.
Over the next five years Dr. Falcon-Lang’s team will search for reasons why this rainforest extinction took place. Understanding how the first rainforests responded to global warming could help shed light on how climatic change may affect present day rainforests.
Additional photos of the amazing fossil forest can be found here. But if you want to see some of the real thing, visit the coal-mining exhibit at the Museum of Science and Industry in Chicago where an actual slab of the gray roof shale is on display.