Pennsylvanian Age Mire Forest

John Nelson and Scott Elrick in the Coal Section of the Illinois State Geological Survey, in cooperation with Bill DiMichele of the National Museum of Natural History Smithsonian Institution, Howard Falcon-Lang of the Univeristy of Bristol and Phil Ames of Peabody Energy recently published a paper about the ecology of a fossil forest in the journal Geology.

ISGS geologists received another bit of publicity from their fossil forest discovery near Danville, Illinois. Discover Magazine honored this discovery by including them in the "Top 100 Science Stories of 2007" in the January 2008 issue. A small summary article and interview with Scott Elrick are contained in the magazine article. The fossil forest was also featured in the July 2007 issue of Outdoor Illinois; an article entitled Underground Science (pdf).

Ecological gradients within a Pennsylvanian mire forest

Pennsylvanian coals represent remains of the earliest peat-forming rain forests, but there is no current consensus on forest ecology. Localized studies of fossil forests suggest intermixture of taxa (heterogeneity), while, in contrast, coal ball and palynological analyses imply the existence of pronounced ecological gradients. Here, we report the discovery of a spectacular fossil forest preserved over ~1000 ha on top of the Pennsylvanian (Desmoinesian) Herrin (No. 6) Coal of Illinois, United States. The forest was abruptly drowned when fault movement dropped a segment of coastal mire below sea level. In the largest study of its kind to date, forest composition is statistically analyzed within a well-constrained paleogeographic context. Findings resolve apparent conflicts in models of Pennsylvanian mire ecology by confirming the existence of forest heterogeneity at the local scale, while additionally demonstrating the emergence of ecological gradients at landscape scale
location map

The location of this fossil forest is just to the south and west of Danville, Illinois, about 30 miles to the east of the ISGS in Champaign, Illinois (see picture to the right). This 300 million year old fossil forest was found directly on top of the Herrin coal seam in the Riola and Vermillion Grove coal mines, and represents the last stages of the peat mire forest responsible for forming the Herrin coal.

If you were walking down a hiking trail through a forest today, looking closely at the vegetation, you might notice that as the landscape changed, gradual changes in the types of plants and trees occured as well. For present day plant biologists examining the ecology of a modern day forest, this is a luxury that is easily taken for granted.

When fossil plant researchers (paleobotanists) want to look at the ecology of an ancient forest, it is rather difficult to simply go walking through an ancient forest! Instead, much study must go into looking at many plant fossils from multiple locations and times and settings, getting one puzzle piece of the ecology here... one puzzle piece of the ecology there. Through diligent study, a picture of ancient environments can be slowly be pieced together and good guesses about ancient ecologies can be made.

However, every once in a while a unique opportunity presents itself where a paleobotanist has an opportunity to do what his plant biologist colleagues do all the time. Walk through a forest... a fossil one! This paper represents the largest study of the plant ecology of a Pennsylvanian age forest to date and shows that subtle ecologic changes in the make-up of the forest were present 300 million years ago.

In the practice of geology in the field, scientists regularly look at vast spreads of time in small geographic slices. For example, standing at the rim of the Grand Canyon and peering across to the other side, your eye takes in millions of years of geologic time... but you are only able to see a thin 'slice' of each unit in profile. Do you want to know what a particular rock unit looks like 500 feet into the side of the canyon walls? The only way to find out is to drill a hole and take a core sample.

Geologic research in an underground mine such as these coal mines turns this notion on its ear!

In this study, we were able to look up at preserved trees and ferns in the mine ceiling, peering upwards at a single slice of time over a huge (relatively speaking) geographic area. This is a unique way to get a 'snapshot' in time look at the forest landscape of 300 million years ago. It's the 'worms eye' view of a fossil forest... allowing a veritable 'walk' through the forest and a chance to see first hand what plant species were present and how they were distributed across the landscape. In other words, a chance to see the ecology of a fossil forest!

If you would like to see a piece of this fossil forest in person, you are in luck! The coal mining exhibit at the Museum of Science and Industry in Chicago has a fossil covered slab of gray roof shale from the Riola mine on display. Now you can take a (mini) walk of your own.

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