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Leaf Fern nodule
Alethopteris sp. or Neuropteris sp
Carboniferous - Pennsylvanian Period (270-310 million years ago)
Mazon Creek, Illinois

Seed fern with large leaves. These 5 m tall trees lived in the drier parts of Carboniferous and Early Permian swamps. The genus "Alethopteris" is a collective navme for plants with similar leaf morphology.

Ironstone concretions containing Mazon Creek fossils are found in various types of natural and man-made outcrops of a rock called the Francis Creek Shale. Plants from the Mazon Creek deposit represent some of the plants that were living in the swampy lowlands near the shore and rivers. When they died they were washed into the bays and were preserved.  Many of the plants found in nodules were also important in producing the large Illinois coal deposits.  In this area fossils are recovered from natural exposures on Mazon Creek, from active and abandoned strip mines, from shaft mines, and from mine spoil piles.

The Francis Creek Shale: Approximately 300 million years ago (during a time geologists call the Pennsylvanian Period) Illinois looked nothing like it does today. Much of it was not even dry land. Much of the area that we now call Illinois was a mixture of swampy lowlands and shallow marine bays.

From the northeast flowed at least one major river system. The river(s) built large deltas through the low swamps and into the shallow bays. The mud that the river(s) carried was deposited in these deltas and bays. This mud turned into a rock called the Francis Creek Shale.

How the Mazon Creek Fossils Formed: When the remains of these plants and animals sank to the bottom of the bays, they were rapidly buried by the mud washing in from the river(s). This process protected the remains from being destroyed. Bacteria began to decompose the plant and animal remains in the mud. The action of these bacteria produced carbon dioxide in the sediments around the remains. The carbon dioxide combined with iron from the groundwater around the remains forming siderite (ironstone). The siderite protected the remains from further damage. The combination of rapid burial and rapid formation of siderite resulted in excellent preservation of the many animals and plants that ended up in the mud.

Importance of Mazon Creek Fossils:  The quality and diversity of fossils recovered in the Mazon Creek nodules makes these localities important worldwide. In most fossil deposits only the hard parts of organisms (shells, bones, teeth, etc.) are preserved. This means that in most fossil deposits only animals that have hard parts are preserved.  Because of the unique conditions of fossilization, Mazon Creek fossils frequently have both hard and softer parts preserved. In addition, many soft-bodied organisms that do not usually fossilize are preserved.

These factors mean that the fossils from Mazon Creek provide scientists with an extraordinary view of biodiversity 300 million years ago.
 

See also:
http://www.museum.state.il.us/exhibits/mazon_creek/about_mazon_creek.html
http://www.museum.state.il.us/exhibits/mazon_creek/alethopteris.html
- “Keys to Identify Pennsylvanian Fossil Animals of the Mazon Creek Area.”, Hay, A., Earth Science Club of
 Northern Illinois, 1989
- “The Fossil Fauna of Mazon Creek”, C.W. Shabica and A.A. Hay, 1997
http://paleo16.ucmp.berkeley.edu/carboniferous/mazon.html
http://alun.uio.no/palmus/galleri/montre/english/plants_liste_e.htm