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.
Most of the fossils come from the Mazon Creek area of Grundy, Will, Kankakee, and Livingston counties. 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.
Fossils like those from Mazon Creek are recovered from a few other areas in Illinois. The Francis Creek Shale is exposed along streams and in strip mines in LaSalle county between the Vermillion River and Marseilles.
Mazon Creek nodules and fossils are also found in museums worldwide. This is because the fossils from the area are abundant and exceptionally well-preserved. Most major natural history museums in the world probably have at least a few nodules.
The most important collection of Mazon Creek nodules is probably at the Field Museum of Natural History.
The Illinois State Museum also has an important collection of Mazon Creek fossils, including numerous type and figured specimens. Some of these fossils are shown in this exhibit.
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.
In some ways the area might have been similar to southern Louisiana (USA) and the adjacent Gulf of Mexico. However, the plants and animals would have been very different from today. They were different for at least two reasons. First, many of the plants and animals that are common today had not yet evolved at that time. Second, the climate would have been tropical. The tropical climate was a result of continental drift; 300 million years ago the area was just a few degrees north of the equator.
Many animals lived in the shallow marine bays. More plants and animals lived in the swampy areas along the rivers. As animals in the bay died they fell to the bottom of the bay. They were joined by plants and animals that died along the river and were washed into the bays.
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.
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.
The Mazon Creek deposit is an example of a Lagerstätten.
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.
These interesting Mazon Creek plants are some of those found in the collections at the Illinois State Museum. NOTE: the number in parentheses following the description is the size(s) of inline image(s) in the page covering that plant.
The Mazon Creek fauna is extremely diverse. According to Nitecki (1979) over 320 species of animal have been identified (or originally described) from the deposit.
Scientists studying the fauna have divided it into two components. These are the Essex and Braidwood faunas. The Essex fauna contains the marine organisms that would have lived in the shallow bays. The Braidwood fauna consists of land and freshwater-dwelling organisms that washed into the bays.
The Essex fauna includes such animals as jellyfish, worms, snails, clams, shrimp, and fish.
The Braidwood fauna includes insects, millipedes, centipedes, scorpions, spiders, spider relatives, amphibians, freshwater fish, shrimps, freshwater horseshoe crabs, and ostracodes.
The following are some of the interesting Mazon Creek animals that are found in the collections at the Illinois State Museum. NOTE: the number in parentheses following the description is the size(s) of inline image(s) in the page covering that animal.
Most textbooks on historical geology, paleontology, or evolution have some discussion of Mazon Creek.
Given the large number and variety of papers on Mazon Creek fossils, choosing a few to list is very difficult. The following list includes a few comprehensive treatments and also includes publications of the Illinois State Museum on Mazon Creek fossils.