Location: Will County, Illinois
Time: 310 to 300 million years ago, during the Carboniferous period
About this Famous Find: Mazon Creek organisms are found at several localities in northern Illinois, some very near downtown Chicago, and occur in a rock formation called the Francis Creek Shale. The fossils are beautifully preserved in rounded concretions and can often be collected from old coal mine dump piles. The excellent preservation of these fossils allows studies of many organisms that are not commonly present in the fossil record, such as soft-bodied animals and those with fragile exoskeletons (insects, spiders, etc.).
The Mazon Creek fauna is well known for the diversity of specimens from the Carboniferous Period, some of whichsuch as the Tully Monster (Tullimonstrum)are found nowhere else in the world. Plants are the most common fossils; more than 400 species from at least 130 genera have been identified and include ferns, seed-ferns, scale trees (gymnosperms), and jointed-stem plants (sphenopsids). Insects, centipedes, millipedes, scorpions, and even small amphibians are sometimes collected as well. Jellyfish, crustaceans, molluscs, worms, and rare fish are found at localities where the rock formed under marine conditions.
We can tell by examining the plants and animals preserved in the Mazon Creek concretions that northern Illinois 300 million years ago was much different than it is today. Constantly shifting marine, freshwater, and brackish environments, including a coal-forming swamp forest, covered the area back then. The fossil plants present in the Mazon Creek show no annual rings in their wood, suggesting that there were few seasonal fluctuations in climate. These can be compared with plants in the tropics today, helping us recognize that Illinois and most of North America lay in equatorial latitudes during the Carboniferous.
Fossils from Mazon Creek can be found in many museums around the world. The Field Museum of Natural History in Chicago has what is probably the most important collection: over 23,000 fossil plant specimens. Mazon Creek fossils can be viewed at the Illinois State Museum, both online and at the museum in Springfield.