Mazon Creek fossils
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The Mazon Creek fossils are conservation lagerstätten found near Chicago, Illinois. The fossils are found in ironstone concretions formed approximately 300 mya. These concretions frequently preserve both hard and soft tissues of animal and plant materials, as well as many soft-bodied organisms that do not normally fossilize. The quality, quantity, and diversity of fossils make the Mazon Creek lagerstätte important to paleontologists worldwide.
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Geology
The Mazon Creek fossils are found in the Francis Creek Shale. The 25 to 30 metres of shale was formed approximately 300 mya, during the Pennsylvanian period. The fossiliferous concretions are usually found within the thickest deposits of Francis Creek. The concretions occur in locallized deposits within of silty to sandy mudstones in the lower four metres of the formation. The paleoenviroment is believed to be a large river delta system deposited by at least one major river system flowing from the northeast. The sediments are believed to derive from the Appalachian orogeny events. The delta had a tropical climate, a result of the area being within 10° north latitude of the equator during the Pennsylvanian.
The remains of plants and animals were rapidly buried by the sediment deposited in the deltaic system. Bacterial decomposition of the remains produced carbon dioxide that combined with dissolved iron from groundwater. This process formed siderite in the sediments surrounding the remains, forming detailed casts of their structure. Lithification of the sediments formed protective nodules of ironstone around the now fossilized remains.
The fossiliferous concretions are found in the Mazon Creek area of Grundy, Will, Kankakee, and Livingston counties. Additional fossils are found in La Salle County, Illinois; between the Vermilion River and Marseilles, Illinois. The ironstone concretions are recovered from exposures along streams, roadcuts, and in active or abandoned coal mine areas.
Flora
The Mazon Creek flora is comprised of over 400 species from at least 130 genera. However, the true number of species is difficult to determine. Paleobotanists name separate plant structures with different names by convention, inflating the number of fossil plant taxa. Paleobotanists are currently determining which taxa are valid.
Mazon Creek flora include: Calamites and other tree-like horsetail relatives; Lepidodendron and other tree-like club moss relatives; extinct gymnosperms related to ginkgoes; ferns; and seed ferns.
Fauna
The Mazon Creek fauna has over 320 species of animal that have been identified. The fauna is divided into two components: the marine Essex fauna and the land and freshwater Braidwood fauna that were washed into the deltaic sediments. The Essex fauna includes jellyfish, sea worms, snails, saltwater clams, shrimp, sea scorpions, and fish. The Braidwood fauna includes insects, millipedes, centipedes, scorpions, spiders, other arachnids, amphibians, freshwater fish, freshwater shrimps, freshwater horseshoe crabs, and ostracodes.
The most famous faunal member is Tullimonstrum, known popularly as the Tully Monster.
External links
- [Illinios State Museum] online exhibits
- [UC Berkeley] Mazon Creek info
- [Northeastern Illinois University] fossil fuana
- [Fossil News] generalized diagram of Mazon Creek Delta and fossil photos
- [University of Bristol] Mazon Creek flora and fauna discussed with photographed fossils
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