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Geology
The Mazon Creek fossils are found in the Francis Creek Shale; the type locality is the Mazon River (or Mazon Creek), a tributary of the Illinois River near Morris, Grundy County, Illinois. The 25 to 30 meters of shale were 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 localized deposits within the silty to sandy mudstones, in the lower four metres of the formation. The paleoecosystem 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 the 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 River area of Grundy, Will, Kankakee, and Livingston counties. Additional fossils are found in LaSalle 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.
The site's importance was realized in the mid-nineteenth century: "the nodules of Mazon Creek, where fragments of plants, even of the softest texture, have been preserved in their integrity".[2]
Flora
The Mazon Creek flora comprises 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, such as Pecopteris.
Fauna
The Mazon Creek fauna has over 320 species of animals 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 ostracods.
The most famous faunal member is Tullimonstrum, known popularly as the Tully Monster.
See also
Notes and references
- ^ Illinois State Museum-Mazon Creek Fossil Exhibit
- ^ A.H.W. in Geological Survey of Illinois, 1866, p 489
- Stephen A. Schellenberg, "Mazon Creek: preservation in late Paleozoic deltaic and marginal marine environments" in Walter Etter, James W. Hagadorn, Carol M. Tang, David J. Bottjer, editors, Exceptional Fossil Preservation: A Unique View on the Evolution of Marine Life (Columbia University Press) 2002
- Charles W Shabica, Andrew A Hay editors. Richardson's Guide to the Fossil Fauna of Mazon Creek
External links
- Illinois State Museum Mazon Creek Fossil Exhibit
- UC Berkeley Mazon Creek info
- Northeastern Illinois University fossil fauna
- Fossil News generalized diagram of Mazon Creek Delta and fossil photos
- University of Bristol Mazon Creek flora and fauna discussed with photographed fossils
- Mazon River Watershed Website
