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Mazon Creek Fossil Beds
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| Nearest city: | Benson Road, Morris, Illinois |
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| Governing body: | Private |
| NRHP Reference#: | 97001272 |
| Significant dates | |
| Added to NRHP: | September 25, 1997[1] |
| Designated NHL: | September 25, 1997[2] |
The Mazon Creek fossil beds are a conservation lagerstätte found near Morris, in Grundy County, Illinois. The fossil beds are located in ironstone concretions, formed approximately 300 million years ago in the mid-Pennsylvanian epoch of the Carboniferous period. 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 in the area, known since the mid-nineteenth century, make the Mazon Creek lagerstätte important to paleontologists, in attempting to reconstruct the paleoecology of the sites.[3] The locality was declared a National Historic Landmark in 1997.[2][4]
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The Mazon Creek fossils are found in the Upper Carboniferous Francis Creek Shale;[5] 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 million years ago, 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. This mode of preservation is known as authigenic mineralisation.
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".[6]
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 includes: lycopsids, related to modern club moss, with arborescent forms named Lepidophloios, Sigillaria and Lepidodendron, and herbaceous forms called Lycopodites and Cormophyton; sphenopsids like Calamites a tree-like horsetail relative, with common foliage names of Annularia and Asterophyllites, and a vine-like form called Sphenophyllum; Pteridophyta as marattitalean tree ferns and Filicales and Zygopteridales understory ferns, with common foliage names of Pecopteris, Acitheca and Lobatopteris; pteridosperms, also known as seed ferns, an extinct group of plants that grew both as trees and smaller shrubs, with features like pinnated leafs similar to true ferns, but reproduced by seeds instead of spores; they had common foliage names Mariopteris, Alethopteris, Odontopteris, Neuropteris, Laveineopteris and Macroneuropteris; extinct gymnosperm Cordaites, believed to be closely related to and sharing many features with modern conifers.
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, cephalopods[5] and fish. The Braidwood fauna includes insects, millipedes, centipedes, scorpions, spiders, other arachnids, amphibians, freshwater fish, freshwater shrimps, freshwater horseshoe crabs and ostracods. The oldest known beetle has been described from the Braidwood fauna.[7]
Three species of centipede have been described from Mazon Creek. They are known from around 60 specimens in total, and belong to three species:[8]
Fossil centipedes are very rare, and Mazoscolopendra and Palenarthrus are the only known Paleozoic scolopendromorphs.[9]
The most famous faunal member is Illinois' state fossil Tullimonstrum, known popularly as the "Tully Monster".
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