Mazon Creek Fossil - Cyclus americanus

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  • Item Category: Antiquities
  • Source: eBay
  • Sold Date: Jul 05, 2007
  • Channel: Auction House
Name: Cyclus americanus, Phylum: Arthropoda; Subphylum Crustacea; Class Maxillopoda; Subclass Halicyna;
Geological Time: Carboniferous (300 m.y.a.)

Size: 15 mm in length on a 30 mm x 33 mm nodule; Pos/Neg specimen.
Fossil Site: Mazon Creek, Braidwood, Illinois

The Mazon Creek fossils are conservation lagerstätten found near Chicago , Illinois . The fossils are found in ironstone concretions formed approximately 300 mya in the mid- Pennsylvanian epoch of the Carboniferous . 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, known since the mid-nineteenth century, make the Mazon Creek lagerstätte important to paleontologists in attempting to reconstruct the paleoecology of the sites.
The Mazon Creek fossils are found in the Francis Creek Shale; the type locality is Mazon Creek , a tributary of the Illinois River near Morris, Grundy County, Illinois . The 25 to 30 meters 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 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 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 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, w fragments of plants, even of the softest texture, have been preserved in their integrity" [1]


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 .


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 scorpiones , and fish . The Braidwood fauna includes insects , millipedes , centipedes , scorpiones , 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 .

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