Mazon Creek fossil beds

The Mazon Creek fossil beds are a conservation lagerstätte found near Morris, in Grundy County, Illinois.

The fossils are preserved in ironstone concretions, formed approximately 309 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 attempting to reconstruct the paleoecology of the sites.

Pit 11, which was located southwest of the town of Braidwood, Illinois, is known for its Essex Biota with a greater abundance of marine species.

[10] 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".

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 leaves 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.

[9] The Essex area also includes the most famous faunal member of the Illinois state fossil Tullimonstrum, known popularly as the "Tully Monster".

The Braidwood fauna includes insects, millipedes, centipedes (three taxa, Latzelia, Mazoscolopendra, Palenarthrus), scorpions, spiders, other arachnids, temnospondyls[15] and lepospondyls,[16] some of which are amphibians (depending on which phylogeny is accepted), actinopterygians (Illiniichthys[17]), shrimps, horseshoe crabs and ostracods.