Mazon Creek Fern
Pecopteris sp.
Carbondale Formation
Middle Pennsylvanian
Grundy County, Illinois
Complete siderite nodule with a very well preserved fern frond.

The Mazon Creek area fossils are found mostly in siderite concretions. The concretions are found in the Francis Creek Shale Member within the Carbondale Formation of the Kewanee Group. The best nodules are a by-product of coal strip-mining operations. The age of the fossil plants is Desmoinesian, Middle Pennsylvanian and are dated at about 307 MYO. Within the area two distinct biotas are noted. One is of terestrial fossils (mostly plants and insects), and another with near-shore brackish or marine fossils such as jellyfish, annelids, fish, and the famous Tully Monster.
The Carbondale paleoenvironment included a wetland flora of seed ferns, true ferns, lycopsids, and horsetails. These plants were buried layers of silt and during burial and subsequent diagenesis, concretions formed around the remains and hardened to preserve the plant impressions.