This fossil fern, Pecopteris sp., is from the Mazon Creek coal beds in Northeastern Illinois. The leaf structure on the fossil is preserved with calcite in a siderite nodule. Siderite nodules form when rapid deposition of sediment onto a nucleus (which can be a piece of plant material, an animal, or even a grain of sand) is cemented with iron carbonate. The Mazon Creek coal beds have siderite nodules that have formed in all shapes and sizes, and most of them contain exquisite fossils.
Pecopteris is a fern from the order Pteridophyta, which no longer exists today. This plant thrived almost 323 million years ago, during the Pennsylvanian period, a time when the forests were dense, damp and best known for their coal-producing qualities. The discovery of this fern, along with many other fern species and horsetails within coal beds is indicative of a humid and swampy climate in the Illinois area during the Pennsylvanian. The paleo-environment of Mazon Creek has been likened to the modern Florida Everglades, where the sea floods the forest and creates vast swamps and mangroves along coastlines.
Along with various plant species an abundance of soft-bodied organisms have also been found at Mazon Creek, one of the most famous being the Tully monster, the Illinois state fossil. Soft bodied organisms are not easily preserved in the fossil record, but the Mazon Creek nodules are a lagerstatten, an instance of exceptional preservation, where soft tissues and features have been preserved extraordinarily well.
The University of Colorado collection of Mazon Creek fossils contains many kinds of plants and animals like: horsetails, ferns, mosses, clams, shrimp and crabs. These specimens are some of the earliest collected items in the invertebrate collections. With the help of a student worker in the Paleontology Section
, the Mazon Creek collection has recently undergone revitalization. The fossils have been re-housed, properly curated, and identified.