The MAZON CREEK biota of northeastern Illinois provides the most complete record known of late Paleozoic life. It ranks among the world’s outstanding fossil assemblages. Occurring in the 280-million-year-old Francis Creek Shale, the Mazon Creek fossil assemblage consists of more than 200 plant and 300 animal species, including representatives of no less than 11 phyla and 23 classes of animals from terrestrial through marginal marine environments.

Mazon Creek is a special type of fossil locality known as a Lagerstatten - a place where many plants and animals have been preserved with amazing soft-tissue detail. There are few places on earth where this type of preservation occurs. Others include the Cambrian Burgess Shale, the Devonian Hunsruck slates, and a site in Kansas.

Mazon Creek fossils are found in concretions of an iron carbonate known as siderite which formed within the mid-Pennsylvanian Francis Creek Shale (Westphalian) when that area was part of an immense equatorial deltaic complex. The study of taphonomy (how fossils form) suggests that the plants and animals were buried quickly by fine clays very shortly after their death. Molecule by molecule, the living tissue was either mineralized or left as a thin shadowy residue and the result is a picture of the past - the living things and the entire ‘Coal Age’ environment in which they lived and died.

There are two fossil assemblages in the Mazon biota: the non-marine to brackish Braidwood assemblage including tetrapods, ferns, insects and scorpions, and the brackish to marine Essex assemblage which is dominated by jellyfish (often called crustaceans), fish, mollusks, crustaceans, worms and oddities like the Tully monster Tullimonstrum gregarium which can be assigned to no living group of animals.

Collection of Mazon Creek fossils has always been a cooperative effort between the landowners, volunteers and scientists. It is the last two groups who have literally "written the book" about their discoveries.

Richardson’s Guide to the Fossil Fauna of Mazon Creek provides an overview of the history of the discoveries, a description of coal mining in the area, an overview of the two faunas and environments, the regional and global significance of the site, a detailed description of the chemistry of taphonomy in siderite, and 25 chapters on each taxonomic group found at Mazon over the years. There are 14 phyla from more than 33 classes comprising about 100 orders including fossils which seem to have left no living descendants and a few fossils which are considered problematica" and have not been assigned to a phylum.