Age: Pennsylvanian, Carboniferous.
Mazon Creek, named after a local river in the Northeast of Illinois in the United States, is a lagerstatte yielding exceptionally well-preserved plants and animals . It forms part of the Francis Creek Shale which was deposited in a deltaic estuarine environment 300 million years ago, during the Middle Pennsylvanian of the Carboniferous period (Figure 1) . Most of the Mazon Creek fossils are preserved in siderite concretions that were retrieved from the Carboniferous Coal Measures and their associated spoil heaps . The Mazon Creek nodules enclose organisms with hard and soft parts, as well as a variety of soft-bodied organisms . The terrestrial and freshwater plants and animals are known collectively as the Braidwood Biota, and the estuarine and marine animals are referred to as the Essex fauna .