Armigatus Fish Fossil Specimen

The Mazon Creek fossils are found in the Mazon Creek, a tributary of the Illinois River near Morris, Grundy County, Illinois during the Pennsylvania age, some 300 million years ago. At that time the area was a delta with a tropical climate, as it was then within 10° north latitude of the equator. Because the fern leaves were rapidly buried by the sediment deposited in the deltaic system, they have retained their leaf structure. Bacterial decomposition of the remains produced carbon dioxide that combined with dissolved iron from the groundwater. This process formed siderite in the sediments surrounding the remains, forming detailed casts of their structure. Lithification of the sediments formed protective nodules of ironstone around the now fossilized remains. The ironstone concretions are recovered from exposures along streams, roadcuts and in active or abandoned coal mine areas. Once found in ample quantities, the concretions are now more difficult to locate.