Grundy County fossil cache a window to Illinois’ prehistoric past

The Mazon Creek Fossil Bed in Grundy County is one of the country’s most pristine fossil caches, located in a shale deposit that was once a tropical river system nearly 300 million years ago. Mazon Creek is distinguished from other fossil beds in the area for its serendipitously perfect conditions, allowing for many rare soft-tissued plants and animals to be preserved in the sediment. 

Mazon Creek fossil beds were discovered in 1857, and they are world famous because they are so abundant and diverse. There are 400 different plant specimens and 320 animal fossils at Mazon Creek. The fossils were found in ironstone concretions of Francis Shale, which is unique to Mazon Creek.  Fossils were also recovered from nearby strip mines, shaft mines, and mine spoil piles. The Illinois State Museum has an important collection of Mazon Creek fossils, including numerous type and figured specimens. 

The 25 to 30 meters of Francis Creek Shale were formed over 300 million years ago at Mazon Creek, which is a tributary of the Illinois River near Morris. The fossils are found inside concretions which are scattered throughout the strip mine overburden and have been dumped in piles across the entire area. The State of Illinois acquired some of the strip-mined land and opened it as a wildlife preserve, but the area is open to fossil hunters too. 

Fossil formation was unique at Mazon Creek, with many animals living in the shallow marine bays and swampy areas along the rivers. As animals died, their remains were eventually washed into bays. The remains were rapidly buried in the mud washing in from the rivers, and this process protected the remains from being destroyed. Carbon dioxide combined with iron from the groundwater around the remains, forming siderite, or ironstone. This combination of rapid burial and formation of siderite resulted in excellent preservation of the many plants and animals. 

Over 300 million years ago, the land that is now Illinois was covered in heavily forested deltaic swamps. The environment of the Pennsylvanian era in Illinois was comparable to the modern Amazon River delta. Most fossils in Mazon Creek were commonly found in other areas of the country, but the ‘Tully Monster’ was unique. This slender soft-bodied creature with a long, narrow snout and sensory organs set away from the body on stalks has come to represent Illinois as the State Fossil. Francis Tully found the first one in 1958, and after taking it to the Field Museum of Natural History in Chicago to be identified, scientists were stumped. The creature eventually became known as the Tully Monster, as nicknamed by museum staff.  The Tully Monster, a free-swimming carnivore, ranged in length from three to 14 inches. 

There is no scientific evidence that dinosaurs once existed in Illinois, but the state’s geologic sediments were being eroded away, rather than actively deposited, during the Mesozoic Era. Illinois was home to a significant number of amphibians and invertebrates dating to the Paleozoic Era, as well as a handful of Pleistocene pachyderms. Examples include the Tully Monster, Amphibamus, Greererpeton, Lysorophus, Mammoth and Mastodon. 

“The window to Illinois’ prehistoric past can be found at the Mazon Creek Fossil Bed in Grundy County,” stated Rep. Jackie Haas (R-Kankakee). “This is history sitting right in front of us and it’s an incredible place to visit, imagine, and learn what animal life was like here 300 million years ago.”