This little piece began as a sidebar to a much longer account of the coal industry in my never-published guide to the history and culture of Illinois. My curiosity about the geologic past was stimulated by my boyhood visits to the Illinois State Museum, where I tried to stretch my tiny brain wide enough to imagine Illinois as (among other things) a dank swamp of horsetails and club mosses the size of trees. 

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Too good to throw away, and it might whet the curiosity of readers new to Illinois geology. Here's a good quick introduction to that subject from the Illinois State Geological Survey. See also my profile of geologist Richard Leary here. 

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Three hundred million years ago, the future continent of North America straddled the equator, and Illinois was a vast lowland forest of primitive, nonflowering plants, part of a massive delta that lay along the edge of tropical southern Louisiana. Dead plants and animals that settled or were washed into low places were covered by mud; compacted under the pressure of its own weight, the muds turned into shale. 

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Remains of plant and animal remains that had fallen into  the mud were preserved when carbon dioxide from their decomposition reacted with iron in the mud to form a hard protective case of what is commonly known as ironstone. These mineral coffins preserved the precise form of even the soft body parts of hundreds of species of plants and land and sea animals. Split open one of these oval or lozenge-shaped ironstone concretions and one finds exquisitely etched portraits of plant leaves, stems, and reproductive structures such as seeds and spores. “Coal balls” contain not just impressions but actual plant remains, a result of their imperfect carbonization; these remains reveal in three-dimensions the structures of the plants entombed in them, even their cellular makeup.

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Outcrops of one shale formation in particular, the Francis Creek Shale laid down approximately during the Pennsylvanian Period, occur naturally along the banks of such Illinois streams as Mazon Creek. The stony pages of these abundant fossils constitute an exceptionally comprehensive and detailed catalog of Pennsylvanian-period vegetation. The Illinois State Museum, with pardonable pride, calls the Mazon Creek deposits “some of the most important fossil deposits in North America.” Mazon Creek nodules and fossils are found in museums worldwide, although the most important collections are at the Chicago’s Field Museum of Natural History and at the Illinois State Museum in Springfield. 

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Among the finds is Tullimonstrum gregarium. The "Tully Monster" was a small soft-bodied animal one foot or so long . It was first found by amateur fossil-hunter Francis Tully in 195. Since then more than one hundred have been found in the Mazon Creek area, and recently they have also been found in open-pit coal mines in central Illinois.

 

The creature was so nicknamed because it looks as it its body was cobbled together with bits of other animals. It had a pair of vertical fins at the tail end of its vaguely submarine-like body, and at the other end  a long proboscis with up to eight small sharp teeth on each "jaw." Two thin antenna-like structures protruded from each side, at the ends of which were round organs "suggestive of a camera-type eye." (A lifelike reconstruction of which may be seen at Chicago’s Field Museum, near the famous Pennsylvanian coal forest display.)

 

For years, the creature defied identification; current speculation is that the Tully Monster was probably an ancient fish akin to today’s bloodsucking lampreys. Because the creature is paleontologically important, prized by collectors, and unique to the Prairie State—and no doubt because its name makes it very box office—it was voted the official Illinois state fossil in 1987. ●