The Tully Monster
Illinois's official state fossil
See Illinois (unpublished)
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.
Too good to throw away, and it might whet the curiosity of readers new to Illinois geology. Here's a good quick introduction from the Illinois State Geological Survey.
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. 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.
Remains within 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. “Coal balls” contain 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.
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 Grand Prairie 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.
Among the finds is Tullimonstrum gregarium. The "Tully Monster" was a small soft-bodied animal was probably an active, swimming carnivore. First found by Francis Tully in 1958, and 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 got its nickname because of its bizarre shape, and because at first it defied identification. (A lifelike reconstruction of which may be seen at Chicago’s Field Museum, near the famous Pennsylvanian coal forest display.) Because the Tully Monster 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. ●