Unearthing Ancient Secrets

So when Francis Tully went hunting for fossils there, he never expected to make a major discovery. In 1955, Tully, a private collector and amateur paleontologist, came upon several bizarre worm-like fossils. The fossil imprint was very unusual; he couldn’t find a description of it anywhere. He knew it was a unique find, but didn’t realize how unique until three years later when he took the fossils to Dr. Eugene S. Richardson at the Field Museum of Natural History in Chicago. Richardson was also stumped by the find and confirmed that Tully had found the remains of a new kind of animal. Thus, the fossil was nicknamed the Tully Monster after its discoverer.

Thanks to fossilized remains, scientists have made invaluable discoveries about our planet’s past and the prehistoric animals that inhabited it. Mazon Creek is a gold mine for such findings because of its ability to preserve soft-bodied animals. Most fossils consist of the remains of plants and animals that have hard parts like teeth, shells, or bones. Animals such as worms and jellyfish are seldom found as fossils even though they probably were as common as they are today.

Tullimonstrum gregarium (Tully's common monster)
photo courtesy of Illinois State Geological Survey

Most Tully Monsters has been found in what is commonly referred to as Pit No. 11 ­ the Peabody Coal Company Strip Mine ­ along the banks of Mazon Creek in Grundy County (80 km/50 mi south of Chicago).

But unlike other species fossilized in Mazon Creek, the Tully Monster is the only one native exclusively to Illinois. This encouraged Illinois to claim the Tully as its own by naming it the “official state fossil” in 1989.

Since Francis Tully’s first find, thousands of other Tully Monster fossils have been unearthed, proving that this worm-like creature had many relatives. What group of animal it belongs to, however, is another question.