A Monster in the Mud
In 1955, amateur fossil hunter Francis Tully was searching the spoil heaps of a coal mine in central Illinois when he cracked open a nodule of rock and found something unknown to science. The creature preserved inside, which lived 307 million years ago, was completely soft-bodied. It had a torpedo-shaped body that reached up to 35 centimeters (14 inches) long, a triangular tail fin, and its eyes sat on the ends of a rigid bar that stuck out sideways from its head. A long, flexible proboscis projected from its head, ending in a claw-like structure with eight small, sharp teeth.
The fossils are found exclusively in the Mazon Creek fossil beds, a geological formation known as a lagerstätte for its exceptional preservation of soft tissues. During the Pennsylvanian Period, this area was a muddy, tropical estuary. When organisms died, they were rapidly buried in silt and an iron carbonate mineral, siderite, formed concretions around them. These hard nodules preserved delicate structures like the bodies of jellyfish, shrimp, and the mysterious Tullimonstrum gregarium—the Tully Monster. Thousands of specimens have since been found, making it a common fossil in this one specific location.
The Classification Crisis
For decades, paleontologists could not place Tullimonstrum on the tree of life. It was compared to mollusks, worms, and arthropods, but none of the classifications fit. The central mystery is whether the Tully Monster was an invertebrate or a vertebrate. The debate appeared to be settled in 2016, when researchers using advanced imaging techniques on over 1,200 specimens identified a notochord—a primitive precursor to a backbone. This feature, along with structures interpreted as gills and teeth similar to modern lampreys, led them to classify it as a jawless fish.
This conclusion, however, was quickly challenged. By 2017, other research teams presented counter-evidence, arguing that the features identified as a notochord and gills were misinterpreted and did not align with the known structures of other vertebrate fossils from Mazon Creek. More recent studies using 3D imaging and chemical analysis have also failed to find definitive evidence of a vertebrate anatomy, suggesting the creature was an invertebrate after all. Today, the Tully Monster remains an enigma. It became the official state fossil of Illinois on August 31, 1989, becoming a scientific puzzle.