Illinois State Fossil - Tully Monster (Tullimonstrum gregarium)
In 1955, a fossil hunter by the name of Francis Tully was exploring the spoil piles of a coal mine near Braidwood, Illinois. He was searching for fossils inside of 300 million year old ironstone nodules found in the Francis Creek Shale. Mr. Tully split one open to reveal something a very strange animal that no one had seen before. It had a long proboscis or snout on its head sporting a claw, spade-shaped fins on its rear and a horizontal bar underneath its head with an eye on each end. This prehistoric oddity would eventually be named Tullimonstrum gregarium (commonly referred to as a Tully Monster) and it would go on to confound scientists for decades.
In the decades since its original discovery hundreds of additional specimens have been found in Illinois, some up to 14 inches long. In 1989 this enigmatic animal was declared the Illinois state fossil which is quite fitting as Illinois is the only location in the world similar animals have been found.
Since its discovery the Tully Monster has been described as a nematode, polychaete worm, mollusc, a stem arthropod and a chordate. Paleontologists couldn’t even decided if it was an early vertebrate or an invertebrate. In the past decade, two papers have sought to investigate the additional specimens using new analytical methods.
These studies discovered several previously unknown anatomical features pointing towards a chordate or early vertebrate identity. Most notably of these was a flexible rod-like structure called a notochord that was previously interpreted as a gut trace. Notochords are a defining characteristic of chordates, the phylum which includes vertebrates. In vertebrates, the notochord is what develops into the spinal column.
The paleontologists also also noted similarities between the teeth of the Tully Monster and those of lampreys and hagfish. Then focusing in on what was believed to be eyes, they also found the presence of both spherical and cylindrical melanosomes, another defining, another defining characteristics of chordates. Based on this research we finally have an understanding of how the tully monster fits into the tree of life.
Or do we... Another more recent study has again put it's identity back into doubt. Focusing in on the melanosomes in it's eyes, it came to the conclusion the structures more closely match those of invertegrates.