Scott Lidgard, a paleontologist at Chicago's Field Museum, said that it goes without saying that the Tully monster, Illinois' official state fossil, was "very, very bizarre" but but no one could say exactly what it was.
But the puzzle ended on Wednesday with a paper published in the journal Nature in which a group of researchers, including Lidgard concluded the Tully monster had a precursor to a backbone, making it a vertebrate, or primitive fish.
He added that the question related to the origin of vertebrates is open and there aren't many good examples of preserved, soft-bodied fishes. The research is also very important for Lidgard as he has been given the charge of the museum?s collection of 1,800 Tully monster fossils, the largest in the world.
"I'm pretty excited because I've had to live with this guy, something I wish I knew what it was, for my 32 years here," he said. "And it's been a puzzle."
The fossilized remains of the Tully monster have not been found outside Illinois, only in the Mazon Creek area, about 50 miles southwest of downtown Chicago. It is believed that over 300 million years ago, the spot was a warm coastal marsh along a long-gone sea near the equator.