For decades, the Tully Monster has baffled everyone since its discovery in 1958 by Francis Tully in the coal mining pits in northeastern Illinois, US. Finally, a team of researchers led by Yale University reveal that the monster was actually a vertebrate with gills and a stiffened rod for body support that lived in Illinois 307 million years ago.

It has now been officially named as Tullimonstrum gregarium. The study published in the journal Nature on March 16 says that it was a sea creature with eyes located at the end of short stalks, a body shaped like a tube and narrow snouts that have toothed jaw or claw at the ends.

Actually, there were thousands of Tully monsters discovered in the area, many of which were donated to the Field Museum of Natural History. The researchers used the 2,000 specimens from the museum to analyse the animals’ morphology and physical features. They found that the gills and rod, which have not been found before, stood as the spinal cord.

This is a holotype fossil specimen of the Tully monster. Photo by Paul Mayer, The Field Museum

This is a holotype fossil specimen of the Tully monster. Photo by Paul Mayer, The Field Museum

The Tully monster became the state fossil in Illinois back in 1989. Many residents have currently begun sticking the image on their vehicles, the species suddenly gaining celebrity status.

“Basically, nobody knew what it was,” added study co-author Derek Briggs, a professor of geology and geophysics at Yale’s G. Evelyn Hutchinson and curator of invertebrate palaeontology at the Yale Peabody Museum of Natural History. “The fossils are not easy to interpret, and they vary quite a bit. Some people thought it might be this bizarre, swimming mollusc. We decided to throw every possible analytical technique at it.”

The Tully monster was probably a predator based on its big eyes and teeth. It is related to the jawless fish that is still alive.

However, scientists still do not know how the Tully monster lived. They add that they are uncertain about how the creature appeared or why it went extinct.