We don’t know what it ate or if it was a predator or scavenger," McCoy said.They analysed numerous fossils of the creature, named Tullimonstrum gregarium, and determined it was not a segmented worm or a free-swimming slug, as once hypothesised, but rather a type of jawless fish called a lamprey.
The notochord previously had been identified as the gut.
Up to about 14 inches (35 cm) long, it had a vertical tail fin and a long, narrow dorsal fin.."I’ve always loved detective work, and in paleontology it doesn’t get much better than this," said paleontologist James Lamsdell of the American Museum of Natural History in New York.
"Tullimonstrum shared its shallow marine environment with fish including sharks as well as jellyfish, shrimp, amphibians and horseshoe crabs.For more than half a century, scientists have scratched their heads over the nature of an outlandishly bizarre creature dubbed the Tully Monster that flourished about 307 million years ago in a coastal estuary in what is now northeastern Illinois.A sophisticated reassessment of the fossils determined it was a vertebrate, with gills and a stiffened rod, or notochord, that functioned as a rudimentary spinal cord and supported its body.It is called the Tully Monster in honour of amateur fossil-hunter Francis Tully, who first found it in Illinois coal-mining pits in 1958 and brought it to experts at the Field Museum in Chicago.
"Our re-study of the specimens has shown that it is a very strange lamprey, a group of eel-like vertebrates that live in rivers and seas today.
"I was blown away when the results started coming in.