A team of paleontologists from the United States has determined that Tullimonstrum gregarium (popularly known as the Tully monster) — a large soft-bodied animal from the late Carboniferous Mazon Creek biota (309-307 million years ago) of Illinois — was a vertebrate, with gills and a stiffened rod that supported its body. According to the team, T. gregarium is part of the same lineage as the modern lamprey.

Tullimonstrum gregarium — named after Francis Tully, the fossil hunter who came across it in coal mining pits in northeastern Illinois — lived during the Carboniferous period, about 307 million years ago.

It was discovered in 1958 and first described scientifically in 1966.

According to scientists, ‘Tully monsters’ look like something out of science fiction — aquatic animals with tube-shaped bodies up to one foot (30 cm) long, skinny snouts ending in a toothed jaw or claw, and eyes at the end of short stalks.

For many years, paleontologists couldn’t determine what kinds of animals these prehistoric ‘monsters’ actually were — they were categorized as soft-bodied invertebrates, with theories ranging from worms to shell-less snails.

But in a paper published this week in the journal Nature, Dr. Victoria McCoy from the University of Leicester and her colleagues from Yale University, the American Museum of Natural History, Argonne National Laboratory, and Yale Peabody Museum of Natural History, announced that Tullimonstrum gregarium is in fact a vertebrate.

“We had a very clear picture of what it looked like, but no clear picture of what it was,” Dr. McCoy said.

Using a collection of 2,000 specimens of Tullimonstrum gregarium, the paleontologists analyzed the morphology and preservation of various features of the animal. Powerful, new analytical techniques also were brought to bear, such as synchrotron elemental mapping, which illuminates an animal’s physical features by mapping the chemistry within a fossil.

The scientists concluded that the ‘Tully monster’ had gills and a notochord, which functioned as a rudimentary spinal cord. Neither feature had been identified in the animal previously.

“The animals are related to the jawless fishes that are still around today by a unique combination of traits, including primitive gills, rows of teeth, and traces of a notochord, the flexible rod-like structure along the back that’s present in chordate animals — including vertebrates like us,” said co-author Dr. Paul Mayer, from the Field Museum of Natural History.

“Tullimonstrum gregarium is so different from its modern relatives that we don’t know much about how it lived. It has big eyes and lots of teeth, so it was probably a predator,” Dr. McCoy said.

Some key questions about ‘Tully monsters’ remain unanswered, however.

“No one knows when the animal first appeared on Earth or when it went extinct,” the scientists said. “Its existence in the fossil record is confined to the Illinois mining site.”