It looks like the most unearthly of aliens, or the figment of a bad dream — and has baffled scientists for nearly 60 years.
The so-called “Tully Monster” had a crane-like neck, tooth-filled jaws resembling a lobster claw, and eyes mounted at either end of a bar across the middle of its back.
Thousands of fossil specimens of the foot-long sea beast, that lived more than 307 million years ago, have been uncovered from a single site close to Chicago in Illinois, US.
Since its discovery in coal mining pits by amateur fossil hunter Francis Tully in 1958, the “monster” has acquired local celebrity status, with its image appearing on the bodywork of trucks and trailers.
Yet scientists had no idea what kind of creature it was — until now.
A new study has shown that the Tully Monster was a primitive vertebrate, an early ancestor of the lamprey, and that it had gills and a rudimentary spinal cord.
Previously there had been speculation that the animal was some kind of nightmarish worm or snail.
Paul Mayer, from the Field Museum in Chicago, which houses 2,000 Tully Monster specimens, said: "The monsters 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.
A wonderful fossil
“The Tully Monster is a wonderful fossil that captures the imagination of every school kid.
“When I talk to school groups, I used to use the Tully monster as an example of a mystery that palaeontologists have been trying to solve ever since it was discovered.”
To date, every specimen of the Tully Monster, Tullimonstrum gregarium, has been found in the fossil-rich Mazon Creek region, 80km south-west of Chicago.
When the creature was alive, the area was a swampy shoreline bordering a tropical sea.
After its discovery the “monster” was described scientifically in 1966, but experts were at a loss to categorise it — even at the level of phylum, one of the major animal groups that include molluscs and chordates.
Prof Derek Briggs, from Yale University, who co-authored the Tully Monster's new description in the journal Nature, said: "Basically, nobody knew what it was.
“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.”
Scientists examined the Tully Monster more closely than ever before using powerful techniques such as synchrotron elemental mapping, which illuminates a fossil animal’s physical features by analysing its chemistry.
The researchers concluded that the Tully Monster had gills and a notochord, which functioned as a primitive spinal cord.
Many questions remain, however. No-one knows when the Tully Monster first appeared on Earth, when it disappeared, or where else it might have made its home.
Mr McCoy added: “It’s 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.”
PA