The Tully Monster

Illinois has many state fossils, but one has been a mystery since it was discovered more than 60 years ago. Now, scientists think they have figured out how to classify that creature. It is called a Tully monster, and it is a soft-bodied animal that swam in rivers more than 300 million years ago. The scientists now think the Tully monster was a creature with a spine. That would make it one of the earliest animals in the group called vertebrates that includes today's fish, birds, reptiles — and us.

The surreal-looking creature, dubbed Tullimonstrum gregarium and also known as the Tully monster, defies easy description.

"It looks like an alien," says Victoria McCoy of the University of Leicester, who authored the study while at Yale.

McCoy studied more than a thousand Tully monster fossils. She and her fellow scientists found that the Tully monster had an early version of a spinal cord, which would make it a vertebrate. They published their study in the science journal Nature on March 16, 2016.

Ancient Cousin To The Lamprey

The study shocked paleontologists, who for decades could not figure out how to classify the Tully monster on the tree of life. Most scientists thought the Tully monster did not have a backbone or spinal cord, which would make it an invertebrate. They thought a Tully monster was like an ancient version of a worm or an animal like an arthropod or mollusk. Instead, the study shows that it was an ancient cousin to a lamprey, a kind of fish that was one of the first animals with a backbone to evolve.

"A vertebrate! Amazing!" wrote Sam Heads of the Illinois Natural History Survey. He wasn't involved with the study.

A pipefitter and amateur fossil-hunter named Francis Tully discovered the Tully monster in 1955. He found it in a fossil bed known as the Mazon Creek formation while he was looking through a coal mine's scrap heap. It has been a mystery ever since.

Unlike Anything Seen Before

Tully thought the fossil was not like anything he had ever seen: an animal with a shovel-shaped tail, a trunk-like snout tipped with a claw-like mouth and eyes at the ends of a rigid bar. "None of the books had it," Tully recalled in a 1987 interview. "I'd never seen it in museums or at rock clubs. So I brought it to Chicago to the Field Museum to see if they could figure out what the devil it was."

Field Museum paleontologists were just as stumped. "No one recognized the creature," wrote staff paleontologist Eugene Richardson in 1966. "We could not even decide what phylum to put it in, and that was a serious and embarrassing matter."

After Tully's death in 1987, Field Museum researcher Mary Carman and scientists from the Illinois State Geological Survey asked legislators to make the Tully monster an official state symbol. The bill was surprisingly in dispute and some called it a waste of time. Still, in 1989, the Tully monster was named a state fossil, one of 40 such fossils with this honor.

Unique And Special State Symbol

"I was proud — very proud — that we helped the people of Illinois choose something that was so unique and special," says Carman. She is now at the Woods Hole Oceanographic Institution.

Regardless of its official title, the Tully monster continued to defy description. To find answers, McCoy partnered with the Field Museum beginning in October 2014 to review thousands of Tully monster fossils, which the museum had started digitizing at the same time.

Right away, the team was drawn to a light-colored band running from the creature's snout to the tip of its tail.

Earlier descriptions had labeled this band as the digestive tract. But several fossils revealed that the band bore the hallmark traits of a notochord, the rubbery tube surrounding some primitive vertebrates' spinal cords.

Features Of A Vertebrate

Once researchers began to see the Tully monster as a vertebrate, many of the Tully monster's strange features made sense. The scientists were able to compare the creature to modern lampreys.

Scans of the fossils showed that the creature's teeth probably were made of keratin, just like lamprey teeth (and human fingernails). A crescent-shaped hole in its midsection corresponded with modern lampreys' nostrils, suggesting that it could smell. In some fossils, the team even spotted the faint outlines of the creature's brain.

"It was about as smart as a modern lamprey," says McCoy. "Not exactly genius level, but enough to survive."

Identifying the Tully monster has been a dream come true for many of the study's authors. They first learned of the creature early in their careers as an example of paleontology's outstanding mysteries.

"It's been such a blast," says Scott Lidgard of the Field Museum, one of the study's co-authors.

Paul Mayer, Lidgard's Field Museum co-worker and a fellow co-author, even took a Tully monster-themed road trip in college, in a failed attempt to find his own fossils of it.

Movement And Eating Habits

The researchers' work isn't over. They now face the difficult task of figuring out how the Tully monster moved, ate and navigated its environment.

"It would be fascinating to watch it swim around," says Mayer. "How does a Tully monster make its living? We don't know."

Even so, the study resolves one of paleontology's most famous cold cases — and will likely change Illinois civics classes forever.