What We Know About the Mysterious 'Tully Monster'

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A reconstruction of the so-called 'Tully Monster,' which also happens to be the state fossil of Illinois. Dotted Yeti/Shutterstock

Ever since its 300-million-year-old fossils were first unearthed in 1958, the alien-like "Tully Monster" has defied classification.

This bizarre creature featured a narrow, trunk-like neck that extended from its head, with a mouth at the end filled with razor-sharp teeth. Its eyes sat further back on the body at the ends of a rigid bar perched on its back, and it swam using cuttlefish-like fins at the tail section.

Needless to say, it looked more like a chimera or hoax than any sort of real creature. It was unlike anything else ever found on Earth.

In April 2016, a Yale-led team of paleontologists said they had determined what this animal was, reports Phys.org.

It's a vertebrate, according to researchers, and its closest living relative is probably a lamprey. With painstaking, high-tech analysis of its fossils, the Yale team was able to establish that the Tully Monster had gills and a stiffened rod or notochord (basically, a rudimentary backbone) that supported its body.

"I was first intrigued by the mystery of the Tully Monster. With all of the exceptional fossils, we had a very clear picture of what it looked like, but no clear picture of what it was," said Victoria McCoy, lead author of the study published in the journal Nature.

"Basically, nobody knew what it was," added Derek Briggs, co-author of the study. "The fossils are not easy to interpret, and they vary quite a bit. Some people thought it might be this bizarre, swimming mollusk. We decided to throw every possible analytical technique at it."

Another study, also published in the journal Nature, showed that the monster's eyes had melanosomes, which make and store melanin. Those structures are typical of vertebrates, according to the researchers, giving that theory more credence.

Or maybe it doesn't have a spine

Tullimonstrum, group of Tully Monsters swimming in the ocean
A artist's concept of a group of Tully Monsters swimming in the ocean. Dotted Yeti/Shutterstock

However, about a year later, a different team of researchers said there was no spine there after all. In their study, published in the journal Paleontology, they said the Tully Monster was likely an invertebrate.

"This animal doesn't fit easy classification because it's so weird," lead researcher Lauren Sallan, an assistant professor in the Department of Earth and Environmental Science at the University of Pennsylvania, said in a statement. "It has these eyes that are on stalks and it has this pincer at the end of a long proboscis and there's even disagreement about which way is up. But the last thing that the Tully Monster could be is a fish."

Sallan and her team said the studies failed to definitively classify the creature as a vertebrate.

"Having this kind of misassignment really affects our understanding of vertebrate evolution and vertebrate diversity at this given time," Sallan said. "It makes it harder to get at how things are changing in response to an ecosystem if you have this outlier. And though of course there are outliers in the fossil record — there are plenty of weird things and that's great — if you're going to make extraordinary claims, you need extraordinary evidence."

So how do we identify the creature?

Tully monster, Tullimonstrum gregarium fossil, Mazon Creek, Illinois.
A Tully Monster fossil, showing its proboscis and eye bar or stalks, found in the Mazon Creek fossil area of Illinois. Kimberly Boyles/Shutterstock

The technology that may make the identification of the Tully Monster possible is a method known as synchrotron elemental mapping, which illuminates an animal's physical features by mapping the chemistry within a fossil.

McCoy — one of the authors on the first study — teamed up with Yale colleague Jasmina Wiemann, who is a specialist in chemical analysis. They studied 32 samples from Mazon Creek rocks, which led them back to McCoy's original conclusion, that the creature was most closely related to a lamprey.

Of course, it's still not a definitive answer.

Thousands of Tully Monster fossils have been found, but all of them have been unearthed at a single site: coal mining pits in northeastern Illinois. So as far as researchers know, these animals could have been distinct to a specific habitat. They were named after their initial discoverer, Francis Tully, and their official scientific designation is Tullimonstrum gregarium.

The Tully Monster is an oddity in any group, Robert Sansom at the University of Manchester, who co-authored the 2017 paper, told New Scientist for a May 2020 article. "If it's a mollusc, it's a weird mollusc. If it's a vertebrate, it's a weird vertebrate."

The fossils have taken on a sort of celebrity status in Illinois, where they have been declared the state fossil — clearly identified or not.

The creatures are so unfamiliar that they're pretty terrifying, and those teeth certainly don't help, but the largest Tully Monster ever found only measures about a foot long. That means that if they were alive today, humans probably wouldn't be on their menu. It's difficult to say much of anything about their behavior, though.

"It's so different from its modern relatives that we don't know much about how it lived," McCoy said. "It has big eyes and lots of teeth, so it was probably a predator."