Tully Monster Mystery Still Baffles After 300 Million Years

Discovered 70 years ago, the 300-million-year-old Tully Monster mystery remains a mystery. Is it a vertebrate or not? Scientists still can’t agree.

What’s the story

An animal that lived 300 million years ago is still keeping scientists up at night. Known as the Tully Monster, this weird sea creature had eyes on a stiff rod, a long snout with clawed teeth, and a fish-like body. First found in 1955 near Chicago, it left fossil experts stumped. Even after seven decades, it refuses to sit neatly in the family tree of life. Some say it was a worm, others a mollusk or even a kind of fish. Victoria McCoy, a paleobiologist who saw its picture in a book as a kid, thought she’d solve the mystery as an adult. But like many others before her, she’s still searching for the truth. The Tully Monster is one of science’s most confusing creatures, and its secrets remain buried in stone.

Creature design
Tully’s looks continue to puzzle scientists

Tully’s looks made it unforgettable—and unplaceable. It had a thin, stretched-out body, two eyes on a rod that looked like a handlebar, and a snout ending in a toothed claw. No wonder early scientists didn’t know what to make of it. In fact, the Field Museum in Chicago couldn’t even decide what kind of animal it was—whether it belonged with worms, mollusks, or arthropods. With over 3,000 fossil samples found in the Mazon Creek beds alone, it’s one of the most common yet most confusing creatures from the Carboniferous Period. Despite being young by fossil standards, around 300 million years old, it continues to mess with scientists’ heads. According to the journal Science, this creature challenges the basics of how we sort life. It doesn’t look like anything we’ve ever seen before—and it might not fit in any known category at all.

Fossil dilemma
Why the record only deepens the mystery

The fossil record is usually pretty helpful. But not when it comes to the Tully Monster. Even though we’ve got loads of fossils—more than 3,500 from the same area—they all miss crucial internal details. Most of the fossils from the Mazon Creek area are squished flat, making it hard to tell what’s what. In 2016, some scientists claimed it had a notochord, meaning it could be a vertebrate like fish or humans. But others argued back that it was an invertebrate with a very tricky body design. In 2023, researchers from the University of Tokyo scanned 150 fossils and said its parts don’t match any known vertebrate. According to the journal Paleontology, they think it may be an invertebrate chordate, like sea squirts or lancelets. That just adds another twist to the story.

Quick Fact Box

• Year discovered: 1955
• Location: Mazon Creek, Illinois, USA
• Age: Around 300 million years
• Known fossils: Over 3,500
• Still unclassified in the tree of life

Classification war
Experts still can’t reach an agreement

In 2016, a team led by McCoy argued it was a vertebrate. They pointed to a structure that looked like a notochord and pigments in the eyes matching known vertebrates. For a moment, it felt like a breakthrough. But right after that, another group said, “Wait a minute.” They found that decay in invertebrates could cause similar pigment patterns. And just like that, the debate raged on. A 2023 scan using 3D lasers gave new data. It showed the creature lacked true vertebrate features, like a skull or muscle segments typical of fish. According to researchers, the fossil’s odd parts don’t line up with anything we’ve seen in the vertebrate world. That keeps the Tully Monster floating in limbo, stuck between two massive branches of the tree of life.

Eye mystery
Strange vision organs deepen the debate

The eyes of the Tully Monster are one of its strangest features. They’re on the ends of a rod that sticks out like a barbell across its back. That alone makes it unique. But when researchers took a closer look, they found melanosomes—tiny structures that hold pigment. In vertebrates, these melanosomes form in distinct layers. When scientists saw similar patterns in the Tully Monster’s eyes, they thought it was a strong sign that it was a vertebrate. But newer studies showed the same patterns could form in squids after death. This raised doubts. According to a Proceedings of the Royal Society B paper, decay changes can fool us into thinking something is a vertebrate when it’s not. The eyes are still a hot topic in research. They might be the key to finally solving this mystery—but right now, they’re just another question mark.

Naming tale
How a hobbyist gave it a famous title

So, who named it the Tully Monster? Back in 1955, Francis Tully found the first fossil. He wasn’t a scientist—just a hobbyist. When he showed the fossil to the Field Museum, they were completely puzzled. For ten years, experts couldn’t figure out what it was. Then in 1966, Eugene Richardson officially named it Tullimonstrum gregarium—a nod to Tully and the fact that the creature seemed common in the fossil beds. It’s now the official state fossil of Illinois! The name might sound spooky, but the creature wasn’t a monster in the scary sense. Scientists just couldn’t place it anywhere in the animal kingdom. That kind of mystery earns you a dramatic name. Today, the Tully Monster is one of the most famous misfits in paleontology, all because a regular guy spotted something weird in a rock.

Body structure
A mashup of features no one expected

Let’s break down what this thing looked like. The Tully Monster had a narrow, flattened body that was soft, not armored. Its most iconic features were its long snout, with what looked like a claw at the end, and those barbell-like eyes. Some fossils show hints of body segments, but not the kind you see in worms or insects. Scientists have been trying to match its layout with known animals. They looked for fins, muscles, gills—even a brain structure. But the evidence doesn’t match anything exactly. In fact, the creature seems to mix features from multiple groups, which only adds to the mystery. A Nature paper mentioned that its body might show signs of evolution trying something totally new. That could explain why no one can pin it down. It’s like finding a tool in your garage and not knowing if it’s a wrench, a screwdriver, or something from another planet.

Modern links
How today’s creatures come close (but not quite)

To understand what the Tully Monster might have been, scientists have compared it to animals alive today. Some say it’s like a lamprey, a jawless fish with a sucker mouth. Others think it might be closer to sea squirts or lancelets, small, tube-like creatures that live in the ocean. But here’s the kicker: none of these matches are exact. The Tully Monster’s combo of features is just too weird. It had fins but not like fish, eyes but in a strange place, and a clawed snout we don’t see in any modern animal. According to the University of Leicester, the pigment structures in its eyes are vertebrate-like, but its body says otherwise. That mix keeps researchers guessing. It’s like trying to solve a puzzle where half the pieces don’t belong to any known picture. That’s why this fossil keeps popping up in science journals again and again.

Timeline oddity
Why its age makes it extra confusing

Usually, we expect bizarre fossils to be ancient—like from the Precambrian, around 540 million years ago, when evolution was still figuring things out. But the Tully Monster showed up much later, about 300 million years ago. That’s odd because, by then, most animals fit into neat categories. You had insects, fish, amphibians—all clearly sorted. So how did something so strange survive that long without relatives? That’s what bugs scientists like McCoy. The Tully Monster seems like it missed the evolutionary memo. Its oddball shape makes sense for a time when animal life was still chaotic, but not for an era that had already seen giant bugs and early reptiles. According to Nature, the creature is too “young” to be so mysterious, which makes scientists think it might actually belong in an existing group—we just haven’t figured out which one.

Hidden clues
Why solving the puzzle could unlock big answers

The Tully Monster isn’t just a scientific headache—it could be a gold mine. Why? Because once we figure out what it is, we’ll learn something big about how life evolved. Think of it like finding a weird branch on your family tree. You don’t just ignore it—you figure out who it was and why they mattered. Scientists believe that solving the Tully puzzle could help us understand how vertebrates or chordates evolved. Maybe it’s a missing link. Or maybe it’s an example of evolution trying out a dead-end design. Either way, it holds clues that could rewrite chapters in biology books. As Science magazine reports, knowing where the Tully Monster fits might reveal how body plans evolved over millions of years. That’s why people keep scanning it, modeling it, and arguing over it. Every detail matters when you’re trying to crack one of evolution’s biggest riddles.

Future search
The next big fossil could change everything

What’s next for the Tully Monster? More digging, better tools, and fresh fossils. McCoy hopes that new finds—maybe from areas outside Mazon Creek—could give us a clearer picture of its inner workings. Right now, we mostly have flat fossils, and they don’t show organs or muscles clearly. Scientists want 3D views. Maybe someone will develop a way to see what’s hidden inside the rocks. Or maybe a new fossil bed will turn up that preserves creatures differently. Until then, the mystery stays alive. But hope isn’t lost. McCoy says, “If we had a really clear picture of the internal organs and musculature, I think that would clarify a lot of things.” She’s still chasing the answer she dreamed of as a child. And maybe someday, one of us—or one of our kids—will find the clue that cracks the case.

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Reference Article – Nature