This 300-Million-Year-Old Fossil Is So Strange, Experts Still Don’t Know What It Is

First discovered in the Mazon Creek fossil beds of Illinois in 1955, the Tully monster (or Tullimonstrum gregarium) has long challenged biologists and paleontologists alike. It lived about 300 million years ago, predating the rise of the dinosaurs, and yet its classification within the tree of life remains hotly debated to this day. In a landmark 2016 study published in Nature, researchers claimed the fossil had characteristics of a vertebrate, possibly related to jawless fish like lampreys. But nearly a decade later, no consensus has been reached.

A Discovery That Defied Classification

The Tully monster’s journey into scientific lore began with Francis Tully, an amateur fossil collector who unearthed the first specimen near Chicago. His find was so unusual, he felt compelled to bring it to the Field Museum for identification. As he later told the Chicago Tribune, “None of the books had it. I’d never seen it in museums or at rock clubs. So I brought it to the Field Museum to see if they could figure out what the devil it was.”

Museum staff, including Eugene S. Richardson Jr., found themselves equally baffled. “We could not even decide which phylum to put it in,” Richardson later wrote — a stunning admission, considering that a phylum is among the broadest taxonomic classifications. This ambiguity was, as he put it, “a serious and embarrassing matter.” In 1966, a decade after the initial discovery, the creature was officially named Tullimonstrum gregarium, not out of newfound clarity, but to honor the fossil’s ubiquity and its original discoverer. Since then, its enigmatic body — with a torpedo-like form, a long proboscis tipped with a claw, and eyes mounted on a transverse bar — has become a staple of paleontological puzzles.

Competing Theories and the 2016 Vertebrate Claim

In the decades following its discovery, scientists attempted to place the Tully monster within several known groups. Some proposed it was a segmented worm, others believed it was a mollusk, and still others argued for a classification among arthropods, which include creatures as diverse as insects and lobsters. This taxonomic tug-of-war continued until Victoria McCoy, now a paleobiologist at the University of Wisconsin, Milwaukee, made headlines with her 2016 Nature paper.

The team led by McCoy identified what appeared to be a notochord, a primitive spinal structure, suggesting the Tully monster was a vertebrate. Additional anatomical traits, such as possible gill structures, muscle segments, and eye composition, pointed toward relatives like lampreys. “With the arrogance of a child,” McCoy says, “I thought: Someday I will answer that question.” Her findings seemed decisive. As she recalls, “At the time, it was viewed as a smoking gun.” But this confidence soon met with skepticism. Later research revealed that some of the eye features thought to be exclusive to vertebrates could actually be produced in cephalopods (e.g., squid) under certain decay conditions — casting doubt on the interpretation.

The Invertebrate Counterargument and a New Wave of Analysis

In 2023, another team led by Tomoyuki Mikami at the University of Tokyo published results that directly challenged the vertebrate classification. Using 3D laser scanning on over 150 specimens, they reexamined the fossil’s internal structures. Their conclusion: the Tully monster lacked definitive vertebrate features. According to Mikami, the previous vertebrate identifications — including supposed brains, teeth, and ray fins — were not consistent with actual vertebrate anatomy.

Instead, they suggested the Tully monster may have been an invertebrate chordate — animals like lancelets or tunicates, which share a distant relationship with vertebrates but do not possess a backbone. This view would place Tullimonstrum among animals that today resemble sea squirts rather than fish. McCoy, though holding to her original conclusions, acknowledged this possibility: “It would not surprise me at all” if the chordate interpretation gained traction, she said. Still, she notes the large size and complexity of the eyes remain difficult to explain under this alternative theory.

Why the Tully Monster Remains Unresolved

Part of the difficulty in resolving the Tully monster’s identity lies in the nature of its fossilization. The Mazon Creek beds, while rich in specimens, rarely preserve fine anatomical details such as muscles or soft organs. “If we had a really clear picture of the internal organs and musculature, I think that would clarify a lot of things,” McCoy admits. “And you just don’t get that at Mazon Creek, except really rarely.” Without such clarity, paleontologists are left to interpret fossil imprints and mineral structures that can be misleading or ambiguous.

What makes this case particularly fascinating is the creature’s age. The Tully monster lived during the Carboniferous period, approximately 300 million years ago. Compared to earlier periods of evolutionary experimentation — such as the Precambrian, over 540 million years ago, when bizarre life forms were more common — the Tully monster is relatively young. That makes its unclassifiable nature even more puzzling. As McCoy puts it, “The Tully monster, by contrast, is very young to be so mysterious. An unclassifiable fossil from 600 million years ago, sure. But 300 million years ago? That’s unusual.” Still, she remains convinced the creature fits somewhere in the current taxonomic framework. “We wouldn’t expect there to be some additional phylum we don’t know about.”