Discovered in the 1950s and first described in a paper in 1966, the Tully Monster (Tullimonstrum gregarium) is an enigmatic creature that lived about 300 million years ago. Unique to the Mazon Creek Formation in Illinois, it became its state fossil in 1989.

Soft-bodied and relatively small (a living specimen would fit in a human hand), with its stalked eyes and long proboscis, it is difficult to compare to all other known fossil or extant animal groups, confounding paleontologists since its discovery.

In 2016, a group of scientists in the U.S. proposed a hypothesis that the Tully monster was an early vertebrate similar in some anatomical details to the modern cyclostomes (a group including today jawless fish like lamprey and hagfish).

Despite considerable effort, studies both supporting and rejecting this hypothesis have been published in recent years, and so a consensus had not been reached.

Now, using 3D imaging technology, a team in Japan believes it has found a definitive answer.

“We believe that the mystery of it being an invertebrate or vertebrate has been solved,” said Tomoyuki Mikami, a doctoral student in the Graduate School of Science at the University of Tokyo at the time of the study and currently a researcher at the National Museum of Nature and Science.

“Based on multiple lines of evidence, the vertebrate hypothesis of the Tully monster is untenable.”

The team studied more than 150 fossilized Tully monsters and more than 70 other varied animal fossils from the Mazon Creek Formation. In the mud layers deposited around 300 million years ago at the bottom of a basin, marine organisms are preserved inside carbonate concretions. The concretions preserved the soft-bodied organism from further decay, but unfortunately the fossils are often flattened due to the weight of the overlying sediment.

With the aid of a 3D laser scanner, the researchers created color-coded, three-dimensional maps of the fossils that showed the tiny irregularities which existed on their surface through color variation (the same technology is often used to study dinosaur footprints). X-ray micro-computed tomography (which uses X-rays to create cross sections of an object so that a 3D model can be created), was also used to look at the Tullimonstrum’s proboscis. This 3D data showed that features previously used to identify the Tully monster as a vertebrate were not actually consistent with those of vertebrates.

According to the scans, the fossil doesn’t show a subdivision of the body in single muscle segments (called myomeres) as a distinguishing feature of vertebrates. Also, the proboscis is a segmented elongation of the animal’s head, showing some similarities to anatomical structures as found today in invertebrates like insects.

“This characteristic is not known in any vertebrate lineage, suggesting a nonvertebrate affinity,” Mikami explains.

Although the researchers are confident from this study that the Tully monster was not a vertebrate, the next step of the investigation will be to answer what group of organisms it actually does belong to.

Problematic fossils like the Tully monster highlight the challenge of piecing together the dynamic history of Earth and the diverse organisms that have inhabited it.

“There were many interesting animals that were never preserved as fossils,” Mikami concludes.

“In this sense, research on the fossils from Mazon Creek is important because it provides paleontological evidence that cannot be obtained from other sites. More and more research is needed to extract important clues from Mazon Creek fossils to understand the evolutionary history of life.”

The study “Three‐dimensional anatomy of the Tully monster casts doubt on its presumed vertebrate affinities” was published in the journal Palaeontology. Material provided by the University of Tokyo.