Photo Credit: PaleoEquii / Wikimedia Commons CC BY-SA 4.0

A long, long, long time ago, there existed a sea creature that was similar to a cuttlefish… but it was also like a slug, and kind of resembled a leech as well. When a fossil of this bizarre creature was discovered hundreds of millions of years later, it was named for its discoverer and called the “Tully monster.” Scientists have been unable to agree on how the creature should be classified, shrouding the specimen in mystery.

Discovery of the Tully monster

The Tullimonstrum gregarium. (Photo Credit: Nobu Tamura / Wikimedia Commons CC BY-SA 4.0 / cropped)

The scientific name for the unclassifiable specimen is Tullimonstrum. The creature’s discovery was first made in 1958 when fossil collector Francis Tully noticed it in the fossil beds of Mazon Creek, located in Illinois. The particular fossil Tully found dates as far back as 307 million years old.

To look at the Tully monster, it’s reasonable to say that it’s reminiscent of a slug. At its mid-body, what are believed to be eyes protrude outward on two stalks, like those of a slug. However, in the place where one might assume the mouth would be located is a long, thin appendage with a claw at its end. This claw appears to have teeth.

The strange characteristics of the Tully monster make it difficult to classify it either as a vertebrate or invertebrate. As a reminder, a vertebrate is a creature with a backbone. Mammals, fish, birds, and reptiles are vertebrates. Invertebrates are creatures without backbones, like insects, octopuses, and crustaceans.

A 2016 study classified it as a vertebrate

A T. gregarium, or Tully monster, fossil. (Photo Credit: Ghedoghedo / Wikimedia Commons CC BY-SA 3.0)

In 2016, a group of scientists conducted a study and said the Tully monster had finally been classified. Their evidence suggested that it was, in fact, a vertebrate. The study focused on prominent features in the creature’s physical shape. Looking at the fossil, the researchers claimed that there was a notochord in the Tully monster.

A notochord is a flexible rod that runs down the length of the body. Although it’s not considered to be a spinal cord, it is considered to be the precursor to a spinal cord. So the presence of the notochord suggested to these scientists that the Tully monster is best classified as a vertebrate. In this way, they likened it to the same group as lampreys.

The study also looked at pigment granules located in the eyes known as melanosomes. By analyzing these, scientists noted that the shape and size were similar to those found in the eyes of other vertebrates, again placing it under that classification.

A newer study challenges this classification

3D model of Tullimonstrum gregarium as a vertebrate. (Photo Credit: Петр Меньшиков / Wikimedia Commons CC BY-SA 4.0)

For a time, the 2016 study was accepted as the closest to a definitive classification scientists were going to reach with the Tully monster. However, a newer study challenged the 2016 outcome, pulling the mystery of the Tully monster back into the scientific discourse.

This newer study also looked at melanosomes in the eyes. Researchers used a particle accelerator, called a synchrotron radiation lightsource, to get a deeper look at the chemical makeup of the melanosomes in the samples from Tully monster fossils and from current vertebrates and invertebrates. The particle accelerator overloads the specimen with intense bursts of radiation that cause the elements inside to become identifiable. Each of the elements has its own specific X-ray signature.

When analyzing the Tully monster’s eye zinc-to-copper ratio, it was determined that the creature more closely resembles invertebrates than vertebrates, contrary to the previous study’s claims. It was also found that the Tully monster’s eyes contained a different type of copper than vertebrates. What’s especially interesting is that even though this unusual creature is different than vertebrates, it’s not identical to invertebrates, either.

The newer study supports the idea that the Tully monster is not a vertebrate, but at the same time, it does not support the idea that it is an invertebrate. It seems scientists are back to square one with the Tully monster, and the mystery continues.