Every now and again, scientists discover fossils that are so bizarre they defy classification, their body plans unlike any other living animals or plants. Tullimonstrum (also known as the Tully Monster), a 300m-year-old fossil discovered in the Mazon Creek fossil beds in Illinois, US, is one such creature.
Artist’s impression of Tullimonstrum. PaleoEquii/Wikipedia, CC BY-SA
Tully appears to be a slug at first impression. Instead of a mouth, the monster possesses a long thin appendage that ends in what appears to be a pair of grabbing claws. Then there are its eyeballs, which protrude on stalks from its body.
Tully is so odd that scientists aren’t sure if it’s a vertebrate (a creature with a backbone, like mammals, birds, reptiles, and fish) or an invertebrate (without a backbone, like insects, crustaceans, octopuses and all other animals). In 2016, a group of experts claimed to have solved the Tully mystery, giving the most conclusive evidence yet that it was a vertebrate. However, my colleagues and I have done a new investigation that calls this conclusion into question, implying that this creature is as enigmatic as ever.
The Tully Monster was discovered in the 1950s by Francis Tully, a fossil collector. Tully has baffled experts since its discovery as to which group of current animals it belongs to. Tully’s real evolutionary links remain a mystery, leading to its designation as the state fossil of Illinois.
The Tullimonstrum fossil. Ghedoghedo/Wikimedia, CC BY-SA
There have been numerous efforts to categorize the Tully Monster. The majority of these studies have concentrated on the look of some of its more noticeable characteristics. These include a longitudinal structure in the fossil that has been interpreted as evidence of a gut, the fossil’s light and dark banding, and the strange gripping claws of its mouth. The Tully Monster’s body plan is so odd that it will considerably expand the diversity of whatever group it eventually belongs to, changing the way we think about that group of animals.
The 2016 study stated that the animal should be classified as a vertebrate because its eyes contain pigment granules called melanosomes that are arranged in the same shape and size as those in vertebrate eyes. However, our research reveals that the eyes of some invertebrates, such as octopus and squid, include melanosomes that are partitioned by form and size in a manner similar to Tully’s eyes, and that these can be retained in fossils.
Particle accelerator research
We used a sort of particle accelerator known as a synchrotron radiation lightsource at Stanford University in California to accomplish this. This enables us to investigate the chemical composition of materials from ancient and modern creatures. The synchrotron uses powerful bursts of radiation to “excite” the elements within specimens. When each element is stimulated, it emits X-rays with a distinct signature. We can tell what atoms were excited and, ultimately, what the specimen is made of by detecting the emitted X-ray fingerprints.
First, we discovered that melanosomes from modern vertebrate eyes have a higher zinc-to-copper ratio than the modern invertebrates we analyzed. Surprisingly, we discovered the similar pattern in fossilized animals and invertebrates found in Mazon Creek.
We then analysed the chemistry of Tully’s eyes and the ratio of zinc to copper was more similar to that of invertebrates than vertebrates. This suggests the animal may not have been a vertebrate, contradicting previous efforts to classify it.
Another possible look for the Tully Monster. Nobu Tamura/Wikimedia, CC BY-SA
We also discovered that Tully’s eyes contain a different sort of copper than vertebrate eyes. However, the copper was not equivalent to that found in the invertebrates investigated. So, while our findings support the hypothesis that Tully is not a vertebrate, they do not definitively classify it as an invertebrate.
What are our next steps? A more comprehensive study of the chemistry of melanosomes and other pigments in the eyes of different invertebrates would be a desirable next step. This may assist to narrow down the animal group to which Tully belongs.
Ultimately the riddle of what kind of creature the Tully Monster is continues. But our research demonstrates how studying fossils at the chemical and molecular levels can play an important part in figuring out the identity of this and other enigmatic creature.