The Illinois state fossil is a strange creature with pointed eyes and a long nose-like appendage with teeth, nicknamed the “Tully monster.” Specimens typically measure just 15 centimeters (about 6 inches), but the tiny creatures have sparked a major decades-long scientific debate about whether they should be classified as vertebrates or invertebrates. That mystery may now have been solved, according to a team of Japanese scientists who say their 3D scans of a generous sampling of fossils rule out the vertebrate hypothesis. They described their findings in a recent paper published in the journal Nature.
The fossil is named (Tullimonstrum gregarious, or “Tully’s common monster”) by Francis Tully, an amateur fossil collector who discovered the specimen in 1955 while scouring the fossil beds of Mazon Creek in Illinois, the only site where monster fossils have been found by Tully. He had never seen anything like this “torpedo” shaped fossil and took it to paleontologists at the Field Museum of Natural History in Chicago for identification. But paleontologists couldn’t figure out how to classify it.
While it may look like a snail at first glance, Tully monster fossils have several unique features, most notably an elongated, flexible proboscis (long nose with teeth) and outwardly bulging eyes on stalks, resembling those of a hammerhead shark. Tully has been compared to gastropods (snails and slugs), conodonts (an extinct group of jawless vertebrates), polychaetes (segmented marine worms), nemertheans (ribbon worms), and nectocarids (a Cambrian squid-like organism) in the following decades . If it were a vertebrate, then Tully’s monster would fill a critical gap in evolutionary history, linking jawless fish (such as lampreys and hagfish) to fish with jaws.
The strongest case for classifying Tully as a vertebrate rests on a pair of 2016 papers, one a detailed morphological study, the other focusing on the anatomy of the creature’s eye. In particular, the authors of the morphological study stated that Tully had a shaft made of cartilage (notochord) similar to a spine; multiple rows of insect-like stylets or piercing teeth near the mouth, similar to lampreys; an elongated segmented body with tail fins; and gills. The same team followed up with a 2020 study in which they used Raman microspectroscopy on several Mazon Creek fossils, concluding that these results also support their vertebrate hypothesis. Those spectral signal results were later replicated in a 2022 study.
The second 2016 study concluded that Tully should be grouped with vertebrates because the pigment granules in the eye, called melanosomes, were arranged in shape and size much like vertebrate eyes. But other scientists questioned that conclusion with a 2019 study investigating the chemical composition of fossil samples of Tully’s monsters, as well as samples of animals still living today. Those scientists used synchrotron radiation to hoe the samples, causing each element to release a unique X-ray signature that could be detected to identify the sample’s composition.
The 2019 study argued that some invertebrates (particularly octopuses and squid) also have melanosomes arranged in shape and size, weakening that element of the pro-vertebrate case. They found that the ratio of zinc to copper in Tully’s eyes was more similar to the ratios found in invertebrates rather than vertebrates. And the copper found in Tully’s eyes differed from the copper found in vertebrate eyes, although it also didn’t match the copper found in invertebrates.
This latest study takes advantage of the extraordinary preservation of body tissue of the Mazon Creek fossils due to rapid burial in a silty deluge with a unique chemistry that essentially buried the remains of dead creatures in a siderite crust and slowed decay. When I saw the specimens in the museum, I realized that there are tiny surface irregularities that have never been studied in detail, co-author Tomoyuki Mikamia graduate student at the University of Tokyo at the time of the study and now with the National Museum of Nature and science told by Scientific American. I thought these could probably be a clue to understanding Tully’s monster.
The Japanese team closely studied 153 Tully monster specimens and 74 other animal fossils found at Mazon Creek. They used a 3D laser scanner to produce color-coded 3D maps to better investigate tiny irregularities on the surface, revealed by color variations. The technique has been used in the past to study dinosaur footprints. They also used X-ray computed tomography (which takes cross-sectional images of an object) to create a 3D model of a Tully monster, the best way to study its distinctive proboscis and other anatomical features in more detail.
The researchers concluded that Tully’s body segmentation “was clearly different” from that of vertebrates such as chordates, in that the segmentation extended in front of its eyes. Other vertebrate-related traits in the 2016 studies, such as gill pouches and fin rays, were either missing from the fossils or not structurally similar to traits seen in vertebrates. teeth found in vertebrates such as lampreys and hagfish.
We believe the mystery of whether it’s an invertebrate or a vertebrate has been solved, Mikami said. Based on multiple lines of evidence, the Tully monster vertebrate hypothesis is untenable. Most importantly, Tully’s monster had segmentation in the head region that extended from the body. This feature is not known in any vertebrate lineage, suggesting a non-vertebrate affinity.
While Mikami et al. are confident in their invertebrate classification for Tully’s monster, they are more vague about exactly what kind of invertebrate it might have been. One possibility is that Tully was a chordate, a small eel-like marine invertebrate. Alternatively, it may have been a kind of protostome, comprising insects and crustaceans, just a radically modified version.
Other scientists find the study interesting but are more cautious about declaring the mystery solved, including paleontologist Victoria McCoy, co-author of one of the 2016 studies supporting a classification of vertebrates, who is now at the University of Wisconsin at Milwaukee. “I was very interested in seeing the application of 3D imaging techniques to Tully monster fossils,” McCoy told Ars. “I was especially excited to see the 3D reconstructions of the teeth, which really helped clarify their morphology. In general, it’s very difficult to interpret the preserved morphology of any Mazon Creek organism, including the Tully monster, and these types of 3D imaging methods can help with that.”
“However, we are still left with a key set of interpretations,” added McCoy. “The Tully monster was a segmented animal and had segments in a W or V shape, with protein teeth and two different morphologies of melanosomes in its eyes. The only phylum that really fits this set of characteristics are the chordates. All The chordate interior, large body size, and large complex eyes of the Tully monster are more consistent with a vertebrate identity in general. However, some of the issues raised in this article, such as the suggestion that the Tully may have segments in front of the his eye, do not strengthen the thesis of a non-vertebrate chordate affinity”.
DOI: Nature, 2023. 10.1111/pala.12646 (DOI information).