This is Mazon Monday post #161. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Discovered in the 1950s and first described in a paper in 1966, the Tully monster, with its stalked eyes and long proboscis, is difficult to compare to all other known animal groups. Unique to Illinois in the U.S., it became its state fossil in 1989. Credit: Takahiro Sakono, 2022.
Phys.org has a story about the new Tully Monster paper that was published in the journal Paleontology last week. The Tully Monster is the State Fossil of Illinois. And, although it is a popular animal, its evolutionary ancestry has been problematic. Since it's discovery in 1958 by Francis Tully, numerous theories of its evolutionary affinity have been proposed including arthropods, worms, vertebrates, mollusks, even conodonts.
For more than half a century, the Tully monster (Tullimonstrum gregarium), an enigmatic animal that lived about 300 million years ago, has confounded paleontologists, with its strange anatomy making it difficult to classify. Recently, a group of researchers proposed a hypothesis that Tullimonstrum was a vertebrate similar to cyclostomes (jawless fish like lamprey and hagfish). If it was, then the Tully monster would potentially fill a gap in the evolutionary history of early vertebrates. Studies so far have both supported and rejected this hypothesis.
Now, using 3D imaging technology, a team in Japan believes it has found the answer after uncovering detailed characteristics of the Tully monster that strongly suggest that it was not a vertebrate. However, its exact classification and what type of invertebrate it was is still to be decided.
In the 1950s, Francis Tully was enjoying his hobby fossil hunting in a site known as Mazon Creek Lagerstätte in the state of Illinois, when he discovered what would later become known as the Tully monster. This 15-centimeter (on average), 300-million-year-old marine "monster" turned out to be an enigma, as ever since its discovery researchers have debated where it fits in the classification of living things (its taxonomic position).
Eugene Richardson, who first called it "Mr. Tully's monster" recognized it didn't fit into any known classification when he described it in 1966. He famously wrote "While this obscure but plentiful animal is being studied, I prefer not to assign it to a phylum." In a later paper with Ralph Gordon Johnson, they wrote "There is no compelling reason to assign Tullimonstrum to any of the known phyla. It could be imagined as an aberrant member of one of several phyla but the critical evidence is not available." There were later attempts to classify it. The latest, by McCoy et al in 2016, again in 2016 by Clements et al, and by McCoy et al again in 2020, classify T. gregarium as a jawless fish similar to a lamprey or hagfish. If that theory is correct, the Tully Monster will fill in a gap in the evolutionary history of early vertebrates. There has been some disagreement (Sallan in 2017), although that study denies the vertebrate ancestry without proposing a viable alternative.
The new research was carried out by a team at the National Museum of Nature and Science. The researchers used 3D laser scanning and x-ray micro-computed tomography to reach their conclusions which refute the vertebrate ancestry of the Tully Monster. They also fail to propose a viable alternative classification.
Three-dimensional anatomy of the Tully monster casts doubt on its presumed vertebrate affinities
Abstract
Tullimonstrum gregarium, also known as the Tully monster, is a well-known phylogenetic enigma, fossils of which have been found only in the Mazon Creek Lagerstätte. The affinities of Tullimonstrum have been debated since its discovery in 1966, because its peculiar morphology with stalked eyes and a proboscis cannot easily be compared with any known animal morphotypes. Recently, the possibility that Tullimonstrum was a vertebrate has attracted much attention, and it has been postulated that Tullimonstrum might fill a gap in the fossil record of early vertebrates, providing important insights into vertebrate evolutionary history. With the hope of resolving this debate, we collected 3D surface data from 153 specimens of Tullimonstrum using a high-resolution laser 3D scanner and conducted x-ray micro-computed tomographic (μCT) analysis of stylets in the proboscis. Our investigation of the resulting comprehensive 3D morphological dataset revealed that structures previously regarded as myomeres, tri-lobed brain, tectal cartilages and fin rays are not comparable with those of vertebrates. These results raise further doubts about its vertebrate affinities, and suggest that Tullimonstrum may have been either a non-vertebrate chordate or a protostome.
Often used to study dinosaur footprints, these color-coded depth maps enabled the researchers to thoroughly investigate the structure of the Tully monster and other fossils from Mazon Creek. Credit: Mikami, 2022.
It will be interesting to see the response to this paper by the vertebrate crowd. Victoria McCoy, lead author on two of the Tully vertebrate papers, gave us an excellent presentation on the latest Tully research at the Mazon Creek Open House back in October 2022.