Tully Monster isn't a vertebrate, so what is it?
The monster is back! Well, not alive, but back in the scientific spotlight.
In 1958 amateur fossil hunter Francis Tully found some strange fossils at the famous Mazon Creek site, about 15 miles southwest of Chicago.
The site, an ancient estuary that formed in the Carboniferous Period about 308 million years ago, has produced fossils of over 300 species of fossil plants and over 350 species of fossil animals, including shrimp, horseshoe crabs, sea cucumbers, jellyfish, insects, spiders, centipedes, millipedes, and fish.
Tully showed his mysterious fossil to paleontologists at the Field Museum, who gave it the scientific name Tullimonstrum in 1966. Commonly called the Tully Monster, the animal had a diamond-shaped fin at its back end, eyes on a horizontal bar across its head, and a long snout that ended in a claw.
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Averaging about 6 inches long, thousands have been found, but only at the Mazon Creek locality.
And despite so many specimens, its biological affinities remain a mystery. Suggestions have included gastropods (snails and slugs), polychaetes (bristle worms) and nemerteans (ribbon or proboscis worms) but the similarities are only superficial.
In 2016 and again in 2020 two different groups of scientists produced evidence they said showed the Tully was a vertebrate, the group of animals having backbones, and was possibly related to jawless fish like lampreys. Those reports didn’t convince many paleontologists – lampreys are known from Mazon Creek, and their characteristic features are lacking in the Tully.
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The new study by five scientists at the University of Tokyo investigated 153 specimens of the Tully and 75 other fossils from Mazon Creek, including vertebrates such as lampreys and jawed fish. And they attacked the problem in a different way – by using a 3-D scanner, similar to how dinosaur footprints are sometimes studied.
The results showed that the body segmentation of the Tully was clearly different from that of fish and other vertebrates, whose muscles have a chevron or V-shaped arrangement called myomeres. Such muscles are not present in the Tully.
Also, the fan of thin bones in the tails of co-occurring fish are prominent, but are lacking in the Tully, and they saw no evidence for a vertebrate-like brain, or vertebrate-like gills.
So, the Tully is not a vertebrate. What is it? We still don’t know, and the mystery of its identity may never be solved.But it could be. To do that we would need to find a rock layer representing a similar environment deposited at the same time but that preserves fossils in a different way. I hope there is such a rock unit somewhere on the planet.
Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University. Contact him at gnidovec.1@osu.edu
This article originally appeared on The Columbus Dispatch: Tully Monster isn't a vertebrate, so what is it?
