Researchers have solved the mystery of the Tully Monster, an oddly configured sea creature with teeth at the end of a narrow, trunk-like extension of its head and eyes that perch on either side of a long, rigid bar.

“We had a very clear picture of what it looked like, but no clear picture of what it was.”

The 300-million-year-old animal—which grew to only a foot long and was a vertebrate, with gills and a stiffened rod (or notochord) that supported its body—is part of the same lineage as the modern lamprey.

“I was first intrigued by the mystery of the Tully Monster,” says lead author Victoria McCoy, who conducted her research as a Yale University graduate student and is now at the University of Leicester. “With all of the exceptional fossils, we had a very clear picture of what it looked like, but no clear picture of what it was.”

illustration of a Tully Monster

For decades, the Tully Monster has been one of the great fossil enigmas: It was discovered in 1958, first described scientifically in 1966, yet never definitively identified even to the level of phylum (that is, to one of the major groups of animals).

Officially known as Tullimonstrum gregarium, it is named after Francis Tully, the amateur fossil hunter who came across it in coal mining pits in northeastern Illinois.

Thousands of Tully Monsters eventually were found at the site, embedded in concretions—masses of hard rock that formed around the Tully Monsters as they fossilized. Tully donated many of his specimens to the Field Museum of Natural History, which collaborated with Argonne National Laboratory and the American Museum of Natural History on the study that is published in the journal Nature.

“Basically, nobody knew what it was,” says coauthor Derek Briggs, professor of geology and geophysics and curator of invertebrate paleontology at the Yale Peabody Museum of Natural History.

“The fossils are not easy to interpret, and they vary quite a bit. Some people thought it might be this bizarre, swimming mollusk. We decided to throw every possible analytical technique at it.”

Using the Field Museum’s collection of 2,000 Tully Monster specimens, researchers analyzed the morphology and preservation of various features of the animal. Powerful, new analytical techniques also were brought to bear, such as synchrotron elemental mapping, which illuminates an animal’s physical features by mapping the chemistry within a fossil.

The researchers concluded that the Tully Monster had gills and a notochord, which functioned as a rudimentary spinal cord. Neither feature had been identified in the animal previously.

“It’s so different from its modern relatives that we don’t know much about how it lived,” McCoy says. “It has big eyes and lots of teeth, so it was probably a predator.”

Some key questions about Tully Monsters remain unanswered. No one knows when the animal first appeared on Earth or when it went extinct. Its existence in the fossil record is confined to the Illinois mining site, dating back 300 million years.

“We only have this little window,” Briggs says.

This text is published here under a Creative Commons License.
Author: Jim Shelton-Yale University
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