June 5, 2023

In the summer of 2017, Joshua Malone, then a student at Augustana College in Illinois, visited a field research camp in Wyoming and picked up some stones. Rounded at the edges and the size of small fists, they were out of place in the fine-grained mud that had surrounded them, and Mr. Malone asked his father, David Malone, a geologist at Illinois State University who was conducting the excavation at the Place if he knew where the rocks came from.

Four years later, the two came up with a surprising answer.

In a study published in Terra Nova magazine earlier this year, the Malones and colleagues say the stones came from a rock formation in southern Wisconsin, about 1,000 miles east of where they were found. What’s even more surprising is their hypothesis of how the rocks made this journey: Researchers say they were carried in the bowels of long-necked dinosaurs.

Known as sauropods, these animals reached lengths of over 100 feet and weighed 40 tons, and regularly swallowed stones known as gastroliths, perhaps to help them digest plants, as some birds and reptiles do today. The hypothesis would explain how the rocks got their smooth and rounded textures. However, the question remains whether they really made the whole journey in the bellies of these large animals.

The gastroliths were found in mudstones from the Jurassic Age in a rock formation called the Morrison. The Morrison Formation is a rainbow of pink and red and filled with dinosaur fossils, including sauropods like Barosaurus and Diplodocus, and meat-eaters like Allosaurus.

But the rocks, which resemble the gastroliths excavated elsewhere, were found alone with no dinosaur remains. To get a clue as to how they ended up in what is now Wyoming, the team crushed the rocks to find and date the zircon crystals inside, much like studying old fingerprints.

“We found that the age of zircon in these gastrolites has different age spectra that correspond to the age in the rocks in southern Wisconsin,” said Malone, who is now a PhD student in geology at the University of Texas at Austin. “We assumed that these stones were picked up somewhere in southern Wisconsin and then transported to Wyoming in the belly of a dinosaur.”

He added, “There has never been a study like this that suggests dinosaur migration over long distances using this technique, so it was a really exciting moment for us.”

The Wisconsin-Wyoming connection suggests a migration hundreds of kilometers longer than previous estimates for sauropod migrations. Changing seasons can lead to migration as animals move in search of food and water. Sauropods in particular would have needed gigantic amounts of these resources to sustain their gigantic lives, according to Michael D’Emic, vertebrate paleontologist at Adelphi University in New York and co-author of the study.

“Sauropods grew rapidly and reached unprecedented size – on par with the growth rates of large mammals today,” he said. “This means that their caloric needs were immense. Given the very seasonal environment they lived in, it is not surprising that they had to travel long distances in search of food.”

However, other scientists say the paper’s hypothesis needs more evidence to be proven correct, as the rocks were not found next to actual dinosaur remains.

“Unfortunately we have no real evidence that these clasts are actually former gastroliths,” said Oliver Wings, geologist and vertebrate paleontologist at Martin Luther University Halle-Wittenberg in Germany. “We cannot rule out the possibility of the stones being transported in dinosaurs’ bellies, but it remains only one of several possibilities.”

Still, Dr. Wings that the team’s new paleontologists’ technique opens the door to date other gastroliths, especially those preserved with real dinosaur skeletons. “It would be amazing if you could use this method on real gastrolites,” he said.

However the rounded rocks got to Wyoming, their discovery helped lead Mr. Malone into a family tradition of studying geology.

“I rejected geology for the first 19 years of my life,” he said. “It wasn’t until this project and when I was out there in this encampment that I began to be interested in maybe going in that direction in my life.”