Several prehistoric stones found in Wyoming four years ago originate in Wisconsin. How did they end up here? Researchers believe they traveled in the belly of a long-necked dinosaur.
We are in 2017, in July. Joshua Malone, then an undergraduate student at Augustana College in Illinois, visits a research camp in Wyoming and comes across rocks. Rounded at the edges and about the size of small fists, they are out of place, lying amid mudrocks, a class of fine-grained siliciclastic sedimentary rocks.
The student then turns to his father, David Malone, a geologist at Illinois State University, who is leading the excavation at the site. He wonders himself. Four years later, he proposes a hypothesis as fascinating as it is surprising.
In a study published earlier this year in the journal New land, Dr. Malone’s team believe these stones originated from a rock formation in southern Wisconsin, about 1600 km east of where they were found. And they probably got there by traveling through the guts of a long-necked dinosaur.
Gastroliths carried by a sauropod
Known as sauropods, long-necked dinosaurs could reach thirty meters in length and weigh nearly forty tons depending on the species. We also know that these animals regularly swallowed gastroliths (or stomach stones), possibly to help them digest plants, much like some modern birds and reptiles do.
As part of this work, the researchers crushed a few rock samples to date the zircon crystals inside. “What we found is that the zircon ages inside these gastrolites have distinct age spectra that match the age of rocks found in southern Wisconsin.“, Explains Mr. Malone.
“We then propose that these rocks were ingested somewhere in southern Wisconsin and then transported to Wyoming in the belly of a long-necked dinosaur.“.
This assumption would then imply a much larger “hike” than previous estimates. We know that seasonal changes can lead to migrations as animals move in search of food and water. And the sauropods, in particular, would have needed large amounts of calories to support their bodies.
“Given the highly seasonal environments in which they lived, it should come as no surprise that they had to migrate such long distances in search of food.“, Argues Dr Malone.
A hypothesis not yet proven
The hypothesis would thus explain how these few rocks acquired their smooth and rounded texture. However, we cannot yet conclude on their origin.
These stones were recovered from the Morrison Formation, a geological formation dating back to the Late Jurassic period that is teeming with dinosaur fossils, including those of sauropods, such as Barosaurus and Diplodocus, as well as carnivores such as Allosaurus. However, they were found alone. In other words, no dinosaur remains were found around.
“Unfortunately, we have no real evidence that these clasts are indeed ancient gastroliths.“, Emphasizes Oliver Wings from Martin Luther University in Halle-Wittenberg, Germany. “We cannot rule out the possibility that these stones were transported into the belly of dinosaurs, but this is only one of several possibilities.“. Also, the hypothesis of this article will need more evidence to be proven.