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Stones Hint at Possible 600-Mile Dinosaur Migration From Wisconsin to Wyoming

Some 150 million years ago, prehistoric plant-eaters may have carried the rocks in their bellies to aid digestion

These polished stones collected in Wyoming may have been carried some 600 miles from Wisconsin inside the stomachs of sauropods. (Joshua Malone)
smithsonianmag.com

A group of polished, ancient stones found in Wyoming may have been carried more than 600 miles in the huge bellies of plant-eating dinosaurs, reports Ashley Piccone for Wyoming Public Radio. The findings, published last month in the journal Terra Nova, could provide a new line of evidence that dinosaurs may have undertaken long overland migrations.

Discovered near Wyoming’s Bighorn Basin in a geologic feature called the Morrison formation, researchers say these smooth, fist-sized rocks are gastroliths, which are rocks swallowed by dinosaurs—and some modern birds and reptiles—that may help grind up fibrous food.

A father-son geology duo collected the proposed gastroliths in 2017 during field research because the stones' shiny appearance looked out of place surrounded by the fine-grained mud-rock that predominates the Morrison, reports Lucas Joel for the New York Times.

"We were walking around just doing some fieldwork in the Bighorn Basin," Joshua Malone, a PhD student in geology at the University of Texas at Austin and the study’s lead author, tells Wyoming Public Radio. "We started seeing these polished stones and we were like, 'those look pretty exotic compared to all the other rock around us.'"

To figure out where the rocks may have come from the researchers ground them up and examined tiny crystals that resided within them called zircons.

The zircon tests suggested the rocks date to the late Jurassic roughly 150 million years ago and that they appear to have originated some 620 miles east.

"We inferred that these were ingested in Wisconsin, or somewhere along this sluggish stream that was flowing from the east to the western United States during that time," Malone tells Wyoming Public Radio. "We figured that once they were ingested, they were carried and eventually deposited out within the Morrison deposition."

Speaking with the Times, Malone adds this is the first time gastroliths have been used to infer the movements of dinosaurs. If confirmed, these findings more than double the longest journeys ascribed to the large plant-eating sauropods or long-necked dinosaurs that may have carried the gastroliths in their stomachs. In 2011, a study of dinosaur teeth suggested the 18-ton Camarasaurus may have migrated nearly 200 miles to find food, reported Ian Sample for the Guardian at the time.

However, the current study’s hypothesis rests on whether these rocks were indeed gastroliths, a claim made more ambiguous by the fact that the rocks were not found in the context of a fossil skeleton, according to the Times.

“Unfortunately, we have no real evidence that these clasts are indeed former gastroliths,” Oliver Wings, a geologist and vertebrate paleontologist at Martin Luther University Halle-Wittenberg in Germany who was not involved in the research, tells the Times. “We cannot exclude the possibility of transport of the stones in the bellies of dinosaurs, but it remains just one possibility of several.”

But Wings tells the Times that the technique detailed in the paper for inferring dinosaur movements could still be a significant contribution to paleontology: “It would be amazing if they could use that method on genuine gastroliths.”

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