Montana State study of turtle fossil narrows timeline of Cretaceous species migration

BOZEMAN - Before leaving on a fossil-hunting trip for a summer 2021 field paleontology class, a Montana State University junior made an apparently fate-tempting plea.

"I kept joking through that whole class, 'Oh, please, just anything but a turtle,'" said Jack Prall, now a doctoral student in MSU's Department of Earth Sciences in the College of Letters and Science.

Prall had prior field experience in his home state of Colorado, where turtle fossils are abundant, so he hoped to find something more novel to him in the Frontier geological formation near Lima in southwest Montana.

"Of course," he said, "one of the cooler finds on that class trip was this turtle."

At the time, Prall didn't know that "this turtle" would turn out to be the oldest confirmed specimen - by about 5 million years - of the Basilemys genus of meter-long, terrestrial turtles that lived in North America during the mid- to late Cretaceous. The journal Historical Biology recently published a paper by Prall and fellow MSU graduate student Brendan Clark, describing how they and their co-authors determined the specimen's age and discussing the scientific significance of the 89-million-year-old animal.

"Most of the oldest confirmed fossils of this turtle get up to 84 million years," said Clark, whom Prall describes as the "resident turtle specialist" among MSU's current group of earth sciences graduate students. "This is definitely the oldest directly dated fossil of Basilemys known so far."

Prall began studying the specimen for an undergraduate research project but, before finishing it, was ready to shift his focus to his unrelated Ph.D. research. In 2024, he invited Clark, who is preparing his master's thesis on another Montana fossil turtle from the mid-Cretaceous, to "fill in the gaps and get this project over the line." Clark agreed, hoping the Basilemys would offer insight into the formation of ecosystems in the Cretaceous. At the time, neither student realized what the fossil would reveal about the large interchange of animals between Russia and Alaska during that period.

The juvenile specimen, whom the students nicknamed "Donatello" in honor of the "nerdy" Teenage Mutant Turtle of cartoon fame, did not disappoint.

To determine how long ago Donatello died, Prall and Clark sought help from fellow graduate students. One of them, Zak Hannebaum, analyzed sediment collected with the fossil in a geochronology class taught by MSU associate professor of geology Devon Orme.

"Once we started getting more and more interesting dates, we brought in Dr. Orme and were able to get a much closer idea of how old this rock is," Prall said. "It fell out to around 89 million, which was really, really exciting when we got those numbers back."

The genus Basilemys belongs to the extinct, Asian Nanhsiungchelyidae family of large, land-dwelling turtles, but Basilemys specimens have been found only in North and Central America, suggesting that the genus evolved after its ancestors migrated toward North America.

"Importantly, Basilemys is the only turtle in its family that is in North America - the rest are from Asia," Prall said. "We have family members going back about 112 million years, but we don't have a good idea of how that dispersal happened or especially when that dispersal happened. This fossil really helps to narrow down that time in which that migration could have occurred and helps strengthen the hypotheses out there about the migrations that are happening during this greenhouse event."

Scientists believe the ancestors of Basilemys began migrating during a period of increasing polar warming between 100 million and 113 million years ago. The paper states that Donatello's location in the Frontier formation indicates Basilemys rapidly colonized western North America south of what is now Alaska and Canada just over 90 million years ago. During that period, polar temperatures averaged 13 degrees Celsius, or 55 degrees Fahrenheit.

Still, because they sometimes would have been exposed to freezing temperatures, it's unknown how the turtles survived in polar regions. The paper suggests they may have engaged in burrowing behavior or entered states of winter dormancy to survive.

Fossil turtles are representative of groups commonly seen later in the Age of Dinosaurs, which ended 66 million years ago. Clark and Prall said seeing them so much earlier in the fossil record sheds light on the origins of ancient ecosystems and provides insight into how turtles responded to a shifting climate in the past, thereby furthering understanding of the evolution of ecosystems in North America during the Cretaceous.

The authors are certain that Donatello lived a lifestyle similar to that of a modern tortoise in balmy conditions at the western edge of a vast, inland sea. But because only a small part of its shell was recovered, Prall said there was no way to determine whether Donatello represents a new, previously undiscovered species of Basilemys.

"Unfortunately, the fossil record gives us what it gives us, but we'll go out and look for more," said Prall, no longer wishing for "anything but a turtle."

Donatello is one of many scientifically significant fossils found in southwest Montana since the turn of the century, including an older turtle fossil that Clark is studying for his master's thesis. Near the Frontier formation lies the older Blackleaf formation, where, in 2004, an MSU graduate student discovered the 95-million-year-old bones of an adult and two juvenile Oryctodromeus cubicularis dinosaurs in a burrow. The fossils offered the first scientific evidence that some dinosaurs dug burrows and cared for their young in dens.

In the Blackleaf in 2021, MSU field paleontology students found the remains of a small, terrestrial crocodyliform, also about 95 million years old, later identified as a new species that shares particular anatomical features with distantly related crocs from the Cretaceous of Africa and South America. The similarities suggest that crocodyliforms were evolving similarly in different parts of the world at the same time.

The sites are managed by the U.S. Department of Agriculture Forest Service, which MSU professor of paleontology David Varricchio credits for furthering scientific discovery by granting access to professional and student paleontologists.

"Allowing researchers and students to work on these specimens gets them preserved in public museums, protecting them for the common good," Varricchio said.

Erik Torgerson, a geologist with Beaverhead-Deerlodge National Forest, said the agency appreciates the stewardship efforts of MSU to preserve paleontological resources on National Forest System lands.

"The work ethic and professionalism of Dr. Varricchio and his students has been exceptional," Torgerson said. "We look forward to continued work with MSU and their contributions, so the paleontological story of southwest Montana does not go unnoticed."