Dinosaurs roamed the northern hemisphere millions of years earlier than previously thought, according to new analysis of the oldest North American fossils

(Press-News.org) MADISON — How and when did dinosaurs first emerge and spread across the planet more than 200 million years ago? That question has for decades been a source of debate among paleontologists faced with fragmented fossil records. The mainstream view has held that the reptiles emerged on the southern portion of the ancient supercontinent Pangea called Gondwana millions of years before spreading to the northern half named Laurasia.

            But now, a newly described dinosaur whose fossils were uncovered by University of Wisconsin–Madison paleontologists is challenging that narrative, with evidence that the reptiles were present in the northern hemisphere millions of years earlier than previously known.

The UW–Madison team has been analyzing the fossil remains since they were first discovered in 2013 in present-day Wyoming, an area that was near the equator on Laurasia. The creature, named Ahvaytum bahndooiveche, is now the oldest known Laurasian dinosaur, and with fossils estimated to be around 230 million years old, it's comparable in age to the earliest known Gondwanan dinosaurs.

            UW–Madison scientists and their research partners detail their discovery Jan. 8, 2025, in the Zoological Journal of the Linnean Society.

            "We have, with these fossils, the oldest equatorial dinosaur in the world — it's also North America's oldest dinosaur," says Dave Lovelace, a research scientist at the University of Wisconsin Geology Museum who co-led the work with graduate student Aaron Kufner.

            Discovered in a layer of rock known as the Popo Agie Formation, it took years of careful work by Lovelace and his colleagues to analyze the fossils, establish them as a new dinosaur species and determine their estimated age.

            While the team doesn't have a complete specimen — that's an exceedingly rare occurrence for early dinosaurs — they did find enough fossils, particularly parts of the species' legs, to positively identify Ahvaytum bahndooiveche as a dinosaur, and likely as a very early sauropod relative. Sauropods were a group of herbivorous dinosaurs that included some famously gigantic species like those in the aptly named group of titanosaurs. The distantly related Ahvaytum bahndooiveche lived millions of years earlier and was smaller — much smaller.

            "It was basically the size of a chicken but with a really long tail," says Lovelace. "We think of dinosaurs as these giant behemoths, but they didn't start out that way."

Indeed, the type specimen of Ahvaytum bahndooiveche, which was full-grown but could have been slightly bigger at its maximum age, stood a little over one foot tall and was around three feet long from head to tail. Although scientists haven’t found its skull material, which could help illuminate what it ate, other closely related early sauropod-line dinosaurs were eating meat and would likely have been omnivorous.   

The researchers found the few known bones of Ahvaytum in a layer of rock just a little bit above those of a newly described amphibian that they also discovered. The evidence suggests that Ahvaytum bahndooiveche lived in Laurasia during or soon after a period of immense climatic change known as the Carnian pluvial episode that has previously been connected to an early period of diversification of dinosaur species.

The climate during that period, lasting from about 234 to 232 million years ago, was much wetter than it had been previously, transforming large, hot stretches of desert into more hospitable habitats for early dinosaurs.

Lovelace and his colleagues performed high-precision radioisotopic dating of rocks in the formation that held Ahvaytum’s fossils, which revealed that the dinosaur was present in the northern hemisphere around 230 million years ago. The researchers also found an early dinosaur-like track in slightly older rocks, demonstrating that dinosaurs or their cousins were already in the region a few million years prior to Ahvaytum.

"We're kind of filling in some of this story, and we're showing that the ideas that we've held for so long — ideas that were supported by the fragmented evidence that we had — weren’t quite right,” Lovelace says. “We now have this piece of evidence that shows dinosaurs were here in the northern hemisphere much earlier than we thought."

While the scientific team is confident they've discovered North America's oldest dinosaur, it's also the first dinosaur species to be named in the language of the Eastern Shoshone Tribe, whose ancestral lands include the site where the fossils were found. Eastern Shoshone tribal elders and middle school students were integral to the naming process. Ahvaytum bahndooiveche broadly translates to "long ago dinosaur" in the Shoshone language.

Several tribal members also partnered with Lovelace and his UW–Madison colleagues as the researchers sought to evolve their field practices and better respect the land by incorporating the knowledge and perspectives of the Indigenous peoples into their work.

"The continuous relationship developed between Dr. Lovelace, his team, our school district, and our community is one of the most important outcomes of the discovery and naming of Ahvaytum bahndooiveche," says Amanda LeClair-Diaz, a co-author on the paper and a member of the Eastern Shoshone and Northern Arapaho Tribes. LeClair-Diaz is the Indian education coordinator at Fort Washakie school and coordinated the naming process with students and tribal elders — a process that started under her predecessor, Lynette St. Clair.

"Typically, the research process in communities, especially Indigenous communities, has been one sided, with the researchers fully benefiting from studies," says LeClair-Diaz. "The work we have done with Dr. Lovelace breaks this cycle and creates an opportunity for reciprocity in the research process."

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