Today's Arctic may feel remote and desolate, but more than 70 million years ago, it was a surprisingly lively place for some of Earth's ancient mammals.
In a new study published May 18 in the Proceedings of the National Academy of Sciences (PNAS), University of Colorado Boulder researchers and collaborators describe three previously unknown species of rodent-like mammals that once scurried alongside dinosaurs in what is now northern Alaska. They found that some of these creatures' ancestors had traveled all the way from modern-day Mongolia, in East Asia, challenging a long-held assumption that the polar regions played only a minor role in evolution.
"While the polar regions don't host the same level of biodiversity as the tropics, they were still very active places for life to flourish, extending far back into deep time," says Sarah Shelley, the paper's first author at the University of Lincoln in the U.K. She conducted the study as a postdoctoral researcher at CU Boulder with senior author Jaelyn Eberle, a professor in the Department of Geological Sciences and curator at the University of Colorado Museum of Natural History.
Shelley, Eberle and colleagues named the three species Camurodon borealis, which roughly translates to "Northern curved-tooth;" Qayaqgruk peregrinus, or "the little wandering hero;" and Kaniqsiqcosmodon polaris, meaning "polar frost ornamented tooth."
The team identified the animals using fossil teeth discovered in the Prince Creek Formation, located near the top of the world in the Arctic Circle. The fossil site dates back 73 million years. Even then, the region experienced months of darkness in winter, freezing temperatures and likely seasonal food shortages. But these little creatures thrived.
"These three new mammal species add to a growing body of evidence that this ancient arctic region was home to unique, polar-adapted species," said Patrick Druckenmiller, a coauthor at the University of Alaska Fairbanks.
All three animals belong to an extinct mammal group called multituberculates. Roughly in the size range between mice and rates, multituberculates were the longest-lived group of mammals known in Earth's history. They persisted for more than 100 million years, from the Jurassic Period to the end of the Eocene Epoch about 35 million years ago. They even survived the asteroid impact that killed all non-avian dinosaurs. By comparison, modern humans (Homo sapiens) have existed for only about 300,000 years.
Scientists have long wondered what allowed multituberculates to outlive many other mammals, and these teeth offered a clue.
The researchers found striking differences in tooth shape among the three species, suggesting that they likely ate different foods. C. borealis had the teeth of herbivores, while Q. peregrinus was an omnivore that probably fed on insects along with some plants. K. polaris, also appeared to have been an omnivore, but might have eaten mostly plants.
In a region with limited food, the ability to evolve and carve out their own diets might have helped different multituberculate species coexist. Such adaptability might also have helped them survive the asteroid impact, Shelley said.
"There's a lot of diversity in the multituberculate group. They lived for an incredibly long time, and I think they can reveal a lot about the resilience of mammals, not just to the mass extinction, but also to climatic stresses that many organisms are facing today," she said.
This discovery also helps paint a more complete picture of the ancient Arctic.
The team found that Q. peregrinus, named after a legendary hero, Qayaq, in the Alaskan Inuit culture, is closely related to a species found in what is now Mongolia, suggesting Q. peregrinus' ancestors traveled from Asia to North America. Shelley estimated that this dispersal happened about 92 million years ago, making it one of the earliest known examples of mammals crossing between the continents.
"This means there was a land corridor between Asia and North America for these little mammals to come through," Eberle said. "And this land bridge was already pretty active as far back as 90 million years ago."
The discovery adds to the growing evidence that species have been migrating and reshaping ecosystems across continents for hundreds of millions of years.
"It really challenges how we think about native species," Shelley said. "Deep time reminds us that a place is not just a point on a map, but a long, layered history of landscapes and inhabitants."
