Today's Arctic is one of the harshest and most sparsely populated regions on Earth, but during the age of the dinosaurs, this high latitude supported a surprisingly rich community of mammals. A recent study published in the Proceedings of the National Academy of Sciences (PNAS) shows that the ancient Arctic was not an isolated evolutionary corner, but a key "evolutionary hub" where early mammals adapted to the polar environment, achieved species diversification, and migrated along transcontinental land bridges.
A team of researchers from the University of Colorado Boulder and several collaborating institutions discovered three previously unknown rodent-like mammal species in the Prince Creek Formation, now located in northern Alaska and within the Arctic Circle, with fossils dating back approximately 73 million years. Teeth morphological analysis shows that some of the species' ancestors came from what is now Mongolia, a discovery that challenges the long-held conventional view that the poles played only a minor role in the evolution of mammals.
The first author of the paper, Sarah Shelley of the University of Lincoln in the UK, said that although today's polar biodiversity is far less than that of the tropics, judging from deep-time geological records, the polar regions are also a stage where life flourishes and evolutionary activities occur frequently. While Shelley was a postdoctoral fellow at the University of Colorado Boulder, she co-authored the research with Jaelyn Eberle, a professor in the Department of Geological Sciences and director of the Museum of Natural History at the University of Colorado.
In this study, the team named three new mammals after their fossilized teeth: Camurodon borealis ("Northern Curved Tooth"), Qayaqgruk peregrinus ("Little Wandering Hero"), and Kaniqsiqcosmodon polaris ("Polar Frost Teeth"). The fossils were collected from the Prince Creek Formation near the "top" of the world, an area that had already experienced long polar nights, freezing temperatures and seasonal food shortages during the late Cretaceous, but these small mammals still managed to survive here.
Co-author Patrick Druckenmiller of the University of Alaska Fairbanks said the three newly discovered species further support the fact that the ancient Arctic fostered a unique community of mammals highly adapted to polar environments. Their existence means that the Arctic ecosystem can not only support dinosaurs, but also support the coexistence of multiple multi-tuberculous toothed mammals with sizes ranging from rat to rat shrew.
Research shows that these three new species belong to the extinct group of multituberculates mammals. They are roughly the same size as modern mice or rats and are considered to be the longest-lasting group in the history of mammal evolution. Polytuberculates emerged from the mid-Jurassic to the end of the Eocene epoch about 35 million years ago, spanning more than 100 million years of evolution and successfully surviving the asteroid impact that wiped out the non-avian dinosaurs.
In comparison, modern humans (Homo sapiens) have only existed for about 300,000 years, a fraction of the life span of multituberculous teeth. For a long time, scientists have been asking: Why can multituberculous teeth be "online for a long time" on geological time scales? This detailed analysis of the teeth provides key clues to this question.
Significant differences in tooth morphology exist among the three new species, suggesting they employ distinct feeding strategies in the Arctic environment. The research team found that the teeth of Camurodon borealis were more suitable for chewing plants, and they speculated that it was a plant-based herbivore; the teeth of Qayaqgruk peregrinus showed omnivorous characteristics, possibly feeding on insects and some plants, while Kaniqsiqcosmodon polaris was also omnivorous, but relied more on plant resources.
In an environment with seasonal food shortages, "division of labor" through differentiated dietary structures may be a key mechanism for the long-term coexistence of multiple polytuberculate species in the Arctic. Shelley believes that this flexibility in diet and ecological niche may be an important reason why they have continued to reproduce throughout geological time through extinction events.
Shelley pointed out that there is extremely high species and ecological diversity within polytuberculates, and their long evolutionary history can provide important insights into the resilience of mammals in the face of catastrophic extinctions and severe climate fluctuations. These deep-time cases not only help scientists analyze ecological changes from the Mesozoic to the early Cenozoic, but also provide a reference for understanding the ability of today's organisms to cope with climate pressure and environmental changes.
The discovery also adds new details to the geographical and biological history of the ancient Arctic. Through phylogenetic analysis, the research team found that Qayaqgruk peregrinus is closely related to a polytuberculous tooth species in present-day Mongolia, which means that its ancestors migrated from Asia to North America.
Shelley estimates that this cross-continental migration occurred about 92 million years ago, making it one of the earliest known records of cross-continental migrations of Asia-North America mammals. Eberle said this evidence suggests that a land corridor for small mammals already existed between Asia and North America at that time, and that this "land bridge" was active 90 million years ago.
The discovery further strengthens a broader understanding that the process of species migrating across continents and reshaping ecosystems has been going on for at least hundreds of millions of years in geological history. Shelley emphasized that this "deep time perspective" challenges our traditional definition of "native species" and reminds people that any area is not a static geographical coordinate, but a complex historical layer superimposed by long-term evolution, migration and environmental changes.