In a groundbreaking study led by the University of Bristol, new evidence has come to light suggesting that mammals were already adapting to a more terrestrial lifestyle millions of years before the asteroid impact that wiped out the dinosaurs. This research, published in the journal Palaeontology, provides fresh insights into the evolutionary changes that were taking place among mammals as the Earth’s ecosystems underwent significant transformations.
For years, it was believed that mammals were largely tree-dwellers or arboreal creatures during the Cretaceous period, a time when dinosaurs dominated the planet. However, this new research challenges that assumption by revealing that many mammal species were already shifting toward a ground-based existence in the years leading up to the mass extinction event. This discovery opens a new chapter in our understanding of how prehistoric mammals adapted to changing environments and what role these adaptations played in their survival after the catastrophic asteroid impact.
Groundbreaking Methodology: Analyzing Fossilized Bone Fragments
The study, which focuses on small-fossilized bone fragments, represents one of the first efforts to track evolutionary trends in entire mammalian communities through microscopic analysis. The team, led by Professor Christine Janis from the University of Bristol’s School of Earth Sciences, meticulously analyzed the ends of limb bones from both marsupial and placental mammals that lived in Western North America. This region boasts the only well-preserved terrestrial fossil record from this time period, making it an invaluable resource for scientists trying to piece together the behavior and ecology of early mammals.
The bones studied were not large, complete skeletons, but rather small fragments—specifically, the articular ends of limb bones, which carry distinct signatures of how an animal moved and interacted with its environment. By examining these fragments, researchers could determine the locomotion habits of these ancient mammals and gain valuable insights into their adaptation to life on the ground.
Professor Janis explained the significance of this approach, saying, “It was already known that plant life changed toward the end of the Cretaceous, with the emergence of flowering plants, or angiosperms, creating more diverse habitats on the ground. What had not been documented was whether mammals were becoming more terrestrial in response to these changing habitats.” This study adds a new layer to that understanding by showing that mammals were not just responding to environmental shifts—they were actively adapting to them, moving from an arboreal to a terrestrial lifestyle long before the asteroid’s impact.
Insights into Mammalian Evolution
Mammals during the Cretaceous period were a diverse group, with species occupying a range of ecological niches. While some were still adapted to life in the trees, others were already making the transition to the ground, where the shifting landscape provided new opportunities for survival. The study found that these early mammals were adjusting their locomotion and skeletal structure to better suit life on land, a crucial step in their eventual dominance of terrestrial ecosystems following the mass extinction event.
The analysis of small bone elements revealed a variety of evolutionary adaptations, including changes in the shape and structure of the limb bones. These changes are indicative of a transition from a primarily tree-dwelling lifestyle to one that involved greater ground-based movement, such as running, digging, or foraging on the forest floor. By comparing these bone fragments with modern mammals, the research team was able to make statistical inferences about how these ancient creatures moved and interacted with their environment.
Interestingly, this study also suggests that environmental factors, rather than interactions with dinosaurs, were more important in driving mammalian evolution during the Cretaceous. While it has often been assumed that the rise of mammals was influenced by competition with or predation by dinosaurs, Professor Janis and her team argue that the changing vegetation and habitat structure were more significant in shaping mammal evolution. “The vegetational habitat was more important for the course of Cretaceous mammalian evolution than any influence from dinosaurs,” she emphasized.
A New Approach to Studying Prehistoric Mammals
While much of the past research into mammalian locomotion and behavior has focused on complete skeletons, this study takes a different approach by examining smaller, less conspicuous bone fragments. By using statistical methods to analyze these tiny fossilized elements, the team was able to track evolutionary trends across entire communities of mammals, rather than focusing on a single species. This approach provides a broader, more holistic view of how mammals were evolving in response to their environment during the Cretaceous.
Professor Janis explained, “We’ve known for a long time that mammalian long bone articular surfaces can carry good information about their mode of locomotion, but I think this is the first study to use such small bone elements to study change within a community, rather than just individual species.” This innovative methodology is not only a step forward in the field of palaeontology but also offers a more detailed picture of the dynamics at play in ancient ecosystems.
This new research also highlights the importance of fossil collections in museum archives. The team used data from museum collections in New York, California, and Calgary to gather and analyze the fossilized bones. These collections, often overlooked in previous studies, have proven to be invaluable sources of information about prehistoric life. By reexamining these specimens with modern analytical techniques, scientists are uncovering new insights into the behaviors and adaptations of ancient mammals.
The Impact of Environmental Change on Mammals
One of the most significant findings of this research is the role that environmental change played in shaping the evolutionary trajectory of mammals. While the asteroid impact that occurred approximately 66 million years ago is often seen as the defining moment in the extinction of the dinosaurs and the rise of mammals, this study suggests that mammals were already adjusting to terrestrial life long before the asteroid struck. The appearance of flowering plants, which created new habitats and food sources on the ground, may have provided the necessary environmental changes that drove this shift toward a more ground-based lifestyle.
This research paints a picture of a dynamic and adaptive group of mammals, capable of responding to changes in their environment over millions of years. As the landscape of the Earth evolved, so too did the mammals that inhabited it, gradually making the transition from life in the trees to life on the ground. The ability of these mammals to adapt to such significant changes in their environment may have played a crucial role in their survival after the mass extinction event.
Conclusion: A Step Forward in Our Understanding of Prehistoric Life
This study represents a major step forward in our understanding of the evolution of mammals during the Cretaceous period. By examining small bone fragments and using innovative statistical methods, the team has provided compelling evidence that mammals were already adapting to life on the ground millions of years before the asteroid impact that reshaped life on Earth. Their findings challenge long-held assumptions about the nature of mammalian evolution and offer new insights into how mammals responded to changing environmental conditions.
As researchers continue to study the fossils of prehistoric mammals, it is likely that even more discoveries will emerge, shedding light on the complex interplay between mammals, their environments, and the catastrophic events that ultimately shaped the course of life on Earth. In the meantime, this study stands as a testament to the power of innovative research techniques and the ever-expanding knowledge we are gaining about the distant past.
Reference: Down to earth: therian mammals became more terrestrial towards the end of the Cretaceous, Palaeontology (2025). DOI: 10.1111/pala.70004