Published in the journal Palaeontology, the research sheds light on how marsupial and placental mammals in Western North America were already adapting to terrestrial environments prior to the asteroid impact that caused the Cretaceous-Paleogene extinction event.

Scientists focused on fossilized limb bone fragments from these mammals, particularly the ends of their bones, which retain clues about how an animal moved. The team compared these fossil elements with those of modern mammals, using statistical methods to identify patterns of terrestrial adaptation.

"It was already known that plant life changed toward the end of the Cretaceous, with flowering plants, known as angiosperms, creating more diverse habitats on the ground. We also knew that tree dwelling mammals struggled after the asteroid impact. What had not been documented, was whether mammals were becoming more terrestrial, in line with the habitat changes," explained lead author Professor Christine Janis from the University of Bristol's School of Earth Sciences.

While previous studies mostly relied on full skeletons, this study is among the first to analyze such small fossil elements across an entire mammal community. The data came from museum collections in New York, California, and Calgary, offering a rich sample of limb fragments from the only well-preserved terrestrial fossil record of the time.

"The vegetational habitat was more important for the course of Cretaceous mammalian evolution than any influence from dinosaurs," Professor Janis added.

The team analyzed bone joint fragments from therian mammals, which include both marsupials and placentals. They excluded earlier mammalian lineages like multituberculates, whose distinct skeletal structures would complicate comparisons.

"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," Janis noted.

Although the project has now concluded, its findings offer valuable new perspectives on how mammals responded to changing ecosystems well before the asteroid strike that dramatically altered Earth's biosphere.

Research Report:Down to earth: therian mammals became more terrestrial towards the end of the Cretaceous

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