Published in the journal Palaeontology, the research highlights that numerous mammalian species were already adapting to life on the ground before the catastrophic event at the end of the Cretaceous. The study draws on the analysis of fossilised limb bone ends from marsupial and placental mammals unearthed in Western North America-the only region with a well-documented terrestrial fossil record from that period.
Researchers focused on the ends of limb bones because they reveal clues about locomotion that can be statistically compared with those of modern mammals. These skeletal elements offer insight into how prehistoric mammals navigated their environments.
"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," said lead researcher Professor Christine Janis of Bristol's School of Earth Sciences.
While earlier investigations relied on complete skeletons to infer mobility, this work pioneers the use of small fossil fragments to detect community-wide trends. The data was gathered from museum collections in New York, California, and Calgary.
Professor Janis emphasized: "The vegetational habitat was more important for the course of Cretaceous mammalian evolution than any influence from dinosaurs."
The team concentrated on bone joint surfaces from therian mammals-a group encompassing marsupials and placentals. They excluded more primitive mammals such as multituberculates from the analysis, as their skeletal structures differ markedly.
"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," added Professor Janis.
Although the project has concluded, its findings offer significant insight into how mammals were already adapting to evolving ecosystems before the mass extinction radically altered life on Earth.
Research Report:Down to earth: therian mammals became more terrestrial towards the end of the Cretaceous
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