A study published in the journal Palaeontology revealed that mammals began transitioning from tree-dwelling to ground-based lifestyles millions of years before the asteroid impact that ended the reign of the dinosaurs. The research, led by Professor Christine Janis from the University of Bristol's School of Earth Sciences, indicated that this shift occurred around 72 million years ago, challenging long-held beliefs about mammalian evolution.
According to Phys.org, Professor Janis and her team analyzed more than 300 small fossilized bone fragments, specifically the ends of limb bones from marsupials and placental mammals found in Western North America. These tiny fossils, known as bone articular fragments, provided insights into the locomotion habits of these ancient creatures. "We've known for a long time that mammalian long bone articular surfaces can carry good information about their mode of locomotion," Professor Janis explained. "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."
The findings suggest that mammals were adapting to terrestrial life well before the end of the Cretaceous period. The team discovered signs that these mammals were moving towards a ground-based lifestyle, with their joints adapting for movement on the ground rather than climbing branches. "The vegetational habitat was more important for the course of Cretaceous mammalian evolution than any influence from dinosaurs," said Professor Janis, according to a report by Phys.org.
The research presents evidence that the spread of flowering plants, known as angiosperms, created more diverse habitats on the ground, prompting mammals to exploit these ecological niches. "It was already known that plant life changed toward the end of the Cretaceous, with flowering plants creating more diverse habitats on the ground," Professor Janis noted. "What had not been documented was whether mammals were becoming more terrestrial, in line with the habitat changes."
By examining the articular surfaces—the tips—of the limb bones, the team could determine whether a specific species was more suited to arboreal or terrestrial movement. These fossils indicate that mammals were already shifting toward a ground-based lifestyle leading up to the asteroid's impact, suggesting that the terrestrial transition played a role in their evolution. This challenges the traditional narrative that mammals only began to thrive after the dinosaurs' extinction.
Janis and her team used statistical data from museum collections in New York, California, and Calgary to analyze these tiny fossils. Their approach marked one of the first times small bone elements have been used to track changes within an entire community. The methods were not applied to more basal mammals, such as multituberculates, because their bones were different.
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