New research from the University of Bristol provides evidence that many mammals were already shifting toward a more ground-based lifestyle leading up to the end-Cretaceous mass extinction that wiped out non-avian dinosaurs 66 million years ago.
Dryolestes, a Late Jurassic relative of Cretaceous therian mammals. Image credit: James Brown / Pamela Gill / University of Bristol.
Present-day mammals mostly live in diverse habitats dominated by flowering plants (angiosperms).
Unlike those provided by coniferous vegetation, woodland and forest habitats have a dense and complex understory as well as larger trees, providing high biodiversity and resources for terrestrial mammals of all sizes.
However, these flowering plant-dominated habitats are relatively new in Earth history, being mainly a feature of the Cenozoic Era (66 million years ago until today), but first developing towards the end of the Cretaceous.
“Mesozoic mammals were small, almost entirely under 5 kg in mass, with the largest no greater than around 10 kg,” said University of Bristol Professor Christine Janis and colleagues.
“Indeed, most mammals today are small, with a median body mass of around 0.5 kg, but in the Cenozoic a diversity of larger mammals evolved.”
“The small size of Mesozoic mammals has often have been attributed to dinosaur suppression but recently the Late Cretaceous radiation of angiosperms, and the vegetational habitat that they provide, has been noted as probably being more important than dinosaurs in explaining the patterns of Mesozoic mammalian evolution and diversification.”
“What was the substrate preference of these mammals, arboreal or terrestrial, and did this change towards the end of the Cretaceous with changing angiosperm habitats?”
In their research, the authors analyzed small bone fragments, specifically ends of limb bones, from ancient therian mammals (marsupial and placental mammals) found in Western North America, the only place with a well-preserved terrestrial fossil record from this time.
They discovered signs that these mammals were adapting to life on the ground.
Ends of limb bones were analyzed as they bear signatures of locomotory habit that can be statistically compared with modern mammals.
“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,” Professor Janis said.
“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.”
While previous studies used complete skeletons to study ancient mammal movement, this research is one of the first to use small bone elements to track changes within an entire community.
“The vegetational habitat was more important for the course of Cretaceous mammalian evolution than any influence from dinosaurs,” Professor Janis said.
“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.”
The study was published in the March/April 2025 issue of the journal Palaeontology.
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Christine M. Janis et al. 2025. Down to earth: therian mammals became more terrestrial towards the end of the Cretaceous. Palaeontology 68 (2): e70004; doi: 10.1111/pala.70004
