When the Dino-Killing Asteroid Hit, Mammals Were Already on the Move

The end-Cretaceous extinction—the massive extinction event widely attributed to an asteroid impact that wiped out all non-avian dinosaurs approximately 66 million years ago—had a profound impact on the evolution of all life on Earth. Specifically, mammals eventually diversified into thousands of distinct species.

By studying mammal remains from the end of the Cretaceous period (approximately 145 to 66 million years ago) and the early Paleogene period (66 to 23 million years ago) an international team of researchers has found new evidence confirming the theory that many arboreal (tree-living) mammals were evolving to live on the ground before the end-Cretaceous extinction. As detailed in a study published Tuesday in the journal Palaeontology, this change in lifestyle may have influenced which mammals survived the dino-killing asteroid.

“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,” Christine Janis, a paleontologist from the University of Bristol and lead author of the study, said in a university statement. “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.”

Janis and her colleagues analyzed fossilized fragments of mammal bones unearthed in western North America for evidence of locomotory habit, or how the animals moved. The bones represent therian mammals: mammals that give birth to live offspring—as opposed to, for example, laying eggs. Therian mammals include marsupials and placentals, the latter being mammals that nourish their young through a placenta.

“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 added. The results of the bone analysis indicate that tree-dwelling mammals had begun adopting a terrestrial lifestyle before the end-Cretaceous extinction.

“The fossils included in this study, from the different time intervals during the latest Cretaceous and earliest Palaeogene, provide support, if not equivocal support, for the hypothesis that therian mammals were becoming more terrestrial toward the end of the Cretaceous,” the researchers wrote in the paper, “as the terrestrial understory habitat, whether in more closed forest or open woodland habitats, probably became more complex and provided a greater diversity of food materials for mammals.”

Furthermore, the team suggests that this change in lifestyle may have determined which mammal species survived the asteroid impact, noting that arboreal marsupials sustained higher levels of extinction than terrestrial placentals. In other words, ground-dwelling mammals seemed to have fared better during the end-Cretaceous extinction.

Ultimately, the study sheds light on the emergence of a lifestyle that, millions of years later, would also play a crucial role in human evolution.