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The mass extinction at the end of the Cretaceous 66 million years ago was characterized by a worldwide ecological catastrophe and a rapid turnover of species. Arboreal (tree-dwelling) species were particularly at risk of extinction due to the large-scale destruction of the forest environment caused by wildfires from the impact of the Chicxulub asteroid.

A new study by scientists at Cornell and Cambridge Universities shows that most of the mammals that survived did not rely on trees, although few arboreal mammals, including the ancestors of primates and marsupials, may have been versatile enough to adapt to tree loss.

“One possible explanation for how the ancestors of the primates survived the mass extinction at the end of the Cretaceous, despite being arboreal, could have been due to some behavioral flexibility, which may have been a critical factor in allowing them to survive,” he said. Jonathan Hughes is from Cornell University's Department of Ecology and Evolutionary Biology.

The earliest mammals appeared about 300 million years ago and may have diversified along with the expansion of flowering plants about 20 million years before the mass extinction at the end of the Cretaceous.

“When the asteroid Chicxulub fell, many of these lines of mammals became extinct,” said Jonathan Hughes. "At the same time, the surviving mammals diversified into new ecological niches that were opened up when dinosaurs and other species became extinct."

In their study, Jonathan Hughes and colleagues used the phylogenies (tree diagrams showing the evolutionary relationships between groups of organisms) of mammals. The researchers then classified each living mammal by phylogeny into three categories - arboreal, semi-arboreal, and non-arboreal - based on their preferred habitats.

They also developed computer models that reconstruct the evolutionary history of mammals. Scientists compared information from living mammals with available fossils to help provide additional context for their findings.

Overall, the models showed that the surviving species were predominantly non-woody during the late Cretaceous extinction period, with two possible exceptions: primate ancestors and marsupials.

The ancestors of primates and their closest relatives were arboreal right before the mass extinction in every model. The ancestors of marsupials were arboreal in half of the model reconstructions.

Scientists have also investigated how a group of mammals could change over time. “We were able to see that in the lead up to the mass extinction at the end of the Cretaceous, around the same time frame, there was a big spike in transitions from woody and semi-woody to non-woody species, so it's not just that we are seeing mostly non-woody species, but and that all species quickly moved away from the trees, ”said Jonathan Hughes.

The results are published in the journal Ecology and Evolution.

PHOTO: Reconstruction of the life of early primate species called Purgatorius mckeeveri (foreground) and archaic ungulate mammals (bottom). Image credit: Andrey Atuchin.

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