The miles-wide asteroid that struck Earth 66 million years ago wiped out nearly all the dinosaurs and roughly three-quarters of the planet’s plant and animal species.

But some creatures survived, including certain rat-sized mammals that would later diversify into the more than 6,000 mammal species that exist today, including humans.

Why did those mammals persist while others perished in the devastating mass extinction that closed the Cretaceous Period? A new study suggests that ground-dwelling and semi-arboreal mammals were better able to survive the cataclysm than tree-dwelling mammals, due to the global devastation of forests that followed the Chicxulub asteroid impact.

A possible exception to that pattern may have been the earliest primates, which likely resembled modern tree shrews and marmosets. Evidence from the new study suggests that primates may have maintained a capacity for arboreal habits through the mass extinction, despite global deforestation.

The mammal study, published online Oct. 11 in the journal Ecology and Evolution, is a follow-up to a 2018 study of birds by some of the same authors that reached similar conclusions about arboreality. Both papers highlight the pivotal influence of the end-Cretaceous apocalypse, which is known as the K-Pg mass extinction, in shaping the early evolutionary trajectories of today’s vertebrate animals.

The asteroid impact triggered a heat pulse that ignited forest fires globally. Dense clouds of debris and soot were ejected into the atmosphere, cooling the planet and likely blocking sunlight, while acid rain poured down.

“Large-scale devastation of forested environments resulting from the Chicxulub asteroid impact likely influenced the evolutionary trajectories of multiple groups, including terrestrial mammals. Our findings are consistent with the hypothesis that predominantly non-arboreal mammals preferentially survived this mass extinction,” said University of Michigan evolutionary biologist Jacob Berv, co-lead author of the new study.

Berv, a Life Sciences Fellow at the U-M Department of Ecology and Evolutionary Biology and Museum of Paleontology, was also a co-author of the 2018 birds paper. He studies systematics, which involves building and analyzing evolutionary trees that reveal relationships among organisms.

For the new study, Berv and colleagues performed statistical analyses of the ecological habits of modern mammals to determine if their ancestors were more likely to live in trees than on the ground, using a process called ancestral state reconstruction. Those analyses showed—as in the bird study—that the mammals that survived the end-Cretaceous mass extinction were mostly ground-dwelling or semi-arboreal.

However, while the signal of selection against arboreality was strong and unambiguous in birds, it is less clear in mammals, said study co-lead author Jonathan Hughes, a mammalogist and a doctoral candidate at Cornell University.

“The fossil record around this time period is pretty sparse,” Berv said. “The statistical models we use make the best guesses they can, but the uncertainty is still significant. In the absence of direct fossil evidence, our conclusions are conditional on the accuracy of our assumptions.”

The other authors of the Ecology & Evolution paper are Daniel Field of Cambridge University, Stephen Chester of City University of New York and Eric Sargis of Yale University. Berv’s work was supported by the National Science Foundation and the Michigan Life Sciences Fellows.

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