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Tree-dwelling mammals endured after asteroid strike destroyed forests
An asteroid strike 66 million years ago wiped out the non-avian dinosaurs and devastated the Earth’s forests, but tree-dwelling ancestors of primates may have survived it. Image courtesy: Daniel Field.

NEW HAVEN, CT.- An asteroid strike 66 million years ago wiped out the non-avian dinosaurs and devastated the Earth’s forests, but tree-dwelling ancestors of primates may have survived it, according to a new study published in the journal Ecology and Evolution.

Overall, the study supports the hypothesis that the widespread destruction of forests following the asteroid’s impact favored ground-dwelling mammals over their arboreal counterparts, but it also provides strong evidence that some tree-dwelling taxa also survived the cataclysm, possibly nesting in branches through the Cretaceous-Paleogene (K-Pg) extinction event.

“We can’t fully understand the composition of life on Earth today without considering the fallout from the asteroid’s impact, which altered the evolutionary trajectories of many animal lineages,” said study co-author Eric Sargis, professor of anthropology in Yale’s Faculty of Arts and Sciences, director of the Yale Institute for Biospheric Studies, and curator of mammalogy and vertebrate paleontology at the Yale Peabody Museum of Natural History. “By reconstructing the ancestors of modern mammal lineages back to the extinction event, we show that ground-dwelling mammals had a selective advantage over arboreal mammals, whose habitat was destroyed, but that some tree-dwellers managed to survive.”

Jonathan Hughes of Cornell University and Jacob Berv ’10 B.S. of the University of Michigan were the lead authors.

The K-Pg mass extinction event occurred when a meteor slammed into Earth at the end of the Cretaceous period. The impact and its aftereffects killed roughly 75% of the animal and plant species on the planet, including whole groups like the non-avian dinosaurs.

For the study, the researchers analyzed patterns of substrate preferences among all mammals still in existence and their ancestors, working backwards from the present day to before the K-Pg extinction event by tracing these traits along numerous phylogenetic trees — diagrams that illustrate the evolutionary relationships among species based on genetic data in this case.

“Our study takes advantage of an ongoing revolution in our understanding of the tree of life, only made possible by researchers working in association with natural history collections,” Berv said. “By integrating data from such collections with modern statistical techniques, we can address new questions about major transitions in evolutionary history.”

The researchers classified each mammalian species as arboreal, non-arboreal, or semi-arboreal. To be considered arboreal, the species had to nearly always nest in trees. Categorizing some species could be tricky. For example, many bat species spend a lot of time among trees but nest in caves, so bats were mostly categorized as non-arboreal or semi-arboreal.

“We were able to see that leading up to the K-Pg event, there was a spike in transitions from arboreal and semi-arboreal to non-arboreal habitat use across our models,” Hughes said.

The work builds on a previous study led by Daniel Field ’17 Ph.D., the senior author of this new paper, which used the same analytical method — known as ancestral state reconstruction — to show that all modern birds evolved from ground-dwelling ancestors after the asteroid strike.

“The fossil record of many vertebrate groups is sparse in the immediate aftermath of the extinction,” said Field, an assistant professor of earth sciences at the University of Cambridge and curator of ornithology at the University of Cambridge Museum of Zoology. “Analytical approaches like ancestral state reconstruction allow us to establish hypotheses for how groups like birds and mammals made it through this cataclysm, which paleontologists can then test when additional fossils are found.”

The analysis helps illuminate ecological selectivity of mammals across the K-Pg boundary despite the relatively sparse fossil record of mammalian skeletal elements from the periods immediately preceding and following the asteroid’s impact, Sargis explained.

How the tree-dwelling ancestors of primates survived the asteroid’s destruction is unclear. It’s possible that some forest fragments survived the calamity or that early primates and their relatives were ecologically flexible enough to modify their substrate preferences in a world mostly denuded of trees, Sargis said.

The analysis also suggests that some marsupial lineages may have resided in trees through the K-Pg extinction event, although the evidence for this finding is less robust than that supporting the conclusion about primates and their close relatives, Sargis said.

Stephen Chester ’13 Ph.D. of Brooklyn College, City University of New York, and a curatorial affiliate of vertebrate paleontology at the Yale Peabody Museum of Natural History, also co-authored the study.

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