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.
For example, the researchers consistently found that Euarchonta, a group that included early primates, tree shrews and gliding mammals called colugos, maintained their arboreal habits through the extinction event and its aftermath.
"We reconstruct the ancestral primates as arboreal in our analyses, and I believe we are the first to suggest that primates possibly maintained arboreality through the K-Pg extinction," Hughes said. "One explanation for this finding could be forested refugia: certain environments like marshes that were less susceptible to total deforestation.
"Another possible answer is that these early primates were behaviorally flexible enough to survive without trees. If they retained their arboreal adaptations, then they could have been among the first mammals to return to trees after forests recovered."
Modern-day primates have been hypothesized to be resilient in the face of rapid environmental change on account of their sociality, cognitive abilities, and dietary and locomotive flexibility. At least some of these and other traits—such as omnivory and small body size—"may have contributed to the survival of representatives of the primate total group when facing the devastation of forests at the end-Cretaceous," according to the authors of the new study.
Evidence from some of the models in the new study also suggests that early marsupials may have held onto tree-dwelling capabilities through the K-Pg boundary.
Marsupials are mammals that today include kangaroos, wombats, bandicoots, opossums and related animals that do not develop a true placenta and that usually have a pouch on the abdomen of the female. They suffered some of the greatest diversity losses and longest recovery times in the wake of the K-Pg impact: They were almost completely eradicated from North America and today are mainly found in Australia, Tasmania and New Guinea.
Sloths and their closest living relatives, anteaters and armadillos, are an example of a group of mammals that began as diggers before diversifying and becoming increasingly arboreal after the K-Pg extinction.
Primates, marsupials and sloths may have been among the first animals back in the trees once forests recovered, according to the new study. Those creatures "may have retained a capacity for arboreal habits across the K-Pg boundary and may have already been adapted to exploit arboreal niches relatively quickly as these habitats recovered," the authors say.
In contrast, arboreal latecomers such as dormice, tree squirrels and bats independently acquired arboreal habits well into the Cenozoic, the geologic era that began after the K-Pg impact 66 million years ago, according to the study.
The study authors caution that well-preserved mammalian fossils from the time of the K-Pg extinction, and during the first million years afterward, are exceedingly rare and usually fragmentary. Unlike the findings of the previous paper on birds, which were strongly supported by both phylogenetic and fossil evidence, "definitive assessments of selective patterns among K-Pg boundary-crossing mammals will remain elusive in the absence of additional fossil evidence," they wrote.
"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 mammal study features two novel methods that were not used in the 2018 bird paper. First, instead of simply modeling transitions between arboreal, semi-arboreal and nonarboreal states among mammals, the researchers looked at changes in the frequency of those transitions through time. Their data revealed a large spike in the frequency of those transitions associated with the K-Pg boundary.
Second, instead of running their simulations on a single "best" evolutionary tree, they ran the simulations over a set of 1,000 credible evolutionary trees—and still found a strong signal of nonarboreal mammals surviving the extinction.
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.
Ecology and Evolution