Wed. Aug 10th, 2022

A short-billed echidna (Tachyglossus aculeatus), Budderoo National Park, New South Wales, Australia.  Echidnas are monotremes, one of the three main groups of mammals along with placentals and marsupials.  The group also includes platypus.  Scientists believe that monotrems were terrestrial creatures before the K-Pg asteroid impact and remained so afterwards.  Image credit: Daniel J. Field

A short-billed echidna (Tachyglossus aculeatus), Budderoo National Park, New South Wales, Australia. Echidnas are monotremes, one of the three main groups of mammals along with placentals and marsupials. The group also includes platypus. Scientists believe that monotrems were terrestrial creatures before the K-Pg asteroid impact and remained so afterwards. Image credit: Daniel J. Field

The mile-wide asteroid that hit Earth 66 million years ago wiped out almost all dinosaurs and about three-quarters of the planet’s plant and animal species.

However, some creatures survived, including certain mammals in rats that would later diversify into the more than 6,000 mammal species found today, including humans.

Why did these mammals survive while others perished in the devastating mass extinction that closed the Cretaceous? A new study suggests that terrestrial and semi-arboreal mammals were better able to survive the disaster than tree-dwelling mammals due to the global destruction of forests that followed the Chicxulub asteroid.

A possible exception to this pattern may have been the earliest primates that probably resembled modern tree tips and marmots. Evidence from the new study suggests that primates may have maintained an ability for arboreal habits through mass extinction, despite global deforestation.

The mammal study, published online October 11 in the journal Ecology and Evolution, is a follow-up to a 2018 study of birds by some of the same authors who reached similar conclusions about arboreality. Both articles highlight the crucial influence of the last Cretaceous apocalypse, known as the K-Pg mass extinction, in shaping the early evolutionary trajectories of today’s vertebrates.

The asteroid affected a heat pulse that ignited forest fires globally. Dense clouds of dirt and soot were pushed into the atmosphere, the planet cooled and probably blocked sunlight while acid rain poured down.

“Large-scale destruction of forest environments due to the Chicxulub asteroid impact probably affected the evolutionary trajectories of several groups, including land mammals. Our results are consistent with the hypothesis that predominantly non-arboreal mammals predominantly survived this mass extinction, ”says Jacob Berv, an evolutionary biologist at the University of Michigan, co-author of the new study.

Berv, a life sciences fellow at the UM Department of Ecology and Evolutionary Biology and the Museum of Paleontology, was also a co-author of the bird paper 2018. He studies systematics, which involves building and analyzing evolutionary trees that reveal relationships between organisms.

For the new study, Berv and colleagues performed statistical analyzes of the ecological habits of modern mammals to determine if their ancestors were more likely to live in trees than on Earth using a process called ancestral state-building. These analyzes showed – as in the bird study – that the mammals that survived the chalk mass extinction were mostly terrestrial or semi-arboreal.

Although the signal for selection against arboreality was strong and unambiguous in birds, it is less clear in mammals, says study author Jonathan Hughes, a mammalian and a doctoral student at Cornell University.

For example, researchers consistently found that Euarchonta, a group that included early primates, tree tips, and sliding mammals called colugos, maintained their tree-dwelling habits through the extinction event and its aftermath.

A chimpanzee (Pan troglodytes) in Kibale National Park, Uganda.  Evidence from the new study suggests that primates may have maintained a capacity for arboreal habits through mass extinction 66 million years ago, despite global deforestation.  Image credit: Daniel J. Field

A chimpanzee (Pan troglodytes) in Kibale National Park, Uganda. Evidence from the new study suggests that primates may have maintained a capacity for arboreal habits through mass extinction 66 million years ago, despite global deforestation. Image credit: Daniel J. Field

“We reconstruct ancestral primates as arboreal in our analyzes, and I think we are the first to suggest that primates may maintain arboreality through K-Pg extinction,” Hughes said. One explanation for this finding could be forested refugia: certain environments such as 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 woodworking adaptations, they could have been among the first mammals to return to trees after forests recovered. “

Modern primates have been thought to be resistant to rapid environmental change due to their sociality, cognitive abilities, and dietary and locomotive flexibility. At least some of these and other traits — such as omnivorous and small-bodied size — “may have contributed to the survival of representatives of the primate group when faced with the destruction of the forests at the end of the Cretaceous,” according to the authors of new study.

Evidence from some of the models in the new study also suggests that early marsupials may have stuck to tree-dwelling capacities through the K-Pg boundary.

And Virginia opossum (Didelphis virginiana), Croatan National Forest, North Carolina.  Opossums are marsupials, one of the groups that may have retained some semi-arboreal properties from the time of K-Pg exposure, according to the new study.  Marsupials suffered some of the greatest diversity losses and longest recovery times in the wake of the impact 66 million years ago.  They were almost exterminated from North America and are found today mainly in Australia, Tasmania and New Guinea.  Image credit: Daniel J. Field

And Virginia opossum (Didelphis virginiana), Croatan National Forest, North Carolina. Opossums are marsupials, one of the groups that may have retained some semi-arboreal properties from the time of K-Pg exposure, according to the new study. Marsupials suffered some of the greatest diversity losses and longest recovery times in the wake of the impact 66 million years ago. They were almost exterminated from North America and are found today mainly in Australia, Tasmania and New Guinea. Image credit: Daniel J. Field

Marsupials are mammals that today include kangaroos, wombats, bandikoots, opossums, and related animals that do not develop a true placenta and that usually have a pouch on the female’s abdomen. They suffered some of the greatest diversity losses and longest recovery times in the wake of the K-Pg influence: they were almost completely eradicated from North America and are found today mainly in Australia, Tasmania and New Guinea.

Sloths and their closest living relatives, ants and armadillos, are an example of a group of mammals that began as diggers before diversifying and becoming more and more arboreal after the K-Pg extinction.

Primates, marsupials and sloths may have been among the first animals left in the trees when the forests recovered, according to the new study. These 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 dormitories, wooden squirrels, and bats independently acquired arboreal habits well into the Cenozoic, the geological era that began after the K-Pg impact 66 million years ago, according to the study.

An American red squirrel (Tamiasciurus hudsonicus) in Calgary, Alberta.  Most of today's wood-dwelling mammals, such as red squirrels, originate from the asteroid impact 66 million years ago, which destroyed forests around the world.  A new study suggests that terrestrial and semi-arboreal mammals were better able to survive the event.  Image credit: Daniel J. Field

An American red squirrel (Tamiasciurus hudsonicus) in Calgary, Alberta. Most of today’s wood-dwelling mammals, such as red squirrels, originate from the asteroid impact 66 million years ago, which destroyed forests around the world. A new study suggests that terrestrial and semi-arboreal mammals were better able to survive the event. Image credit: Daniel J. Field

The study authors warn that well-preserved mammalian fossils from the time of the K-Pg extinction and during the first million years thereafter are extremely rare and usually fragmentary. Contrary to the findings of the previous paper on birds, which were strongly supported by both phylogenetic and fossil evidence, “final assessments of selective patterns among K-Pg cross-border mammals will remain elusive in the absence of further fossil evidence,” they wrote.

“The fossil record around this 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 conditioned by the accuracy of our assumptions. ”

The mammalian study contains two new methods that were not used in the bird paper from 2018. First, instead of simply modeling transitions between arboreal, semi-arboreal and nonarboreal conditions among mammals, the researchers looked at changes in the frequency of these transitions through the time. Their data revealed a large increase in the frequency of these transitions associated with the K-Pg limit.

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 that non-arboreal mammals survived 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 Michigan Life Sciences Fellows.

/Disclosure. This material comes from the original organization / author (s) and may be of a point-in-time nature, edited for clarity, style and length. The expressions and views expressed are those of the author (s). See full here.

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