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Long-term global climate shifts are correlated with the successive rise and fall of different evolutionary mammal groupings, according to recent research by a team that includes Professor of Biology Christine Janis.

The researchers looked at North American mammal populations in the Cenozoic Era — which covers the past 65 million years — and found correlations with long-term climate patterns in an article published in the Proceedings of the National Academy of Sciences.

The scientists identified six associations of large mammals that shared similar patterns of population rise and fall and grouped them together as faunas. They then compared the evolution of these faunas to past temperatures inferred from oxygen isotopes. They found that, with the exception of two faunas, the transitions between groups were correlated with climatic changes.

"When that group first appeared, rose to prominence and then disappeared is tied by and large with measurable changes in the global climate," Janis said. The two faunas that showed a lesser correlation were perhaps influenced by migrations of large mammals to North America from other continents.

Janis cautioned against using these results to predict future trends in mammal populations in the context of global warming. 

Still, the findings help "us better understand something about how organisms are responding to global climate change," said David Polly, associate professor of geological sciences at Indiana University at Bloomington, who commented on an earlier draft of the article.

In an email to The Herald, John Alroy, future fellow in the Department of Biological Sciences at Macquarie University in Sydney, Australia, criticized some of the statistical methods used in the study — including performing "no correction for variation in sample size" — as well as the study's failure to look at small mammals.

Janis defended her work. "We did not include the small mammals with this study because it is well known that the collection biases for small mammals are different to those of the larger ones," she wrote in an email to The Herald. 

"However, we do intend to include them at a later date — in fact to study them separately and see if the same patterns hold up," she wrote.

"The fact that they could quantify the faunas in the way that they did and then be able to correlate the record with the oxygen isotope curve was clearly innovative and produced new results," said Thompson Webb III, professor emeritus of geological sciences at Brown, who also commented on an earlier version of the manuscript.

Future research directions include looking at herbivores versus carnivores, as well as studying the significance of certain kinds of fauna to "understand the underlying structure of what's causing those kinds of animals to group together," Janis said.


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