During the dawn of the “Age of Mammals”, around 56 million years ago, the global temperature rapidly stabbed upwards. In about 100,000 years temperatures rose over 40 degrees Fahrenheit. Paleontologists know this as the Palaeocene/Eocene Thermal Maximum (PETM for short), and there’s hardly a better place to see its effects than the deserts of Wyoming. The fossil localities here are famous for producing a detailed sequence of species from this time, recording how life responded to rapid climate change.
But what does this change really mean? Up until now, paleontologists have considered two hypotheses. It could be that the larger mammal species evolved to become smaller over time in a straight-line fashion called anagenesis. Then again, the smaller species could be closely-related immigrants that had lived in warmer habitats and were able to thrive as the larger species went extinct. Either way, the hypothesis is that smaller-bodied mammals were probably better able to shed heat and cope with altered nutritional values of plants that go along with high CO2 levels.
But the new study by Brian Rankin and colleagues looks at another possibility – species selection. The logic is the same as that of natural selection, but bumped up one level. Just like individuals, the argument goes, some species will vary in ways that make them more successful in splitting off descendant species than others. In this case, large-bodied mammal species would have suffered in the hothouse world, while the species that were already small would have spun off an increased number of new lineages. While the large species went extinct, the small species would have proliferated.
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