After dinosaurs went extinct 65 million years ago, land mammals tended to supersize and then hit a limit, roughly at the same time worldwide.
“Globally, there is a really clean pattern that shows up,” says paleoecologist Felisa A. Smith of the University of New Mexico in Albuquerque. In the broadest survey yet of big-mammal body size, she and colleagues found that mammal groups around the world tended to give rise to giant species at about the same time. The largest, for example, the hornless rhinoceros-like Indricotherium transouralicum, would have weighed about seventeen tons.
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