CROSSING A CLIMATE THRESHHOLD

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Mysterious Fossil Mammals

The most remarkable thing about this greenhouse era in North America was the sudden diversification of modern mammals from just a few predecessors. Ancestors of today's barnyard residents-cows, pigs, and sheep-roamed among tropical ferns and palm trees, along with the early relatives of deer, camels, and hippos. Rhinos and miniature horses galloped across the landscape. And cat-sized primates with large eyes and long tails jumped from tree to tree catching insects.

Paleontologists had long wondered whether climatic changes led to this flourishing menagerie. Now Koch and Zachos have evidence implying that the new mammals were ushered into North America by a quick pulse of high temperatures in the already warm world. To get that evidence, the pair had to connect clues from land and sea...