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EVOLUTION
To paleontologists who study mammals, you are what you eat with. Teeth are often the only remains of tiny, extinct mammals, but they can reveal an animal's diet as well as its place on the family tree. The most important advance in mammalian dental evolution has long been regarded as the tribosphenic molar-a Cuisinart-like tooth that could both slice and grind. This was considered a key innovation, shared exclusively by placental mammals and marsupials, that helps explain their extraordinary success ever since the Cretaceous period.
Now three paleontologists propose that the tribosphenic molar evolved not once, but twice-a highly provocative idea. "It shakes a bedrock principle that we've held for a long time," says Andy Wyss of the University of California (UC), Santa Barbara. In the 4 January issue of Nature, the trio argues that this kind of molar independently appeared in the Southern Hemisphere in fossil relatives of the monotremes, an extremely ancient group of mammals that includes the platypus. Because the hypothesis is based on extremely limited evidence, many paleontologists are reacting cautiously. "I think many people would tend to take it with a grain of salt right now," says Michael Woodburne of UC Riverside. But Bill...