One ancient swimming mammal fossil and two evolutionary surprises, including a third set of teeth

What may have looked like a cross between a platypus and a beaverlike rodent and lived around 120 million years ago? Dongoconodon platycauda, a newly discovered fossil mammal from northeastern China. Beyond its unusual features, such as evidence of webbed paddling feet and a rudderlike tail, this semiaquatic mammal had a surprise waiting for scientists in its jaw: a third generation of teeth.

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