A new fossil from China proves that the mammals that lived during the Jurassic era were more diverse than previously thought. The 164-million-year-old creature, dubbed Castorocauda lutrasimilis, had a tail like a beaver, the paddling limbs of an otter, seallike teeth and probably webbed feet. And although most Jurassic mammals discovered thus far were tiny, shrewlike animals, C. lutrasimilis, would have weighed in at approximately a pound. Roughly the size of a small, female platypus, it is the largest mammal from this time period on record.