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.

Chinese archaeologists led by Qiang Ji of Nanjing University found the well-preserved fossil, including impressions of soft tissue and fur, in the Jiulongshan Formation in Inner Mongolia. Other fossils had hinted that mammals might not just have been small terrestrial creatures until the demise of the dinosaurs 65 million years ago but the beaver-tailed animal definitively pushes back the date of mammalian adaptation to an aquatic lifestyle by at least 100 million years. "Based on its relatively large size, swimming body structure, and anterior molars specialized for [fish] feeding, Castorocauda was a semiaquatic carnivore, similar to the modern river otter," the team writes in the paper announcing the find in today's issue of Science.

The discovery also highlights how little is known about early mammals. Most are represented by teeth and jaws alone. "We stand at the threshold of a dramatic change in the picture of mammalian evolutionary history," argues mammalogist Thomas Martin of the Senckenberg Institute in Frankfurt, Germany in an accompanying commentary. "The potential of fossil-rich deposits like the Jehol group in Liaoning Province in China or the Jiulongshan Formation in Inner Mongolia is only just beginning to be exploited."