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.