Fossil Discoveries in China continue to astonish paleontology, and re-write the text books. The fossil of the semi-aquatic mammal named Castorocauda lutrasimilis was discovered in the Middle Jurassic Jiulongshan formation of Inner Mongolia, China, as reported in Science by Qiang and co-workers. This is yet another discovery alongside the 2005 discovery of Repenomamus giganticus in China that showed land mammals had reached a meter in length during the Cretaceous Period, setting aside the dogma that dinosaur-age mammals were all, timid, nocturnal and shrew-like creatures.

An amazing aspect of Castorocauda lutrasimilis is a full pelt of fur, the oldest fur ever found in the fossil record. The exquisitely preserved fossil appears like a combination of platypus, otter, and beaver phenotypes, some half meter long and weighing more than a pound. It is the largest mammal ever found from the Jurassic Period that spans the time from 200 million to 145 million years ago. The Castorocauda had webbed feet and a broad flat tail apparently adapted for swimming, and teeth appropriate for catching fish. It is now the earliest mammal known to have partly lived in the water; it was some 100 million years later when whales and manatees ultimately moved to the water.