The discovery of a beaver fossil, thought to have lived when dinosaurs roamed the earth, could change scientists' theories on evolution.
The Castorocauda lutrasimilis, which was unearthed in China, is a previously unknown species of mammal.
At 164 million years old it has the oldest mammal remains ever found.
But as scientists thought mammals didn't evolve until dinosaurs became extinct 65 million years ago they now have to rethink their theories.
The fossil was found by German scientists in China and is 45 cm long.