Dino Beaver, a.k.a.: Castorocauda lutrasimilis
A prehistoric beaver was recently unearthed in Mongolia. Scientists believe this may be the oldest aquatic mammal ever discovered, but it is NOT a precursor to modern day beavers and otters.
From the article on the FOX web site:
Although Castorocauda bears many similarities to beavers, otters, and platypuses, it's not an ancestor of any of them. It is what scientists sometimes call a "dead-end species."But then, aren't they all?
Read more about the ancient beaver here: A Mammal Swam With the Dinosaurs