Need help from someone who knows American Beavers well?

Mind the caps lol, but anyways I'm having trouble researching exactly the oldest ancestor of the American Beaver. Now, I understand that there was one found in China before the Dinosaurs, but there others that I'm not so sure of. Also, can anyone verify how exactly the American Beaver evolved? From my understanding, it was cases of geographic isolation with the separation of North America and Eurasia, but again, the info is very wide on this. Also, can anyone specify animals closely related to this beaver? I understand the squirrel is, but I need an exact species name. Thanks!!!