Jurassic Beaver?

No, not the Caltech mascot...

This L.A. Times story says paleontologists have found a 164 million year-old mammal. It's aquatic, furry, had a beaver-like tail, and weighed 500-700 grams, which is a lot heavier that previously known Jurassic mammals. It's not directly related to the beaver, though it has some similarities, and would push back the history of aquatic mammals by a hundred million years or so.