Ancient "Beavers"

We know what you're thinking...but this isn't that kind of Web site. We're referring to a newly discovered fossil, for your information. It seems that early mammals were a lot bigger and more complex than paleontologists originally thought. The latest issue of Science reports that the Middle Jurassic–era fossil in question, Castorocauda lutrasimilis, turned up in Inner Mongolia. During its lifetime, the critter weighed around a pound—huge compared to other mammals of that era, which have been roughly the size of mice. C. lutrasimilis had—in addition to a warm-blooded metabolism and a fur pelt with guard hairs—a flat, beaver-like tail and webbed feet. These features indicate that mammals exploited aquatic habitats far earlier than previously believed. —Martha Harbison