This week's Nature (subscription link) has a fantastic letter describing two new Cretaceous-era mammalian fossils found in the Yixian formation in China. Instead of being rat-like losers, both of these new fossils represent mammals with an attitude: one was the size of a cat, and had the fossilized remains of a dinosaur where its stomach had been (see livejournal> Jim Haze for a longer description.). The other, R. giganticus, was even bigger- the size and weight of a large dog.
These finds may provoke re-thinking of the idea that mammals were being kept exclusively to the margins by dinosaurs during the Cretaceous era; indeed, in the fossil bed where the finds were made, the R. giganticus fossil is the bigger than most of the dinosaur fossils. In this local context, then, it's the mammals who were top dog.
The Yixian formation in northeast China continues to yield spectacular fossil finds, upending all sorts of ideas about the folkways of the Cretaceous era. For an idea of the detail of preservation in these fossil beds, check out this Pharyngula post from a while back. Phenomenal!
QUICK UPDATE: The BBC has a good summary, and linkouts to other media.