What were the mammal ancestors like 65 million years ago?

Question

What we know about our mammal ancestors that were alive (and survived) at the extinction event 65 million years ago?

Answer 1

Many of them looked like little rodents. However, several distinct mammalian lineages were already present, including Monotremes, marsupials and placental mammals. Throughout the whole mesozoic era, mammals were already quite diversified! Also, even though most of the mammals that survived the K/T boundary were rather small, there were already some larger mammals around in the Cretaceous.

Here is a nice picture of a symmetrodont:

See a nice page on early mammals.

Answer 2

A recent genetic analysis suggests that they were probably bigger that we thought before :

http://www.newscientist.com/article/dn22343-mammals-ancestor-was-not-as-puny-as-we-thought.html