Myself, I think it's a no brainer that the mammals were heavily evolving in the time of the dinosaur. They weren't these ignored things in the shadows.
Let's drop a few facts:
- Mammals evolved a heat regulation system. This allowed them to live in a wider variety of environemnts.
- Mammals evolved a hibernation mechanism. This allowed them to live in a wider variety of environments.
- Mammals evolved milk glads. This allowed adult mammals to eat nutrient poor food and convert it into nutrient rich food. This allowed them to live in a wider variety of environments.
- Mammals are placental. That allows them to put a minimal amount of energy into their young. If times are good, they rear the young. If times are bad, they can abort their young.
- Mammals rear litters fast, often multiple litters in the same year.
What does that tell us:
- Mammals were often living in the marginal, difficult environments.
- Mammals needed to watch their energy expenditure.
- Mammals had to reproduce fast.
- Mammals were under pressure.
- Pressure drive evolution.
Why do creatures go live in difficult environments? Because of competition. Those dino vacuum cleaners were good at sucking down all the highest energy food. The other source of competition was othe rmammals. So many mammals were left with roots, nuts, insects, and small places that the bigger dinos just didn't go and other mammals weren't very specialized at eating.
Let's look at dinos. Why were there so many small, fast carnivores with small teeth and agile hands. Could it be that their favorite food was small, fast, and plentiful? Like, say, MAMMALS? You can bet on that. Entire branches of carnivores pretty much specialized on these critters as lunch. Small carnivores didn't survive on carrion alone and I severly doubt that these fine-boned creatures made of habit of hunting mega-game.
So, the dinos munched on the mammals. That's a big pressure. You can bet those small carnivores put a continuous burden on the mammal population. Why else so nicely explains a species which exists to corner-case survival.
Not surprisingly, after the meteor hit, mammals were in a wonderful position to survive. They were already good at survivng in marginal environments.