Mammals Took A While To Flourish

Even after the asteroid hit, it wasn't a quick take-off:

But it was not until at least 10 million to 15 million years afterward that the lineages of living mammals began to flourish in number and diversity.

Sparky says that's because flourishing is hard work. Cloudy says flourishing is actually pretty easy, if you are a bunny, but is willing to listen to arguments about the matter.