The post-Cretaceous extinction saw the world initially as relatively tropical climate, moving towards a gradual cooling (which eventually led to the quasi-modern ice ages). Mammal diversification flourished in the newly-cleansed world. Continental plates brought the globe closer and closer to their modern locations. Indeed, the stage was being set over these 40 million years or so for a tremendous change around the corner...
A nice clean start for the Earth and the most excellent rise of the mammal.