This period saw the splitting-up of the Mesozoic landmasses, which came close to their modern-day positions. Earth's climate changed a lot: There was a cooler time towards the beginning, then a warming trend and then a cool interval again. Most mountain ranges that we know and love today were created then. All of this helped with the evolution of mammals.
Grass, grass, grass! The trees and ferns of previous eras were no match for this new and abundant flowering plant. Grass caused a huge evolutionary change in the eating habits of herbivores, and it was well suited for the Cenozoic's climate change.
Cenozoic Era: Tertiary Period