As stated in Part 1, grass enormously affected evolution of most Cenozoic mammals. Since grass is rich in minerals, it was a prime candidate of food for the herbivores, such as brontotheres (Creature Feature - Oligocene). These herbivores, in turn became a prime candidate of food for the carnivores. Grass, though, had its drawbacks. Its cell walls were much tougher than the previous plants of the times. Therefore, mammals had to adapt. Some developed a 3-chamber stomach, called a rumen. This helped break down the cellulose found in grass even more. For more information about these herbivores, see the Oligocene epoch's Creature Feature: Brontotheres and the Herbivores. Or, to find out how grass affects humans, see Grass - Part 3.