During the tertiary period, the ancestors of modern horses and cattle evolved with adaptations for eating what new food source?

The answer to the given question above would be GRASS. During the tertiary period, the ancestors of modern horses and cattle evolved with adaptations for eating GRASS. As they began to specialize in grass eating, and in living on the grass-lands, their physiologies and anatomies underwent a number of changes and this includes changes in their teeth. Hope this answer helps.

Organism, organ system, organs, tissue, cells