How did the spread of grasslands affect horse evolution?

The spread of grasslands during the Miocene Epoch acted as a primary driver for significant evolutionary changes in the horse family, shifting them from forest-dwelling browsers to specialized grazers of open country.12

As grasses became widespread, they provided a vast new food supply that was much coarser than the succulent leaves consumed by earlier ancestors like Eohippus. This dietary shift necessitated major anatomical adaptations:1

• Dental Evolution: Horses developed high-crowned (hypsodont) teeth with complex, crested surfaces and cementum to resist the abrasive nature of silica-rich grass. These teeth were adapted for the side-to-side grinding motion required to process tough vegetation, with long crowns that grew out as the surfaces wore down over the animal's life.12
• Locomotion: To survive in open grasslands, horses evolved for speed and efficiency. Their limbs became longer and more rigid, with fused lower leg bones and a reduction in the number of toes. The development of a spring mechanism in the feet allowed for more efficient running, which was essential for navigating open terrain and escaping predators.12

These adaptations allowed the lineage leading to the modern genus Equus to thrive across diverse environments.12