Larger size would be an advantage for survival in this new habitat because

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"Horses are much larger today than their two-foot tall ancestor, mesohippus. Geologists believe the change in size was the result of changing environments, from tropical forests to a grassland habitat. Larger size would be an advantage for survival in this new habitat because "

larger horses had longer legs and could run faster to escape predators.

Explanation:

A pasture habitat left horse ancestors more exposed to their predators than a tropical forest. Because of all this exposure and the imminent danger of being reached by a predator, the horses have adapted and evolved to ensure their survival. In a grassland environment, long legs are a favorable feature, because they allowed horses to run faster to escape predators.