Horses competed with rhinos for food

(Feb 8, 1999) Five million years ago ancient horses -- some as small as dogs -- competed for food with rhinoceroses and elephants in a Florida that resembled equatorial Africa.

Scientists have used the fossilised teeth of six species of ancient horses to reconstruct the animals' diets and paint a picture of prehistoric Florida.

The modern-day horse was brought to the Americas by Spanish explorers. But prehistoric horses roamed North America millions of years earlier until dying out about 10,000 years ago.

Scientists studied the fossilised teeth of those ancient horse ancestors, found in central Florida phosphate mines. The six species ranged from a three-toed, dog-sized animal to a one-toed, zebra-sized equine called Dinohippus mexicanus, among the closest relatives of modern-day horses.

Paleontologists had thought ancient horses were mostly grass eaters because 20 million years ago they evolved long, high-crowned teeth, perfect for snipping gritty grass. The five million-year-old Florida horses shared those long teeth.

The researchers found that some of the ancient horses ate only grass, some a mix of grass, leaves and shrubs and, surprisingly, the dinohippus, the one related to modern horses, ate mostly from trees and shrubs.

Some horses were tiny because they competed for food not just with other horses but with rhinoceros and elephant-like beasts with similar grazing habits.

Not much later, 4.8 million years ago, North America experienced a mammal extinction that killed off huge beasts like rhinoceroses, as well as three of the horse species.

The smallest horses that originally ate mostly grass became extinct. The bigger scrub eaters, including Dinohippus, survived -- and then they switched their diet to the grass their smaller relatives left behind.