Herds of tiny, deerlike mammals known as Leptomeryx fed in the forests and grasslands of North America for more than 13 million years. Leptomeryx was not much bigger than a hare and probably just as nimble, darting around on its tiny hooves in the undergrowth. This little planteater flourished in huge numbers and was probably an easy snack for predators of the time.
■ When: 38–25 million years ago (Paleogene)
■ Fossil location: USA
■ Habitat: Grasslands
■ Length: 1 ft (0.3 m)
■ Diet: Mostly plants
Leptomeryx was a ruminant—a hoofed mammal that brings up food from its stomach to chew a second time. During the period in which it lived, the climate changed and grasslands replaced forest. Fossils show that the teeth of Leptomeryx became stronger at the same time—perhaps it was evolving to live on rougher foods such as grass, which contains tiny flecks of glass to protect itself from herbivores. Leptomeryx was built like a deer but had no antlers, although males had small tusks formed from their canine teeth.
On their toes
Hoofed mammals are divided into two groups depending on how many toes they have. Horses, rhinos, and tapirs have an odd number of toes; antelopes, deer, hippos, and pigs have an even number of toes.
Chevrotains (also called mouse deer) are found in the rainforests of Southeast Asia and Africa. Although it’s not clear whether they are close relatives of Leptomeryx, they are about the same size and, like Leptomeryx, lack horns or antlers but have small tusks. These deerlike animals usually live in pairs.