For a span of about 20 million years, "Eohippus" thrived with few significant evolutionary changes. The most significant change was in the teeth, which began to adapt to its changing diet, as these early Equidae shifted from a mixed diet of fruits and foliage to one focused increasingly on browsing foods. During the Eocene, an "Eohippus" species (most likely "Eohippus angustidens") branched out into various new types of Equidae. Thousands of complete, fossilized skeletons of these animals have been found in the Eocene layers of North American strata, mainly in the Wind River basin in Wyoming. Similar fossils have also been discovered in Europe, such as "Propalaeotherium" (which is not considered ancestral to the modern horse).