Hyracotherium (hyrax-like beast), also known as Eohippus or the dawn horse, is an extinct genus of very small (about 60 cm in length) perissodactyl ungulates that lived in the woodlands of the northern hemisphere, with species ranging throughout Asia, Europe, and North America during the early Tertiary Period and the early to mid Eocene Epoch. This small, dog-sized animal is the oldest known horse and was once considered to be the earliest known member of Equidae before the type species was reclassified as a palaeothere, of a perissodactyl family related to both horses and brontotheres. This animal appeared during Cenozoic era, the most recent era of geologic time, from about 65 million years ago to the present. The Cenozoic Era is characterized by the formation of modern continents and the diversification of mammals and plants. Grasses also evolved during the Cenozoic. The climate was warm and tropical toward the beginning of the era and cooled significantly in the second half, leading to several ice ages. Humans first appeared near the end of this era.