The Eocene is the second of five epochs in the Tertiary Period — the second of three epochs in the Paleogene — and lasted from about 55.8 to 33.9 million years ago.* The oldest known fossils of most of the modern orders of mammals appear in a brief period during the early Eocene and all were small, under 10 kg. Both groups of modern ungulates, Artiodactyla and Perissodactyla, became prevalent mammals at this time, due to a major radiation between Europe and North America.