Belong to: Tertiary Period
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The Eocene epoch is part of the Tertiary Period in the Cenozoic Era, and lasted from about 54 to 38 million years ago (mya). 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.