Eocene Epoch, a division of geologic time that began about 54,000,000 years ago and lasted about 17,000,000 years. It is the second epoch of the Tertiary Period. During the Eocene, mammals were the dominant form of animal life. Among them were the ancestors of today's horses, pigs, whales, rats, camels, and monkeys. Reptiles and birds were also common. Plants included elm, maple, beech, and walnut trees.