The Epoch of the Tertiary Period is a time period that lasted from 66 million to 2.6 million years ago and is divided into three sub-periods: the EOCENE (66-33.9 million years ago), the Oligocene (33.9-23 million years ago), and the Miocene (23-2.6 million years ago). The EOCENE is characterized by the emergence of modern mammals, the Oligocene by the expansion of grasslands, and the Miocene by the cooling of the climate.