The third geological epoch of the Cenozoic
era. The Oligocene occurred approximately 34–23 million years
ago. During the Oligocene epoch, the climate cooled. Many modern mammals
evolved, including early elephants and Mesohippus, an ancestor
of the modern horse. Only a few archaic mammals, such as titanotheres, survived
into the epoch, and they became extinct before it ended.
AND PLANETARY SCIENCE