of the thirdrelating to or dating from a geological epoch, the third epoch of the Tertiary period, lasting from about 35.5 million to about 23.5 million years ago, and marked by extensive movements of the earth's crust that produced new mountain ranges, diminished forests, rising grasses, and the appearance of the first primates
Oligocene noun
[Greek oligo- + kainos new or recent; from the scarcity of new life forms during this period]