I.

Introduction

Oligocene Epoch, third and final division of the Paleogene Period of the Cenozoic Era, spanning an interval from about 34 million to 23 million years ago. Like the Eocene Epoch, which preceded it, and the Miocene, which followed, the Oligocene (Greek, “little life”) was originally defined by the percentage of modern species of shellfish (10-15 percent) found in strata of this age.