Oligocene Formation refers to a time period of 25 to 35 million years ago. Current-Day mammals related to that time were small and evolved to a present, larger size. Most larger mammals of that time became extinct along with others that did not survive... Also see dig storyboard » Oligocene Dig Oreodont Skeleton, squirrel skull and Ictops skulls (insectavors). Turtles, turtle skull and Rhino leg. Oreodont skulls. Four turtles in rock.