| Summary: | Life During the Oligocene Epoch: North Dakota Scrubland and Savanna The early Eocene Epoch greenhouse conditions gradually came to an end by the early Oligocene, about 34 million years ago. This worldwide deterioration of climate and cooling resulted in the formation of ice sheets on Antarctica. It has been estimated that global temperatures dropped by almost 22�F during this time. In North Dakota, the climate changed from the equable, humid warm temperate to subtropical conditions during the Paleocene Epoch and early Eocene Epoch to cooler and drier conditions at the end of Eocene and during the Oligocene Epoch, about 34 million to 30 million years ago. By the Miocene Epoch, the climate was semi-arid to arid and cool temperate with pronounced dry seasons, not much different than western North Dakota today. The swampy environments of the Paleocene and early Eocene began to disappear, along with the plants and animals that had inhabited them. Alligator fossils are found in the Eocene Chadron Formation in North Dakota, indicating that the climate was still warm and wet enough to support them. But conditions were changing. This mural at the North Dakota Heritage Center shows life in the North Dakota scrubland plain about 30 million years ago. Large mammal in the foreground is the giant pig-like animal Archaeotherium. (Figure 18) After that time, these reptiles had essentially disappeared in North Dakota, never to return. By the Oligocene Epoch, about 30 million years ago, the subtropical, swampy forests had given way to a mostly treeless plain, similar to a savanna, in North Dakota. (Figure 18) This open plain was mostly a scrubland consisting of shrubs, herbaceous plants, and possibly some grasses. True savanna habitats, dominated by grasslands similar to areas in Africa today, did not become established in North Dakota until the Miocene Epoch. The climate became seasonal and temperate, with an annual rainfall within the sub-humid to semi-arid range. Ponds and lakes punctuated the open plain, and gallery woodlands grew along the Chapter One - History of Ancient Life in North Dakota 25
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