(began 58 million years ago)
In the Eocene Epoch, mammals emerged as the dominant land animals. In addition, they took to the air and the sea. The increase in the diversity of mammals that had begun in the Paleocene continued at a rapid pace. There were many variations in life forms. For example, some of the earliest giant forms of mammals arose in the Eocene. Some were successful, some not. Although the fossil record reveals that many mammals were quite unlike anything we can see today, increasingly there were many forest plants, freshwater fish, and insects that were much like those today.
Many freshwater fish swam in North American lakes of the Eocene. Gars (bottom), herring (middle), and sunfish (top), are similar in appearance to these Eocene fish. Fossil Butte NM
One of the first bats, the only type of mammal ever to develop the power of active flight, took to the air more than 50 million years ago. Fossil Butte NM
The delicate bones of shorebirds, including frigate birds are preserved in the fine-grained sediment of Eocene lake deposits.
Fossil Butte NM
Ancient tapirs such as Heptodon browsed not far from the shores of Fossil Lake. Unlike modern tapirs, Heptodon had a very small snout. Fossil Butte NM
Coryphodon, with its short stock limbs and 5-toed hoofed feet, closely resembled a tapir. The brain was very small, and in males the tusks were very large. Coryphodon lived on land not far from the shores of Fossil Lake.
Fossil Butte NM
The first horse-like creatures lived in Eocene forests. They were barely bigger than today's domestic cat. Throughout the Cenozoic Era their size increased; their legs became longer and their feet changed from many-toed to single-hoofed, for faster running. Their teeth became adapted from browsing to grazing. Fossil horses occur at many National Park System sites.
continue to Oligocene