66-1.6 Million Years Ago
During the Paleocene and Eocene epochs, lasting more than 30 million years, deposition continued in Southern Illinois. Plants began to develop their modern forms and mammals diversified. A period of mountain building worldwide, the Tertiary witnessed the rise of the Alps, Himalayas, and Rocky Mountains. A long-term decline in global temperatures began during the end of the Eocene Epoch. The poles had been warm enough to support reptiles before carbon dioxide was scrubbed from the atmosphere by Azolla ferns. The absorbed gas was sequestered when the ferns were buried on the sea floor. Carbon dioxide levels fell 80% over 800,000 years. Continental drift during the Eocene altered circulation patterns in the atmosphere and ocean, leading to drier and more seasonal weather and the return of ice to the poles. Animals like Aepycamelus of the Miocene Epoch (23-5 million years ago) roamed North American prairies before the onset of the Quaternary Ice Age.