The Green River Formation One of the most important fossil sites for understanding the Eocene is found at Green River, located in western Colorado, eastern Utah and southwestern Wyoming in the United States. During the Eocene, this region was located at much the same latitude it is today, though global climate was more equable. Therefore, the climate in which the organisms lived differs somewhat from that of the present-day western United States. The fossils, especially plants, found at this site indicate that the climate was moist temperate or sub-tropical, with temperatures ranging from 15 to 20 degrees Celsius. In addition to the plants, another piece of evidence suggesting that the climate was sub-tropical was the presence of fossilized crocodiles.
Crocodiles can only survive in areas with a constant, warm temperature.
If you were able to visit the Green River locality during the Eocene, you would see palms, cat-tails, sycamores, and other familiar plants from North America, but you would also see some that are today more common in, or restricted to, eastern Asia. A series of large inland lakes extended across the region, and it is in the bottoms of these lakes that various plants and animals were buried and fossilized. These lakes later dried up as the local climate changed, and many of the plants and animals living here went extinct. At right, a sycamore leaf Platanus wyomingensis . Crocodiles and sycamores are among the fossils that provide evidence for a warmer and wetter climate in the Eocene of Western North America.