The Eocene Epoch (meaning "dawn of the recent") encompasses that time between approximately 55 million years ago and 34 million years ago. It was during the Eocene that mammals took over the large-animal niches previously held by the dinosaurs.
The Eocene was a time of warm climate and significant vulcanism was present in the western U.S. and central Mexico. Sea levels were high, and much of the southeastern United States was submerged. Europe was separated from Asia by a narrow strait or sea.
By the Early Eocene, practically all of the modern eutherian (truly warm-blooded) mammal orders are present.
Classic Fossil-Bearing Sites
Messel Oil Shale, Germany
Green River Formation, Western U.S.
The Green River Formation, found in various western states, provides wonderful and prolific samples of fossils. The area around Kemmerer, Wyoming provides world-class fish fossils of Eocene age.
Several remnants of extraterrestrial impacts of Eocene age are found throughout the world, including the largest one since that time. A 20 km crater is found in Logancha, Russia (50 mya), a 17 km crater is found in Belorus (40 mya), a 28 km crater has been discovered in Labrador, Canada (38 mya), and a 10 km structure is found in Ontario, Canada (37 mya). The 100 km remains of a massive impact in Popigai, Russia, dated to about 39 million years ago, occurred near the end of the epoch.
Development of Grasses
As commonplace as it seems now, it wasn't until the Early Eocene that grasses developed, and with them a host of grass-living animals. With new growth originating near the root, rather than at the tip of the plant, grasses are wonderfully protected from otherwise catastropic damage caused by grazing and fire. They quickly regenerate and create a renewable resource for plant-eaters.
Grass plains developed in those areas frequently ravaged by fire (from lightning strikes, for instance), and animals rapidly evolved to utilize this new environment.