Eocene

The Eocene (IPA: /ˈiːəsiːn, ˈiːoʊ-/ EE-ə-seen, EE-o-) is a geological epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. During this time, Earth experienced warm climates and many changes in plant and animal life.

The Eocene spans the time from the end of the Paleocene Epoch to the beginning of the Oligocene Epoch. The start of the Eocene is marked by a brief period in which the concentration of the carbon isotope ¹³C in the atmosphere was exceptionally low compared with the more common isotope ¹²C. The average temperature of Earth at the beginning of the Eocene was about 27 degrees Celsius, much warmer than today.

The end of the Eocene is set at a major extinction event called the Grande Coupure (the "Great Break" in continuity) or the Eocene–Oligocene extinction event. This change may be related to the impact of one or more large bolides in Siberia and in what is now Chesapeake Bay. Like other geologic periods, the strata that define the start and end of the epoch are well identified, though their exact dates are slightly uncertain.

Etymology

The word "Eocene" comes from two Ancient Greek words: ἠώς (ēṓs), meaning 'dawn', and καινός (kainós), meaning 'new'. This name reflects that this time period marked the beginning of many modern forms of life.

Scottish geologist Charles Lyell first used the term "Eocene" in 1833. Later, other scientists like British geologist John Phillips and Austrian paleontologist Moritz Hörnes helped shape our understanding of this time in Earth's history. Eventually, international experts standardized these terms to help us study Earth's past more clearly.

Geology

The Eocene was a time of big climate changes on Earth. It started with a very warm period called the Paleocene–Eocene Thermal Maximum, which caused many tiny sea creatures to die. Later, the climate cooled down, but there were still warm periods in the middle of the Eocene.

Scientists divide the Eocene into three parts: early, middle, and late. They study rocks from this time to learn more about Earth's history. In North America, scientists also use special names for plant fossils from this period to understand how forests changed over time.

Palaeogeography and tectonics

During the Eocene, the continents continued to move toward their current positions. At the start of this time, Australia and Antarctica were still connected, and warm ocean currents helped keep the Earth's climate warm. Later, Australia separated from Antarctica, changing ocean currents and helping cool the planet.

The northern supercontinent Laurasia began to break apart, with Europe, Greenland, and North America moving away from each other. In western North America, large lakes formed between rising mountains, creating important fossil sites. Meanwhile, India collided with Asia, beginning the formation of the Himalayas.

Climate

The Eocene epoch had many different climate conditions. It was one of the warmest times in Earth’s history, with very little ice at the poles. Temperatures were much the same around the globe, unlike today when it is much colder at the poles than near the equator.

The climate began to warm after the end of the Paleocene-Eocene Thermal Maximum about 56 million years ago. It reached its warmest point around 49 million years ago, called the Eocene Optimum. During this time, the oceans were about 150 meters higher than they are today because there was so little ice.

After the Eocene Optimum, the climate began to cool, and ice started to form again at the poles. By about 34 million years ago, the climate had shifted to what is called an “icehouse” climate, with large ice sheets forming in Antarctica.

Flora

During the Eocene epoch, forests covered much of the Earth, even near the poles. Tropical forests grew across Africa, South America, India, Southeast Asia, and China. Paratropical forests spread over North America, Europe, and Russia, with different types of trees at higher latitudes.

Polar areas also had forests, with fossils of trees like swamp cypress and dawn redwood found in the Arctic. Palm trees grew as far north as Alaska and northern Europe during the early Eocene. By the end of this time, the climate cooled and dried, causing forests to thin in some places. Grasses began to spread, and deciduous trees became more common in northern regions, while rainforests remained only in equatorial areas such as South America, Africa, India, and Australia.

• Nuphar seeds, Nymphaeaceae, Ypresian

• Iodes tree seed, Icacinaceae, London Clay

• Mastixia tree seed, Nyssaceae, London Clay

• Ocotea tree seed, Lauraceae, London clay

• Macginitiea leaf, Platanaceae, Clarno Formation, Oregon

• Flower, Florissant Formation, Colorado

Fauna

During the Eocene, plants and marine life became quite modern. Many modern bird groups first appeared during this time, and the oceans were warm and full of fish and other sea creatures.

The Eocene period saw the oldest known fossils of most modern mammal groups. New mammal groups arrived in North America, including artiodactyls, perissodactyls, and primates. These mammals had long, thin legs and hands that could grasp, along with teeth adapted for chewing. Most Eocene mammals were small, under 10 kg, likely because the hot temperatures favored smaller animals that could manage the heat better. Rodents were widespread, and early forms of many modern mammals like horses, bats, elephants, and rodents appeared. Important fossils from this time have been found in western North America, Europe, Patagonia, Egypt, and southeast Asia. Whales became very diverse, and the first manatees and dugongs began to evolve.