Eocene

The Eocene epoch (56-34 Ma) is a major division of the geologic timescale and the second epoch of the Palaeogene period in the Cenozoic era. 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 the emergence of the first modern mammals. The end is set at a major extinction event that may be related to the impact of one or more large bolides in Siberia and in what is now Chesapeake Bay. Still, as with other geologic periods, the strata that define the start and end of the epoch are well identified, but their exact dates are slightly uncertain.

The name Eocene comes from the Greek eos (dawn) and ceno (new) and refers to the "dawn" of modern ('new') mammalian fauna that appeared during the epoch.

Tertiary sub-era
Paleogene period
Paleocene epoch		Eocene epoch		Oligocene epoch
Danian	Selandian	Ypresian	Lutetian	Rupelian	Chattian
Thanetian		Bartonian	Priabonian	Rupelian	Chattian

Eocene subdivisions

The Eocene is usually broken into lower and upper subdivisions. The Faunal stages from youngest to oldest are:

Priabonian	(37.2 ± 0.1 – 33.9 ± 0.1 Ma)
Bartonian	(40.4 ± 0.2 – 37.2 ± 0.1 Ma)
Lutetian	(48.6 ± 0.2 – 40.4 ± 0.2 Ma)
Ypresian	(55.8 ± 0.2 – 48.6 ± 0.2 Ma)

Eocene climate

Marking the start of the Eocene, the planet heated up in one of the most rapid (in geologic terms) and extreme global warming events recorded in geologic history, called the Paleocene-Eocene Thermal Maximum or Initial Eocene Thermal Maximum (PETM or IETM). This was an episode of rapid and intense warming (up to 7°C at high latitudes) that lasted less than 100,000 years [1]. The Thermal Maximum provoked a sharp extinction event that distinguishes Eocene fauna from the ecosystems of the Paleocene.

The Eocene global climate was perhaps the most homogeneous of the Cenozoic; the temperature gradient from equator to pole was only half as much as it is today, and deep ocean currents were exceptionally warm compared to today. [2] The polar regions were much warmer than today, so mild that warm temperate forests extended right to the poles. They were also much wetter than today. The polar regions may have been at least as mild as the modern-day Pacific Northwest. Tropical climates extended as far north as 45 degrees latitude away from the Equator.

Climates remained warm through the rest of the Eocene, although slow global cooling, which eventually led to the Pleistocene glaciations, started around the end of epoch as ocean currents around Antarctica cooled.

Eocene paleogeography

During the Eocene, the continents continued to drift toward their present positions.

At the beginning of the period, Australia and Antarctica remained connected, and warm equatorial currents mixed with colder Antarctic waters, distributing the heat around the world and keeping global temperatures high. But when Australia split from the southern continent around 45 mya, the warm equatorial currents were deflected away from Antarctica, and an isolated cold water channel developed between the two continents. The Antarctic region cooled down, and the ocean surrounding Antarctica began to freeze, sending cold water and icefloes north, reinforcing the cooling.

The northern supercontinent of Laurasia began to break up, as Europe, Greenland and North America drifted apart.

In western North America, mountain building started in the Eocene, and huge lakes formed in the high flat basins among uplifts.

Europe saw the Tethys Sea finally vanish, while the uplift of the Alps isolated its final remnant, the Mediterranean, and created another shallow sea with island archipelagos to the north. Though the North Atlantic was opening, a land connection appears to have remained between North America and Europe as the faunas of the two regions are very similar.

India continued its journey away from Africa, and began its collision with Asia, folding the Himalayas into existence.

Detailed maps of Tertiary Western North America: Eocene
Map of Eocene Earth

It is hypothesized that the Eocene hothouse world was due to runaway global warming from released methane cathrates deep in the oceans. The cathrates were buried beneath mud that was disturbed as the oceans warmed. Methane (CH4) has ten to twenty times the greenhouse gas effect of Carbon Dioxide (CO2).

Eocene flora

At the beginning of the Eocene, the high temperatures and warm oceans created a moist, balmy environment, with forests spreading throughout the earth from pole to pole. Apart from the driest deserts, Earth must have been entirely covered in forests.

Polar forests were quite extensive. Fossils and even preserved remains of trees such as swamp cypress and dawn redwood from the Eocene have been found in Ellesmere Island in the Canadian Arctic. As aforementioned, the preseved remains found in the Canadian Arctic are not fossils, but actual pieces preserved in oxygen-poor water in the swampy forests of the time, and then buried before they had the chance to decompose. Even at that time, Ellesmere Island was only a few degrees in latitude further south than it is today. Fossils of subtropical and even tropical trees and plants from the Eocene have also been found in Greenland and Alaska. Tropical rainforests grew as far north as the Pacific Northwest and Europe.

Palm trees were growing as far north as Alaska and northern Europe during the early Eocene, although they became less and less abundant as the climate cooled. Dawn redwoods were far more extensive as well.

Cooling began mid-period, and by the end of the Eocene continental interiors had begun to dry out, with forests thinning out considerably in some areas. The newly-evolved grasses were still confined to river banks and lake edges, and had not yet expanded into plains and savannas.

The cooling also brought seasonal changes. Deciduous trees, better able to cope with large temperature changes, began to overtake evergreen tropical species. By the end of the period, deciduous forests covered large parts of the northern continents, including North America, Eurasia and the Arctic, and rainforests held on only in equatorial South America, Africa, India and Australia.

Antarctica, which began the Eocene fringed with a warm temperate to sub-tropical rainforest, became much colder as the period progressed; the heat-loving tropical flora was wiped out, and by the beginning of the Oligocene the continent hosted deciduous forests and vast stretches of tundra.

Eocene fauna

Mesonyx, a carnivorous ungulate

The oldest known fossils of most of the modern mammal orders appear within a brief period during the early Eocene. At the beginning of the Eocene, several new mammal groups arrived in North America. These modern mammals, like artiodactyls, perissodactyls and primates, had features like long, thin legs, feet and hands capable of grasping, as well as differentiated teeth adapted for chewing. Dwarf forms reigned. All the members of the new mammal orders were small, under 10 kg; based on comparisons of tooth size, Eocene mammals were only 60 per cent of the size of the primitive Paleocene mammals that had preceded them. They were also smaller than the mammals that followed them. It is assumed that the hot Eocene temperatures favored smaller animals that were better able to manage heat.

Both groups of modern ungulates (hoofed animals) became prevalent due to a major radiation between Europe and North America; along with carnivourous ungulates like Mesonyx. Early forms of many other modern mammalian orders appeared, including bats, proboscidians, primates, rodents and marsupials. Older primitive forms of mammals declined in variety and importance. Important Eocene land fauna fossil remains have been found in western North America, Europe, Patagonia, Egypt and South-East Asia. Marine fauna are best known from South Asia and the southeast United States.

Reptile fossils are also known from the Eocene, such as the fearsomely enormous crocodile Deinosuchus, which lived as far north as Wyoming during the Eocene and grew much larger than the modern-day saltwater crocodile. Python fossils and turtle fossils are also known from North America.

During the Eocene plants and marine faunas became quite modern. Many modern bird orders first appear in the Eocene.

Eocene oceans

The Eocene oceans were warm and teeming with fish and other sea life. The first Carcharinid sharks appeared, as did early marine mammals, including Basilosaurus, an early species of whale that is thought to be descended from land animals, the hoofed predators called mesonychids, of which Mesonyx was a member.