The Eocene Epoch

An element of the paradigm of the Grande Coupure was the apparent extinction of all European primates at the Coupure: the recent discovery (Kohler and Moya-Sola 1999) of a mouse-sized early Oligocene omomyid, reflecting the better survival chances of small mammals, further undercut the Grand Coupure paradigm.

Whether this abrupt change was caused by climate change associated with the earliest polar glaciations and a major fall in sea levels, or by competition with taxa dispersing from Asia, few would argue for an isolated single cause. More spectacular causes are related to the impact of one or more large bolides (in present day Siberia and Chesapeake Bay). Improved correlation of northwest European successions to global events (Hooker et al. 2004) confirms the Grande Coupure as occurring in the earliest Oligocene, with a hiatus of about 350 thousand years (ka) prior to the first record of post-Grande Coupure Asian immigrant taxa.

They further noted that the only families to cross the faunal divide undiminished were the marsupial family Herpetotheriidae, the rodent families Theridomyidae and Gliridae (dormise) and the artiodactyl family Cainotheriidae.

“Pre-Grande Coupure faunas are dominated by the perissodacty] family Palaeotheriidae (distant horse relatives), six families of artiodactyls (cloven-hoofed mammals) (Anoplotheriidae, Xiphodontidae, Choeropotamidae, Cebochoeridae, Dichobunidae and Amphimerycidae), the rodent family Pseudosciuridae, the primate families Omomyidae and Adapidae, and the archontan family Nyctitheriidae... Post-Grande Coupure faunas include the true rhinos (family Rhinocerotidae), three artiodactyl families (Entelodontidae, Anthracotheriidae and Gelocidae) related respectively to pigs, hippos and ruminants, the rodent families Eomyidae, Cricetidae (hamsters) and Castoridae (beavers), and the lipotyphlan family Erinaceidae (hedgehogs). The speciose genus Palaeotherium plus Anoplotherium and the families Xiphodontidae and Amphimerycidae were observed to disappear completely”.

The Grande Coupure marks a break between endemic European faunas before the break and mixed faunas with a strong Asian component afterwards. J.J. Hooker and his team (2004) summarized the break:

The Grande Coupure was given its name in 1910 by the Swiss palaeontologist Hans Georg Stehlin to characterize the dramatic turnover of European mammalian fauna, which he placed at the Eocene-Oligocene boundary. A comparable turnover in Asian fauna has since been called the “Mongolian Remodelling”.

The Grande Coupure, or “great break” in continuity, with a major European turnover in mammalian fauna about 33.5 mya, marks the end of the last phase of Eocene assemblages, the Priabonian, and the arrival in Europe of Asian species. The Grande Coupure is characterized by widespread extinctions and allopatric speciation in small isolated relict populations (called “dispersal-generated origination” in Hooker et al. 2004).

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. The first sirenians, relatives of the elephants, also appeared at this time.

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

Reptile fossils are also known from the Eocene, such as the 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.

As noted above, most mammal orders appeared in the early Eocene over a relatively brief period of time. Prothero (1992) reported that an examination of the entire mammalian fauna from the early Eocene (about 50-55 mya) of the Bighorn Basin of northwestern Wyoming showed that most species did change gradually through time. He also found that mammals from the Ecocene-Oligocene (30-35 mya) beds of the Big Badlands of South Dakota and related areas were static through millions of years and if speciated, did so abruptly. They were remarkably stable even over an interval of well-documented climatic change. Such findings accord well with the view of punctuated equilibrium, as advocated by Stephen Jay Gould and Niles Eldredge, as well as theistic views of creation.

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 is best known from South Asia and the southeast United States.

The oldest known fossils of most of the modern mammalorders appear within a geologically 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 ten kg; based on comparisons of tooth size, Eocene mammals were only 60 percent 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.

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.

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.

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.

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.

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. The preserved 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.

As I said before at the beginning of the Eocene, the high temperatures and warm oceans assumedly 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.

The Eocene epoch (meaning dawn of recent life) ranges from 54.8 million years ago to 33.7 million years ago. It is a part of the Paleogene period comes after the Paleocene epoch and before the Oligocene epoch. The SU and Yale research team found that average Eocene water temperature along the subtropical U.S. Gulf Coast hovered around 80 degrees Fahrenheit. and the modern temperatures in the study area average 75 degrees Fahrenheit (Astrobio 2011). This epoch is said to have started with lots of rain and no ice and said to have ended cool with ice forming. Earths continents were also moving closer to where they are today at this time. The supercontinent Laurasia broke into the pieces that are known today as Europe, Greenland, and North America and the Rocky Mountains in western North America were also formed during this time. Gondwana another supercontinent which contained land that is now South America, Africa, India Australia and Antarctica had also broken apart, but Australia and Antarctica were still connected until around 45 million years ago when they split apart. Australia and India went north India eventually collided with Asia to create the Himalayan mountains. Africa and south America were both isolated continents (boone,corrie).