The Eocene epoch is the second and among the longest epochs in the Cenozoic era, lasting from 55 to 34 million years ago. It marked a milestone in mammal diversity in which the modern orders of mammals become apparent in the fossil record. It is marked with two major changes to the planet: the Thermal Maximum and Le Grande Compure.

Thermal MaximumEdit

The Thermal Maximum towards the very end of the Paleocene (55 million years ago) marked a dramatic change on the environment and the fauna. After the Cretaceous, North America and Eurasia are still linked by Greenland, Scandinavia, and the Canadian Arctic. Interchange within Eurasia was limited due to the a seaway separating Europe from Asia. Around Greenland, a magma plume forced a rift between North America and Eurasia. However with magma close to the seafloor, it heated methane deposits in the deep seabed and sent it to the surface. This methane release triggered a swift change in climate. The world grew warmer to such a degree that most of the planet was tropical rainforest, and forests became dominant even at the poles.

This affected the animal life as well. The warming also made some land bridges close to the Arctic and Antarctic circles now crossable. An influx of mammals from Asia spread into North America. This also continued into Europe as well. At this point, the fauna of the Northern Hemisphere includes similar mammals. To the south, marsupials crossed into Australia as well as other animals.

Le Grande CompureEdit

Le Grande Compure or "The Great Turnover" marked close to the end of the Eocene and the demise of many Eocene animals. There are several causes for this. Antarctica became an isolated continent once South America and Australia broke off from it and moved north. With no warm currents deflected off of other continents reaching Antarctica, it slowly began to freeze. There have also been suggestions that two meteorite impacts hastened the cooling process. Whatever the case, the results were dramatic for the mammals:

  • The mesonychid and oxyaenid creodont mammals died out
  • Of the whales, only the modern groups seen today survived
  • The diversity of perissodactyl hoofed mammals reduced to a handful of major survivors
  • A portion of artiodactyls died out, but enough survived to dominate the fauna
  • The condylarths died out
  • Many archaic mammals, such as pantodonts and multituberculates, went extinct or grew rarer almost to the point of extinction
  • Only a few primates survived in the northern hemisphere, though they were later dominant in Africa. By the Oligocene, no primates existed in Europe and North America


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Rose, Kenneth D. The Beginnings of the Age of Mammals, 2006