Biology Index
By Richard Bruce BA, MA, and PhC in Economics

A summary of on the Epochs of the Age of Mammals, the Cenozoic

I have had some difficulty remembering the epochs of the age of mammals, which makes it difficult to read the literature on that period. So I have tried to straighten the whole thing out in my mind, and now I am writing my thinking down on this page. That way both the general public and I will be able to refer to it. This page contains relatively little in the way of new insight, I am simply organizing information.

The age of mammals, the cenozoic, has been split into seven epochs.

Those seven epochs are as follows:

  1. Paleocene
  2. Eocene
  3. Oligocene
  4. Miocene
  5. Pliocene
  6. Pleistocene
  7. Holocene

Paleocene ~ 65-56 Million Years Ago

The comet or meteor hit and the dinosaurs, except the birds, were wiped out roughly 65 million years ago. This left the relatively tiny mammals to seize the niches the dinosaurs had dominated. The mammals rapidly evolved larger sizes.

It is my impression that many of the larger mammals were evolution's rather crude early drafts.

Eocene ~ 56-34 Million Years Ago

Something made the earth much hotter for 100 thousand years at the transition between the Paleocene and Eocene. This is called the Paleocene-Eocene Thermal Maximum. Many species went extinct as a result. It is the official explanation that the heat got them.

Even after the hot spell the Eocene was very warm. From the Eocene through the Pleistocene, the world generally became colder.

In the tropical world of the Eocene the mammals were somewhat smaller than before or after the Eocene. The mammals were on average only 60 percent of the size of the late Paleocene mammals.

The earliest fossils of most of the modern mammalian orders are found in the Eocene.

Oligocene ~34-23 Million Years Ago

The Oligocene began with an extinction event. The earth may have been hit by a large object.

As the earth became cooler and dryer the mammals became very large. This was the era of indricotherium, the giant hornless rhino which is the largest land mammal that ever lived, with the possible exception of an type of elepant that lived much later. It is also the era of brontotherium, the giant animal that looks like a rhino with a forked horn.

This period of relatively unsophisticated giants seems to be similar to the Jurassic for the age of dinosaurs, which was also an age of unsophisticated giants.

Miocene ~23-5.3 Million Years Ago

The animals in the Miocene were quite different from the Oligocene but quite similar to those we have today. The climate was cooling but still warmer than ours. Much of the Miocene was as warm as the earlier Oligocene.

Pliocene ~5.3-1.8 Million Years Ago

The climate is about the same as ours, and the animals are also quite similar. For the animals think of a hairy version of today's Africa.

Pleistocene ~1.8 Million Years Ago To 11 Thousand Years Ago

The Pleistocene has commonly been called the ice age. Glaciers advanced and it was very cold most of the time. But there were also warmer interglacial periods, like the one we are in now.

Holocene ~Last 11 thousand Years

The Holocene is the current interglacial period. Perhaps because of man, this interglacial period has lasted longer than previous ones. Agriculture developed fairly early in the Holocene, and for about the last half of the Holocene there have been human civilizations and written language.

One might suspect that the Pleistocene would have continued without us.

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