I understand the three divisions of the Mesozoic. In the Triassic the pre-mammals fight it out with the pre-dinosaurs, the dinos win. In the Jurassic the dinos are in charge and reach gigantic size. In the Cretaceous a new set of dinosaurs takes over and rule until the comet or meteor smacks the earth and takes them out. But the books do not give us such an easy to follow narative for the age of mammals. This is the problem that I am trying to solve here. I am hoping to provide an easy to follow narative of the seven epochs of the age of mammals, the Cenozoic.
The cenozoic, has been split into seven epochs.
- Paleocene - 65 to 56 million years ago - Mammals take over.
- Eocene - 56 to 34 million years ago - The earth gets hot
- Oligocene - 34 to 23 million years ago - Mammals get big
- Miocene - 23 to 5.3 million years ago - Mammals similar to today
- Pliocene - 5.3 to 1.8 million years ago - Like today without us
- Pleistocene - 1.8 million to 11 thousand years ago - Ice age
- Holocene - last 11 thousand years - Age of man
Paleocene ~65-56 Million Years AgoThe comet or meteor hit and the dinosaurs were wiped out roughly 65 million years ago. This left the relatively tiny mammals to seize the larger land niches. The mammals rapidly evolved larger sizes.
It is my impression that many of the larger mammals were rather crude early drafts by evolution.
Eocene ~56-34 Million Years AgoSomething 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. I suspect that many of those early drafts simply were not able to compete.
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 mammal orders are found in the Eocene.
Oligocene ~34-23 Million Years AgoThe 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. 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.
Miocene ~23-5.3 Million Years Ago
The animals in the Miocene were quite different from the Oligocene but quite similar to those we are familiar with. Note that the modern orders had been around since the Eocene, but the casual layman might not be able to tell that. The Miocene animals look like and probably acted roughly like modern animals.
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 AgoThe climate is cooler than the Miocene, about the same as ours, and the animals are also fairly similar. For the animals think Africa with more hair.
Pleistocene ~1.8 Million Years Ago To 11 Thousand Years AgoThe Pleistocene has commonly been called the ice age. Glaciers advanced and it was very cold most of the time. But there were 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 half of the Holocene there have been human civilizations and written language.
One might suspect that the Pleistocene, the ice age, would have continued without humans. This may be the era when one social animal, man began to dominate the climate.
So there is your summary. The climate reached a maximum temperature at the boundry of the Paleocene and the Eocene and trended colder at least until we showed up. The mammals reached their maximum size in the middle of their reign, the Oligocene, just as the dinosaurs reached their maximum size in the middle, the Jurassic.
I hope this provides an easy to remember narative.
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