Oligocene

The Oligocene epoch was a prehistoric period within the Cenozoic era that took place from about 34 to 23 million years ago. The name Oligocene, which means "epoch of few recent life-forms," reflects the fact that the animals of the time underwent little significant evolution during this period. The most notable characteristic of the Oligocene epoch was instead the ripple effect caused by certain geographical changes that ultimately marked the beginning of a period of global cooling. However, there were a few key life-forms that did develop during the Oligocene epoch. Specifically, the first elephants with trunks appeared in the Oligocene, as did the earliest of the grasses that would eventually spread to form vast grasslands in the subsequent Miocene epoch. In short, the Oligocene was a brief, but important, era in Earth's development that played a key role in the formation of the modern world.

Background

The Oligocene epoch was part of the Cenozoic era, which is the third documented era in Earth's history. The Cenozoic era stretches from about 65 million years ago to the present time. All of Earth's contemporary inhabitants emerged during this period. Earth's current geography also took shape during the Cenozoic era.

The Cenozoic era began at the end of the Cretaceous period, which was the final stage of the Mesozoic era. The end of the Cretaceous period was marked by the Cretaceous-Paleogene extinction event, which led to the disappearance of the last non-avian dinosaurs. From there, the Cenozoic era is divided into three different periods: the Paleogene, the Neogene, and the Quaternary. The Paleogene period included the Paleocene, Eocene, and Oligocene epochs. The Neogene period included the Miocene and Pliocene epochs. Finally, the Quaternary period included the Pleistocene and Holocene epochs. While most scientists believe that Earth is still in the Holocene epoch, others argue that the massive shift in environmental conditions caused by humans has actually marked the beginning of a new epoch.

In the early stages of the Cenozoic era, Earth had a much warmer climate than it does today. As the era unfolded over time, climate conditions changed quite dramatically. During the Paleogene period, most of Earth's climate was tropical. As the Paleogene period ended with the Oligocene epoch, the global climate began to cool. This trend continued through the Pleistocene epoch of the Quaternary period. Earth's geography also changed significantly during the Cenozoic era. Starting in the Paleogene period, large oceans began to form as the continents gradually drifted apart. This also led to some regional climate change.

To those interested in the study of living things, the Cenozoic era is also sometimes known as the age of mammals. This is because the extinction of giant mammals during the period allowed smaller species to thrive. Small horses, rodents, elephants, dogs, cats, and pigs all rose to prominence in the Paleogene period. The Neogene period saw the rise of early primate and humans, as well as cattle, sheep, and goats. In the Quaternary period, saber-toothed cats, woolly mammoths, and cave bears all become common.

Overview

The Oligocene epoch is divided into two separate stages: the Rupelian and the Chattian. These two geologic stages are determined by the rock ages that correspond with the Oligocene period. In terms of geology, the Oligocene is most noted for an increase in volcanic activity and plate tectonic movement. This geological movement also translated into geographic movement. The most important geographic change of the Oligocene was the breakup of the last remaining segment of the Gondwanaland supercontinent that with Laurasia once formed the larger supercontinent of Pangea. During the Oligocene, the last segment of Gondwanaland separated into Australia and South America. Both also separated from Antarctica and drifted northward as Antarctica moved farther south. This separation eventually led Antarctica to settle over the South Pole and caused the polar ice cap to grow. As a result, the waters surrounding the polar ice cap began to cool. This, in turn, led to a dramatic drop in temperatures around the world and the onset of greater seasonal climate changes. All of this resulted in the beginning of a ten-million-year span of decreasing global temperatures and sea levels.

Terrestrial life underwent a number of key changes during the Oligocene epoch. Most importantly, a variety of newly evolved grasses began to spread across the plains of the Northern and Southern Hemispheres. The spread of grasses soon led to the early development of sprawling grasslands. Such burgeoning grasslands provided an ecological niche in which grazing mammals could thrive as never before. As a result, mammalian species like proto-camels, early horses, and primitive beavers became increasingly common. Some early evolutionary ancestors of modern rhinoceroses also began to appear. One such species, called the Indricotherium, was the largest land mammal to ever live. The early ancestors of modern cats and dogs also began to appear during the Oligocene. In South America, which was isolated from the other continents, various species of giant mammals, Pyrotherium, and the marsupial Borhyaena thrived. Common bird species of the Oligocene included so-called "terror birds" such as the Psilopterus and giant penguins like the Kairuku. Common Oligocene insects included termites, butterflies, bees, ants, and spiders.

In terms of marine life, the Oligocene epoch was an especially fruitful age for transitional whale species such as the Aetiocetus, Janjucetus, and Mammalodon. Prehistoric sharks also continued their dominance of marine habitats during the Oligocene. Most notably, the massive Megalodon first appeared during this period. The pinniped family of marine mammals, which today includes seals and walruses, also began to evolve during the Oligocene. Another notable group of Oligocene marine animals were the foraminiferans, which were amoeba-like protists that had a complex shell. Among the most common of these species were the large, lens-shaped nummulites.

Although the spreading of grasses and the growth of grasslands was the biggest development in the evolution of flora during the Oligocene, it was not the only important change. Prior to the Oligocene, many of the tropical forests found in non-equatorial regions began turning into deciduous forests. This trend continued in the Oligocene, as temperatures around the world dropped even lower.

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