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Oligocene

The Oligocene epoch (39 to 22 million years ago) is the transition period between the earlier and later Tertiary period (65 to 2 million years ago).

A key feature of evolution is the ripple effect created by geographical changes that influence climate and therefore vegetation and ultimately the ways in which animals develop. The most important geographical event separating the Oligocene from the preceding tropical and fairly stable Eocene was the splitting off of the Australasian landmass from Antarctica. As the oceans encircled the growing polar ice cap, the waters cooled. This cooling effect was spread around the globe by circulating currents that produced a dramatic drop in temperatures and, equally important, a new climate marked by seasonal fluctuations. Many animals of the Eocene that depended on a warm climate became extinct in the Oligocene, which is sometimes called "the great divide." Changing seasons favored the rise of homeothermic (warm-blooded) animals, such as mammals, over those who could not control their metabolic temperatures, such as reptiles. Thus the lizards, turtles, and crocodiles who survived did not flourish in the way that mammals like rodents and all modern-hoofed animals did.

The climatic changes produced changes in vegetation as well. Most forests in northern latitudes (45°) became cooler, mixed coniferous-deciduous, in which the most successful mammals tended to be short-legged, stumpy-bodied browsers and scavengers. Fortunately for humans, tropical zones, though greatly diminished, still existed in parts of South America and Africa, where a few primates survived on the year-round fruit supply. Plentiful food sources and tropical climate allowed for the survival of the early primates.

Insect groups expanded to include the social ants and termites, followed rapidly by the appearance of insectivores. Now that whole colonies of foods were available in a single place, the previously scarce mouse-size mammals who fed on this food also grew in size and number.

The growth of the polar ice cap locked up more and more of the ocean water, causing sea levels to drop and connecting parts of Europe and Asia that had been separate. This allowed a mingling of species throughout Eurasia from which a number of herbivores did not recover. Archaic predators such as the condylarths and creodonts, which were hoofed flesh eaters, began to decline and were replaced by giant, flightless, carnivorous birds. At over 2 meters (7 feet) tall, with deadly claws and ferocious, hatchet-like beaks, Diatryma and Phorusrhacus were the fearsome top predators of the Oligocene. They too disappeared, possibly because they were unable to protect their ground-dwelling young from the small, fast mammals that came along in the Miocene.

Just as the linking of landmasses tended to produce uniformity, so isolation produced spectacular diversity. Australia had sailed off with a few ancestral marsupials, mammals whose infants crawl into a pouch, or

Era	Period	Epoch	Million Before Years Present
Cenozoic	Quartenary	Holocene	0.01
		Pleistocene	1.6
	Tertiary	Pliocene	5.3
		Miocene	24
		Oligocene	37
		Eocene	58
		Paleocene	66

marsupium, where they are suckled and grow to independence. Given an entire continent in which to experiment, the marsupials exploded in a riot of shapes and sizes, filling every conceivable evolutionary niche from herbivores and carnivores to scavengers and insectivores. Only a few of these species survive to the present.

South America also separated from the other land masses and developed its own unique mammals. The edentates (toothless mammals), which included anteaters, sloths, and armadillos, were enormous, slow-paced vegetarians and insectivores. For example, glyptopons (armadillos) were 3 meters (10 feet) long and baluchitheriums (rhinoceroses) were 5.5 meters (18 feet) tall and 8.2 meters (27 feet) long. A bizarre assembly of hoofed animals also flourished in this region until the Isthmus of Panama formed at the end of the Cenozoic (2 million years ago) and linked North and South America. This two-way land bridge allowed a few herbivores from the south to move north, but on the whole, the invasion of ruthless carnivores and more efficient ungulates (hoofed mammals) signaled the end of most of the uniquely southern mammals.

In the Atlantic and Pacific Oceans, separated by the Isthmus of Panama, whales continued to thrive, spreading from Europe to New Zealand where they were joined by sea cows and the first seals.