Tertiary

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The Tertiary era, from 65 to 2 million years ago, consists of six epochs: the Paleocene, Eocene, Oligocene, Miocene, and Pliocene, which representchapters in the story of the mammal's rise to dominance of land and oceans. The Tertiary follows the great Cretaceous extinction in which the dinosaurs, who had dominated the terrestrial food chain for hundreds of millions of years, inexplicably vanished, leaving only a few reptiles and mammal-like creatures as survivors.

The ancestors of the mammals, the therapsids, had been evolving into a broad range of ecological niches since the Permian-Triassic periods, some 260 million years ago. During the Mesozoic reign of dinosaurs, these mammals had dwindled almost into nonexistence, a few rat-sized species eking out a nocturnal insectivorous living, staying out of the way of predators.

This long period of trying to avoid being eaten may actually have produced the very features that later allowed mammals to spread across the entire planet. Smaller animals had a greater need for maneuverability, selecting for skeletal changes toward speed and flexibility of joints and spine. Smaller mammals need proportionately greater energy to maintain a constant body temperature, thereby selecting for more efficient teeth and jaws as well as digestive systems. And what may have seemed like their greatest drawback, the birth of helpless young who need a period of parental care, actually produced offspring who were uniquely flexible in their behavior patterns and able to be taught by their parents.

In the Tertiary with the dinosaurs gone, mammals along with birds underwent a cycle of massive evolutionary expansion into the greatest range of shapes and sizes to ever populate Earth.

The story of evolution parallels that of geography. During the Permian period (250 million years ago) the supercontinent of Pangaea allowed for migration of plants and animals across the whole of Earth. When Pangaea, driven by the forces of plate tectonics, began to break up into separate continents, each chunk of land took with it a random cargo of the original inhabitants. Separation breeds diversity and all of the earliest archetypes (orginal ancestors of a group of animals), grazers, browsers, carnivores, insectivores, and canopy dwellers were free to evolve in wildly different ways.

In the first epoch of the Tertiary, the Paleocene (65-55 million years ago), mammals still consisted of survivors from the Cretaceous, including the monotremes, primitive egg-laying mammals.

Condylarths, the ancestors of the ungulates, or hoofed animals, were widely present in the Paleocene. This group included carnivores and scavengers, as well as more common herbivores. Some rodent-like early primates also appeared in the Northern Hemisphere during this epoch.

In the Paleocene seas, sharks became the most abundant fishes, while gastropods and bivalves replaced the once-dominant ammonites.

By the Eocene, also known as the "dawn of early life" (55-39 million years ago), Pangaea had begun to break apart. Australia had split off, carrying a load of marsupials, mammals who give birth to immature young who then crawl into a pouch (marsupium) in which they suckle and grow. Freed from competition with placental mammals, the marsupials diversified into every ecological niche across the whole of the Australian continent. Limestones in northern Queensland reveal a tropical rainforest of marsupials for every niche.

Eocene seas chronicle the momentous return of the first mammals to the oceans they had emerged from several hundred million years earlier. The legs of the first whales began to change to flippers and increase in size, thanks to the new weightless environment.

In the Oligocene (39-22 million years ago), the Antarctic ice cap was beginning to form, provoking a marked cooling effect and a pattern of seasonal fluctuations. These changes apparently favored homeothermic, or warm-blooded animals, because the turtles, lizards, and crocodiles of the time did not undergo the explosion of evolution (cycles of immense activity and then decline in evolution) that the mammals underwent.

By the Miocene (22-5 million years ago), a dryer, warmer climate again produced changes in vegetation which rippled through the world of herbivores and predators. Teeth patterns of Miocene fossils suggest that the vast deciduous forests and their leaf-browsing inhabitants were being replaced by vast grasslands and grazing animals. These early ruminants (cud-chewing animals) included several types of deer and the first horses. Predation tends to shape evolution, and the new open plains encouraged longer, swifter legs in horses or burrowing capabilities in smaller animals closer to the ground. Condylarths and creodonts, the flesh-eating ungulates, had begun to decline, replaced by other orders of carnivores that were faster, and had sharper teeth and claws.

By the Pliocene, (5-2 million years ago) the continents had shifted into more or less their present-day locations. The isthmus of Panama had arisen to reconnect North and South America, allowing animals that had developed independently for millions of years to mingle. The two-way traffic across the isthmus sent the ponderous sloths and glyptodonts (giant armadillos) north. Highly evolved predators (such as sabre-toothed cats) traveled south, leaving numerous extinctions of South American marsupials in their wake. The isthmus also separated the ocean into two, Atlantic and Pacific, causing differentiations in marine species.

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This is the complete article, containing 973 words (approx. 3 pages at 300 words per page).