Tertiary Period

In geologic time , the Tertiary Period (also sometimes referred to in terms of a Paleogene Period and a Neogene Period), represents the first geologic period in the Cenozoic Era . The Tertiary Period spans the time between roughly 65 million years ago (mya) and 2.6 mya. When referred to in terms of a Paleogene Period and a Neogene Period, the Paleogene Period extends from approximately 65 mya to 23 mya, and the Neogene Period from 23 mya to 2.6 mya.

The Tertiary Period contains five geologic epochs. The earliest epoch, the Paleocene Epoch , ranges from approximately 65 mya to 55 mya. The Paleocene Epoch is further subdivided into (from earliest to most recent) Danian and Thanetian stages. The second epoch, the Eocene Epoch ranges from approximately 55 mya to 34 mya. The Eocene Epoch is further subdivided into (from earliest to most recent) Ypresian, Lutetian, Bartonian, and Priabonian stages. The third epoch of the Tertiary Period, the Oligocene Epoch ranges from approximately 34 mya to 23 mya. The Oligocene Epoch is further subdivided into (from earliest to most recent) Rupelian and Chattian stages. Following the Oligocene Epoch, the Miocene Epoch ranges from approximately 23 mya to 5 mya. The Miocene Epoch is further subdivided into (from earliest to most recent) Aquitanian, Burdigalian, Langhian, Serravallian, Tortonian, and Messinian stages. The last epoch of the Tertiary Period is the Pliocene Epoch . The Pliocene Epoch is further subdivided into Zanclian and Placenzian stages.

The onset of the Tertiary Period is marked by the K-T boundary or K-T event—a large mass extinction. Most scientists argue that the K-T extinction resulted from—or was initiated by—a large asteroid impact in the oceanic basin near what is now the Yucatan Peninsula of Mexico. The remains of the impact crater , termed the Chicxulub crater, measures more than 105 mi (170 km) in diameter. The impact caused widespread firestorms, earthquakes, and tidal waves. Post-impact damage to Earth's ecosystem occurred as dust, soot, and debris from the collision occluded the atmosphere to sunlight. The global darkening was sufficient to inhibit photosynthesis. Widespread elimination of plant species caused repercussions throughout the food chain as starvation resulted in extinction of the largest life forms with the greatest metabolic energy needs (e.g., the dinosaurs).

At end of the prior Cretaceous Period and during the first half of the Tertiary Period (i.e. the Paleogene Period), Earth suffered a series of intense and large impacts. Large impact craters (greater than 25 mi or 40 km in diameter) include the Kara and Popigal craters in Russia, the Chesapeake crater in Maryland, and the Montagnais crater in Nova Scotia.

The last major impact crater with a diameter over 31 mi (50 km) struck Earth near what is now Kara-Kul, Tajikistan at end of the Tertiary Period and the start of the Quaternary Period .

The extinction of the dinosaurs and many other large species allowed the rise of mammals as the dominant land species during the Tertiary Period.

At the beginning of the Tertiary Period, North America and Europe were separated by a widening ocean basin spreading along a prominent mid-oceanic ridge. North America and South America were separated by a confluence of the future Pacific Ocean and Atlantic Ocean, and extensive flooding submerged much of what are now the eastern and middle portions of the United States. By the start of the Tertiary Period, water separated South America from Africa , and the Australian and Antarctic continents were clearly articulated. The Antarctic continent had begun a southward migration toward the south polar region. At the outset of the Tertiary Period, the Indian subcontinent remained far south of the Euro-Asiatic continent.

By the middle of the Tertiary Period (approximately 30 mya), the modern continental arrangement was easily recognizable. Although still separated by water, the Central American land bridge between North and South America began to reemerge. Antarctica assumed a polar position and extensive ice accumulation began on the continent. The Indian plate drove rapidly northward of the equator to close with the Asiatic plate. Although still separated by a shallow strait of water, the impending collision of the plates that would eventually form the Himalayan mountain chain had begun. The gap between North America and Europe continued to widen at a site of sea-floor spreading along a prominent mid-Atlantic ridge. By the middle of the Tertiary Period, the mid-Atlantic ridge was apparent in a large suture-like extension into the rapidly widening South Atlantic Ocean that separates South America from Africa.

By the end of the Tertiary Period, approximately 2.6 mya, Earth's continents assumed their modern configuration. The Pacific Ocean separated Asia and Australia from North America and South America, just as the Atlantic Ocean separated North and South America from Europe (Eurasian plate) and Africa. The Indian Ocean washed between Africa, India, Asia, and Australia. The Indian plate driving against and under the Eurasian plate uplifted both, causing rapid mountain building. As a result of the ongoing collision, ancient oceanic crust bearing marine fossils was uplifted into the Himalayan chain.

Climatic cooling increased at the end of the Tertiary Period, and modern glaciation patterns became well-established.

See also Archean; Cambrian Period; Dating methods; Devonian Period; Evolution, evidence of; Evolutionary mechanisms; Fossils and fossilization; Historical geology; Holocene Epoch; Jurassic Period; Mesozoic Era; Mississippian Period; Ordovician Period; Paleozoic Era; Pennsylvanian Period; Phanerozoic Eon; Pleistocene Epoch; Precambrian; Proterozoic Era; Silurian Period; Supercontinents; Triassic Period

Tertiary

The Tertiary era, from 65 to 2 million years ago, consists of six epochs: the Paleocene, Eocene, Oligocene, Miocene, and Pliocene, which represent chapters 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.

see also Geologic Time Scale.

Nancy Weaver

Bibliography

Asimov, Isaac. Life and Time. Garden City, NY: Doubleday & Company, 1978.

Fortey, Richard. Fossils: The Key to the Past. Cambridge, MA: Harvard University Press, 1991.

———. Life: A Natural History of the First Four Billion Years of Life on Earth. New York: Viking Press, 1998.

Friday, Adrian, and David S. Ingram, eds. The Cambridge Encyclopedia of Life Sciences. London: Cambridge University, 1985.

Gould, Stephen Jay, ed. The Book of Life. New York: W. W. Norton & Company, 1993.

Lambert, David. The Field Guide to Prehistoric Life. New York: Facts on File, 1985.

McLoughlan, John C. Synapsida: A New Look Into the Origin of Mammals. New York: Viking Press, 1980.

Steele, Rodney, and Anthony Harvey, eds. The Encyclopedia of Prehistoric Life. New York: McGraw Hill, 1979.

Wade, Nicholas, ed. The Science Times Book of Fossils and Evolution. New York: The Lyons Press, 1998.

ter·ti·ar·y / ˈtərshēˌerē; -shərē/ • adj. 1. third in order or level: most of the enterprises were of tertiary importance the tertiary stage of the disease.2. (Tertiary) Geol. of, relating to, or denoting the first period of the Cenozoic era, between the Cretaceous and Quaternary periods, and comprising the Paleogene and Neogene subperiods.3. Chem. (of an organic compound) having its functional group located on a carbon atom that is itself bonded to three other carbon atoms. ∎  Chem. (chiefly of amines) derived from ammonia by replacement of three hydrogen atoms by organic groups.• n. 1. (the Tertiary) Geol. the Tertiary period or the system of rocks deposited during it.2. a lay associate of certain Christian monastic organizations: a Franciscan tertiary.

Tertiary The older geological period of the Cenozoic era (compare Quaternary). It began about 65 million years ago, following the Cretaceous period, and extended to the beginning of the Quaternary, about 2 million years ago. It is subdivided into the Palaeocene, Eocene, Oligocene, Miocene, and Pliocene epochs in ascending order. The Tertiary period was characterized by the rise of the modern mammals and the development of shrubs, grasses, and other flowering plants.

Tertiary First sub-era of the Cenozoic Era, which began about 65Ma ago and lasted until 1.64 Ma. The Tertiary followed the Mesozoic and comprises five epochs: the Palaeocene, Eocene, Oligocene, Miocene, and Pliocene.