Cenozoic era


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Era Period Epoch Approximate duration
(millions of years)
Approximate number of years ago
(millions of years)

Cenozoic Quaternary Holocene 10,000 years ago to the present  
Pleistocene 2 .
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Cenozoic era

(sēnəzō`ĭk, sĕn–), last major division of geologic time (see Geologic TimescaleGeologic Timescale
Era Period Epoch Approximate duration
(millions of years)
Approximate number of years ago
(millions of years)

Cenozoic Quaternary Holocene 10,000 years ago to the present  
Pleistocene 2 .
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, table) lasting from 65 million years ago to the present. The Cenozoic is divided into the Tertiary (from 65 million years ago until 2 million years ago) and Quaternary (2 million years ago to the present) periods. Early in the Cenozoic, Greenland began to separate from Europe; Antarctica and Australia, and Africa and India also separated. The great Alpine-Himalayan mountain systems were formed; rifting with associated volcanic activity occurred in Africa, Europe, Asia, and Antarctica. In the late Cenozoic, the Cascade Range of volcanoes extended from southern British Columbia to N California, and represented a new volcanic arc superimposed on older structures. The volcanic arc is still periodically active today. Following the disturbances of the late Mesozoic eraMesozoic era
[Gr.,=middle life], major division of geologic time (see Geologic Timescale, table) from 65 to 225 million years ago. Great crustal disturbances that marked the close of the Paleozoic and the beginning of the Mesozoic eras brought about drastic changes in the
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, the geography of North America had by the beginning of the Cenozoic attained substantially its present form. The only areas subjected to inundation by shallow marine waters were the Atlantic and Gulf coasts and a small area on the Pactific coast. Most of today's common plants and insects can be recognized in early Cenozoic fossils. Modern grasses appeared in the mid-Cenozoic. The animal life of the Cenozoic was dominated by mammals, which developed rapidly and were most numerous in the Tertiary periodTertiary period
, name for the major portion of the Cenozoic era, the most recent of the geologic eras (see Geologic Timescale, table) from around 26 to 66 million years ago. The name Tertiary was first applied about the middle of the 18th cent.
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 and then declined, with the exception of a few specialized types, in the Quaternary periodQuaternary period
, younger of the two geologic periods of the Cenozoic era of geologic time (see Geologic Timescale, table) from 2 millon years ago to the present. Comprising all geologic time from the end of the Tertiary period to the present, it is divided into the
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. Theories explaining the decline or extinction of mammals during the PliocenePliocene epoch
, fifth epoch of the Cenozoic era of geologic time (see Geologic Timescale, table), from 5.1 to 2 million years ago. By the beginning of the Pliocene, the outlines of North America were almost the same as in recent time.
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 and PleistocenePleistocene epoch
, 6th epoch of the Cenozoic era of geologic time (see Geologic Timescale, table). According to a classification that considered its deposits to have been formed by the biblical great flood, the epoch was originally called the Quaternary.
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 epochs of the quaternary have ranged from a change in climate to the predation of humans. Cenozoic land mammals were never as large as the Mesozoic dinosaurs, but many were larger than today's mammals, and included beavers that grew to lengths of more than 7 ft (2 m), sloths as large as elephants, and birds up to 7 ft (2 m) in height. Early humans are thought to have evolved in Africa during the Pliocene and were widespread and accomplished toolmakers by the beginning of the Pleistocene. Around 1.6 million years ago, Homo erectus, a robust ancestor of modern humans, evolved (see human evolutionhuman evolution,
theory of the origins of the human species, Homo sapiens. Modern understanding of human origins is derived largely from the findings of paleontology, anthropology, and genetics, and involves the process of natural selection (see Darwinism).
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). H. sapiens evolved more than a million years later in Africa and moved to Asia and Europe, supplanting earlier Homo species by 28,000 years ago, developed a form of agriculture in the Middle East c.10,000 years ago, and started writing and building the first cities c.4,000 years ago. See also geologygeology,
science of the earth's history, composition, and structure, and the associated processes. It draws upon chemistry, biology, physics, astronomy, and mathematics (notably statistics) for support of its formulations.
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.

Cenozoic Era

 

the youngest group of strata of the earth’s crust on the stratigraphic scale and the era corresponding to it, the most recent in the earth’s geological history, encompassing the present epoch as well. The name was proposed by the English geologist J. Phillips in 1841. The Cenozoic era is subdivided into the Paleogene, Neocene, and Anthropogenic (Quaternary) systems or periods. Until 1960 the first two were classified in the Tertiary system or period. The span of the Cenozoic era is estimated at 60–70 million years, of which 40–45 million years are allotted to the Paleogene and 20–25 million to the Neocene; the duration of the Anthropogenic period is estimated by some at between 0.6 and 1.7 million years and by others at as much as 3.5 million.

General description. During the Cenozoic the present distribution of continents and oceans occurred. The very beginning of the era saw the completion of the breakup of the formerly unified southern continental mass, Gondwana, into the separate contiental blocks of South America, Africa, the Indian subcontinent, Australia, and Antarctica, divided by the newly formed basins of the Indian Ocean and the southern part of the Atlantic Ocean —a process that had been under way since the Mesozoic. By the middle of the Cenozoic, Eurasia and Africa formed the continental mass of the Old World, joined by the mountain structures of the Mediterranean geosynclinal belt. In the Paleogene the vast Tethys Sea, stretching from Gibraltar to the Himalayas and Indonesia, occupied this geosynclinal belt. In the middle of the Paleogene the Tethys Sea spread to the neighboring platforms, flooding vast areas of present-day Western Europe, the southern part of the European USSR, Western Siberia, Middle Asia, North Africa, and the Arabian Peninsula. Beginning in the late Paleogene the seas gradually receded from these areas. By the end of the Neocene, as a result of Alpine tectogenesis, there had developed in the Mediterranean belt a system of young folded mountains, including the Atlas and Andalusian mountains, the Pyrenees, Alps, Apennines, Dinaric Alps, Stara Planina (Balkan Mountains), Carpathians, Caucasus, Hindu Kush, Pamirs, Himalayas, and the mountains of Asia Minor, Iran, Burma, and Indonesia. The Tethys gradually began to break up, forming in the course of long evolution the basins of the Mediterranean, Black, and Caspian seas.

Young folded mountain systems also formed on the periphery of the Pacific Ocean. Among them are the chains of mountainous island arcs, unique in structure and development, along the western edge of the Pacific (the Aleutian, Kamchatka-Kuril, Japan, Philippine, and New Zealand arcs), as well as the mountains along the coast of Antarctica and the Andes of South America. Arcs of mountainous islands also formed in the Atlantic (the Antilles and the southern Antilles arc between Cape Horn and Antarctica). Together with the adjacent deep-water ocean trenches, the island arcs form geosynclinal systems that continue to develop today. Most of them are separated from neighboring continents by the basins of marginal seas (the Bering, Okhotsk, Japan, South China, Tasman, and Caribbean seas and the Gulf of Mexico) that also developed during the Cenozoic.

In the northern half of the Pacific geosynclinal belt, vast areas consisting of Mesozoic folded structures (the Sikhote-Alin’ and Verkhoiano-Chukotka folded regions and most of the Cordilleras of North America) experienced periodically intensified general uplifts with relative displacement of large blocks along fractures. In North America these uplifts were sometimes accompanied by massive lava discharges (the basalt sheets of the Columbia Plateau in the United States and the Meseta Central in Mexico). Here, block movements also encompassed the edge of the adjacent ancient North American (Canadian) Platform, thus creating a chain of block mountains, the Rocky Mountains, parallel to the Cordilleras.

In Eurasia arched uplifts and block displacements along fractures encompassed even larger areas of folded structures of various ages, causing the development of mountainous relief in areas that had earlier been leveled by prolonged denudation. Thus, there originated the mountain belt that includes the Tien-Shan, Altai, Saian, Iablonovyi, and Stanovoi ranges and the mountains of Central Asia, Tibet, the Scandinavian Peninsula, and the Urals. At the same time fracture systems of great length developed on the continents of the Old World, accompanied by linear rifts expressed in the relief as deep valley-like depressions often containing large bodies of water. The largest rift system, the East African rift zone, stretches in a north-south direction from the graben of the Jordan Valley and Dead Sea to the lower reaches of the Zambezi River in Africa. The development of this rift system was accompanied by intensive volcanic activity in East Africa, which continues to the present. The smaller Baikal rift system includes the basin of Lake Baikal and the graben-type depressions extending from it.

During the Paleogene the climate was significantly warmer than it is today. Even in the arctic region there were mixed forests, and the vegetation over most of Europe, northern Asia, and North America was tropical or subtropical.

The vast uplifts of continents in the second half of the Cenozoic caused most of the shelf of northern Eurasia and North America to dry up, partially isolated the Arctic Ocean, and created large mountainous areas on land. As a result, contrasts between the climatic belts appeared, and a general cooling of the earth’s climate occurred. This cooling process, which had already begun at the end of the Paleogene, culminated in several periods of vast continental glaciation in the middle latitudes of the northern hemisphere during the Anthropogenic period. The terrestrial animals and plants that are now typical of the subpolar region migrated south as far as the Crimea and Northern Caucasus. Vast ice sheets developed in North America, Europe, and northern Asia. In the southern hemisphere the glaciers of the Andes and New Zealand increased greatly in size, and Tasmania was subjected to glaciation. The last ice sheet in North America and Europe disappeared between 10, 000 and 12, 000 years ago.

Cenozoic life. Terrestrial vegetation was altered in the middle of the Cretaceous period, when angiosperms (flowering plants) became dominant. By the beginning of the Cenozoic there had appeared most of the families and many of the genera of angiosperms existing today; as the climate changed they formed the communities typical of the different climatic belts. Beginning in the mid-Paleogene, grass formations of the savanna and steppe type appeared, and at the end of the Neocene there developed taiga-type coniferous forest formations and, later, forest-tundras and tundras. At the end of the Mesozoic and beginning of the Cenozoic the reptiles that had predominated during the Mesozoic became extinct and were replaced by mammals that, with birds, constituted the nucleus of the terrestrial vertebrates of the Cenozoic era. On most continents the higher placental mammals became dominant, and only in Australia (which became isolated before these mammals appeared on a large scale) did unique marsupials and, to some extent, monotremes develop. During the early Paleogene, mammals were represented almost exclusively by small primitive forms. By the middle Paleogene almost all the orders existing today had appeared, as well as several groups that subsequently became extinct. A great variety of mammals evolved and flourished. At the end of the Paleogene there were both very small mammals and large ones, including some larger than modern elephants. Mammalian fauna was particularly rich during the Neocene. Some mammals returned to the sea—earliest of all the cetaceans, which may have appeared prior to the Cenozoic and which by the beginning of the Eocene occupied about the same position among marine fauna as today. Much later, probably toward the end of the Paleogene, there appeared the pinnipeds (walruses, seals), which were descended from predatory mammals. The order Chiroptera is known to have existed from at least the middle of the Paleogene. A very ancient order, existing from the beginning of the Cenozoic, is that of the primates. Their long evolution culminated in the appearance of higher hominoid apes in the Neocene and the emergence of primitive man at the beginning of the Anthropogenic Period.

Invertebrate fauna differed less sharply from that of the Mesozoic. Beginning in the mid-Cretaceous period the appearance of flowering plants resulted in a great increase in the number and variety of insects, and their rapid development continued into later times. It is also possible that the change in vegetation during the Cenozoic caused a sharp increase in the number and variety of terrestrial air-breathing gastropods. Among marine invertebrates the major occurrence at the end of the Mesozoic and beginning of the Cenozoic was the complete extinction of the formerly widespread ammonites and belemnites, which had been dominant among Mesozoic invertebrates. Subsequently the composition of invertebrate fauna resembled that of the present day in that the dominant groups were bivalves, gastropods, sea urchins, hexacorals, and other groups similar to modern forms. An exception was the appearance and rapid development in the Paleogene of nummulites, a distinct group of large benthic foraminifers whose shells formed entire layers of Paleogene limestone in the Tethys zone and adjacent regions. By the beginning of the Neocene the nummulites were almost completely extinct, and only a few of their descendants, much smaller in size, have survived to the present in the tropical seas.

Minerals. There are comparatively few ore deposits of endogenic origin among Cenozoic minerals because the deep-lying interiors of the young folded mountain ranges with which they are associated have not as yet been fully uncovered by denudation. Most important are petroleum and natural gas, deposits of which are concentrated primarily in the foredeeps framing the alpine folded mountain systems (Mesopotamia, Ciscarpathia, Ciscaucasia). Cenozoic sedimentary strata also contain large deposits of brown coal, oolitic brown iron ore, manganese ore (Chiatura, Nikopol’), rock and potassium salts (Ciscarpathia, Transcarpathia), bauxites, phosphorites (Tunisia, Algeria, Morocco), and various building materials.

REFERENCES

Strakhov, N. M. Osnovy istoricheskoi geologii, part 2. Moscow-Leningrad, 1948.
Gignoux, M. Stratigraficheskaia geologiia. Moscow, 1952. (Translated from French.)
Krishtofovich, A. N. Paleobotanika, 4th ed. Leningrad, 1957.

E. V. SHANTSER

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