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Translations of Encyclopedia about Geology

 

The Cenozoic of the earth

The Cenozoic of the earth started roughly 65 million years ago. It is divided into Tertiary (from latin, tercier, third system), and Quaternary (from Latin, kvarter, fourth system).

The Tertiary era is then subdivided, in accordance with the share of the surviving kinds in the total fauna, into five epochs The earlier Cenozoic (Paleogene) is then divided into Paleocene, Eocene, and Oliigocene, while the youngar Tertiary (Neogene) is further subdivided into Miocene and Pliocene. The scientists originally concluded that the Tertiary era ended 600.000 years ago. Today, the science is of the opinion that the dividing time line between the Tertiary and the Quaternary should be somewhere between 2,5 and 2 million years ago.

Important sediments containing fossils of land as well as marine mammals are characteristic for that time period. It was time of formation of mountains such as Alps, Pyrenees, and Himalayas, and of other movements of the earth's crust. A prominent depression of the earth's surface extended from the Mediterranean across Europe all the way to Norway. From time to time, sea waters penetrated far inland. Volcanic activity was quite common at that time.

During the early Tertiary era the climate was still warm, almost tropical. Mighty forests flourished and produced raw material for today's bituminous coal deposits in northern Europe and North America. During that time, however, a gradual cooling trend set in and the pole was covered by ice for the first time. Steppes were forming in the Mediterranean and the sea levels fluctuated. Shallow coastal seas containing numerous sedimentary deposits were constantly forming. Rhine and Parisian basins are examples of these formations.

Fossils of that time, which differ greatly from the fossils of the Cretaceous, attest to a prominent evolution of mammals. They spread all over the globe by adapting to the diverse conditions on land, in the water and in the air. Primitive forms of animals similar to rodents, marsupials, and insectivores were among the mammals of the earlier Tertiary era.

Unable to compete with the placental mammals, the marsupials soon died out. They survived to the present time only in Australia, which was already separated from the continent, thus no small mammals could colonize that part of the world.

Insectivores are perceived as the initial form in the evolution of modern proboscideans, fliers (chiroptera), rodents, primates, beasts of prey (creondots), and ungulates (condylarthra). Permanent addition to the different modern animal kinds, in particular during the later Tertiary era, was supported by the climate changes. For example, when the American continent lifted, the climate there became drier. Forests gave way to extensive lowlands covered by grassy prairies. In the open spaces, the beasts of pray could detect easily the insectivores. Only the fastest ones could escape. Animals that could run on their toes were faster and evolved into the ungulates.

The mammals dominated the oceans as well. Sirenians (sirenia), whales, and seals evolved independently of each other. They shared their environment mainly with bony fish, which already included prehistoric forms of bass, pike, herring, eel, tench, and barbel. In the meantime, the reptiles of the earth's Mesozoic died out. Only turtles, crocodiles, therapsids, snakes, as well as certain amphibians such as frogs and turtles, survived. In the case of birds, the majority of the orders, to which the birds of today belong, evolved at that time.

During the Tertiary, the insects were evolving also. Fossils of mosquitoes, beetles, dragonflies, ants, crickets, and bees were found. The flowers of the angiosperms provided new sources of nutrition supporting their evolution.

Angiosperms formed a new group of plants. Although the earlier ones persisted, their share in the total vegetation was decreasing. In the second, cooler period of the Tertiary, coniferous forests were growing mainly in the northern latitudes. This is where amber, in which many insects were captured, was formed.

Quaternary (kvarter, Latin for fourth system) is the youngest and simultaneously the shortest period of time in the history of our earth. It is divided into Pleistocene, which started some 2 to 2,5 million years ago. and approximately 10.000 years ago entered the present geological time called Holocene.

Although this time period is very short when compared to the age of the earth, it is of special significance because humans evolved during this period. The Quaternary, therefore, is denominated as the time of "human formation" (Anthropozoic). A fast-changing climate was another reason for delimiting this period.

Throughout almost the entire history of the earth the climate at least at latitudes close to the equator was evenly tropical and warm. During the Quaternary the cold and warm time periods began to alternate. The first ice age occurred as early as 600.000 years ago in the northern hemisphere. That is why the beginning of the Pleistocene was originally set at that time. A longer-lasting period of cooling, however, preceded the ice age, and the beginning of that period was therefore set at approximately 2 million years ago.

The cool periods had a profound impact on the flora and fauna, while during the warm periods the temperatures were similar to what we experience today. Eurasian plants and animals could not recede easily to the south due to impassable mountains such as Alps and Carpathians, which were covered by ice. In North America, however, they could retreat all the way to the Gulf of Mexico and when the ice age concluded, they would expand again in northerly direction. This is why we find a greater diversity of species in North America than in Europe where, according to estimates, 50 to 80 percent of different species died out.

The fauna left behind fossils, which are classified by warm and cold periods. The warm periods are characterized by forest elephant and forest rhinoceros. In addition, the prehistoric form of our domesticated gaur, prehistoric gaur, lived in this warm climate.

In cold periods, the predominant animals were the mammoth, which developed from the genus Elephantidae, giant deer, muskox, reindeer, and woolly rhinoceros. In addition, cave bear and lion inhabited the cool tundra. Some of these animals froze so quickly that the contents of their stomach were preserved.

The evolution of the humans is also closely linked to the ice age. The human environment narrowed as a consequence of more inclement weather and the ice masses which covered the continents, especially in the northern regions. As the seas receded, the humans discovered new places to live. They found sufficient food supply on the coasts in the form of fish, shells, and marine mammals. The humans settled on the islands of south Asia, as well as Australia and America (by crossing a land bridge called Beringia).

During the Holocene, there was a new global warming and the latest melting of the ice masses, up to this point in the history of the earth, took place. Reindeer and muskox followed the retreating ice. Mammoth, woolly rhinoceros, and giant deer probably did the same. It is still unclear why these animals died out. The successful hunting activities of the humans during the Stone Age may have been a contributing factor.

Retreating glaciers left moraines in the foothills of Europe, Asia, and North America. The sea level rose again and the climate turned more humid. Numerous lowlands became swamps. The plants again grew faster and larger, and new forests flourished. In addition to birches and conifers, which already existed in the latter part of the ice age, there were new forests of beech and oak.

The humans started to engage in agriculture and animal husbandry. They cleared forests and had an indelible impact on both fauna and flora

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