Cenozoic Era

During the Cenozoic era, the last 66.4 million years of Earth's history, all aspects of the Earth's modern environment developed. By studying the geologic record from this era, scientists can distinguish between environmental changes caused by a normal progression of geologic phenomena and those related to human activity.

Modern Earth Environment

The Cenozoic era began approximately 66.4 million years ago, extending into modern times. It is subdivided into three periods—the Paleogene, Neogene, and Quaternary. The Paleogene extends from 66.4 to 23 million years ago, the Neogene from 23 to 2.6 million years ago, and the Quaternary from 2.6 million years ago to the present. Relative time units are based on geologic events and phenomena such as fossils. Their position in geologic history is determined by the relative position of these events and phenomena in the rock record. The values in years are determined by radiometric methods after the relative units are recognized.

The Cenozoic era represents a time in Earth's history when the modern environment began to develop. The geologic setting, geography, atmosphere, climate, oceans, fauna, and flora began to exhibit a decidedly modern appearance. In previous eras of Earth's history, many environmental conditions were very different from those of the present, as were the fauna and flora. Despite the differences, there is a progression in geologic history that can be followed. The Earth of the Cenozoic era most closely resembles the present-day Earth. Earth's Cenozoic era is closer in time and contains the recent epoch of Earth's history. The further back one goes in time, the greater the differences. To fully understand and appreciate the development of the Earth through the Cenozoic era, a brief survey of the conditions and phenomena of the epochs of the Cenozoic era, from the oldest to the youngest, is necessary.

The Paleocene epoch (66.4 to 57.8 million years ago) is the first epoch of the Cenozoic era. The transition from the preceding Cretaceous period of the Mesozoic era is one of the most pronounced in the geologic record. Great physical and accompanying biological changes took place. Whatever the cause of such changes, the setting of the Paleocene was very different from that of the Cretaceous. Worldwide, the beginning of the Paleocene is marked by a regression of the seas and a fall in sea level. This change resulted in reduced shallow seas for the dwelling of marine organisms and a decrease in humid conditions on the continents. The geographic setting also changed with the continued enlargement of the Atlantic Ocean Basin and the northerly drift of North America. This northerly drift resulted in the increase of the temperate zone in North America at the expense of the subtropical zones, causing a cooling of the climate from the tropical conditions of the Cretaceous period.

Another important geographic change in the Paleocene epoch was the decrease in size of Tethys, the worldwide warm-water ocean roughly parallel to the equator. In North America, mountain building that began in the late Cretaceous continued in the Rocky Mountains region. Although sea level fell worldwide in the earliest Paleocene, a large transgression, or sea-level rise, soon began. This transgression was large enough that the sea reached into the present-day High Plains of North and South Dakota. It is estimated that the sea level was more than 250 meters above its present level during this episode. When this transgression and the following regression ended, the sea never again rose to this level. Since the end of this transgression-regression interval, the sea level has never risen above the coastal plains.

Life in the Paleocene epoch was very different from that of the Cretaceous period. Marine life was not as diverse in the Paleocene, and many prominent forms of marine invertebrates common in the Cretaceous were extinct by the Paleocene. Many of the marine organisms looked very much like modern marine invertebrates and are the ancestors of modern biota. On land, a fauna that had been previously dominated by the dinosaurs was now dominated by the mammals. Most of these mammals were small and did not closely resemble many of the modern mammals. Others, such as the rodent Paramys, which resembled a modern squirrel in appearance, are clearly the ancestors of modern mammal faunas. The flora of the Paleocene was dominated by flowering plants.

The Eocene epoch (between 57.8 and 36.6 million years ago) contained two major tectonic events of global significance. The first major event was the beginning of the closure of Tethys. This was caused by the collision of two continents, India and Asia. Although mountain building did not begin in the area at this time, the two continents were near each other. Mountain building began in the Mediterranean region during the Eocene because of the African plate moving relative to the Eurasian plate. Another significant tectonic development was the separation of Australia from Antarctica. This event allowed the development of a cold current around the Antarctic continent and a climatic isolation of this landmass. In North America, volcanic activity was extensive throughout the West.

Environmental conditions in the Eocene showed a continued cooling and drying trend from the Paleocene. Large basins between the mountains built during the Paleocene were filled with river and lake sediments during the Eocene. Sea levels remained higher than at present, but the epicontinental seas in North America were confined to the coastal plain regions.

One of the major innovations in marine life during the Eocene epoch was the whale. The whales developed from land carnivores that had adapted to a marine existence. On land, the most significant development was the appearance of the grasses. These plants enabled the widespread development of savannas (semiarid grasslands) and various grazing animals. Included among these are the earliest members of the horse, elephant, and rhinoceros lineages. Also present were the first very large mammals, the titanotheres, and early ancestors of the camels. The Eocene also saw the first development of the dog, cat, and weasel families and the existence of numerous birds.

Oligocene and Miocene Epochs

The Oligocene epoch (between 36.6 and 23.7 million years ago) saw a continuation of the tectonic and environmental conditions present in the Eocene. An important change occurred at the Earth's sea level in the middle of the Oligocene. The sea level fell radically to a point well below the modern sea level. This change is likely related to the development of continental glaciers in Antarctica and resulted in further cooling and drying of the continents.

Life of the Oligocene is characterized by the success of the large land mammals such as titanotheres and rhinoceros. One member of the rhinoceros family, Indricotherium, was the largest mammal to walk the Earth, standing about 5.5 meters tall at the shoulder. The mammal fauna of the Oligocene continued to become more modern in appearance; monkeys and other apes began to develop at this time.

The Miocene epoch (between 23.7 and 5.3 million years ago) represents a time of marked change in Earth's history. Although the fauna and flora were very modern in appearance throughout the Cenozoic era, the Miocene marks a time when fauna and flora began to resemble modern ones closely. In the marine realm, invertebrates resembled modern ones, and many modern species of invertebrates trace their origins to the Miocene epoch. The expansion of the whales in the Miocene was the greatest marine-related change. On land, the flora began to be dominated by small, nonwoody plants. These plants were well adapted to life under somewhat dry, cool conditions. Many modern common families of wildflowers began their rapid expansion in the Miocene. Small mammals, such as the rodents, and the perching birds became more common. Large mammals were present, but very large forms, such as the titanotheres, were extinct by the Miocene. In the Miocene, two major additions to the carnivores, the bear and hyena families, developed.

The tectonic and environmental conditions of the Miocene are closely akin to modern processes. One major worldwide tectonic event was the beginning of the uplift of the Himalayas as India began to move beneath the Asian continent. Another important event was the closure and evaporation of the entire Mediterranean Sea. Whether this event, known as the Messinian Event, was caused by tectonic processes or was related to a worldwide drop in sea level, or some combination of the two, is still debated among scientists. No matter the cause, such a phenomenon represents a major change in the Earth's environment. As a result, the development of widespread continental glaciation began in the Southern Hemisphere. Global conditions produced by this process were lower sea level and dryer conditions in the Northern Hemisphere.

Pliocene, Pleistocene, and Holocene Epochs

The Pliocene epoch (between 5.3 and 1.6 million years ago) represents a time of equable climate over much of the Earth, partly because of a rise in sea level that began at this time. Other major events that began in the Pliocene were the renewed uplift of the Rocky Mountains and the uplift of many of the other mountain ranges throughout the American West. Two new locations for seafloor spreading began to develop. The Red Sea began to expand at this time, as did the Gulf of California. Continued volcanism between North and South America caused a land bridge to develop between the two land masses. A sharp climatic change began in the late Pliocene with the widespread development of continental glaciers. Life of the Pliocene resembled the life of the Miocene in many ways. One important development was the first well-preserved fossils of the hominids, the family to which the human species belongs.

The Pleistocene epoch (between 1.6 million years ago and 10,000 years ago) is a time in Earth's history dominated by glaciers. The Pleistocene is commonly called the “ice age,” but there were many advances and retreats of the great continental ice sheets during this epoch. Much of the modern Earth's surface acquired its present appearance as features such as the Great Lakes and many present-day river systems were formed. During the Pleistocene epoch, the last of the truly large mammals became extinct. Forms such as mammoths, mastodons, and giant sloths disappeared from the fauna. Some of these extinctions appear to be related to the expansion and success of the human species. A part of this expansion includes the arrival of humans in the Americas more than 30,000 years ago, possibly over a land bridge between Siberia and Alaska, resulting from lower sea levels in the present-day Bering Sea.

The Holocene epoch began about 10,000 years ago. This date is approximately when the last glacier retreated from the temperate latitudes. The Holocene epoch is not recognized by all geologists as different from the Pleistocene. The question is whether the present conditions on the Earth mark a fundamental change in the Earth's climate or whether they mark another warm period between glacial episodes. One thing the Holocene characterizes is the importance of humans as geologic agents and the ability of humans to reshape the environment. For this reason, many geologists recognize the Holocene as a unique epoch in Earth history.

Principal Terms

epicontinental sea: any body of marine water that is present on the continents; epicontinental seas were more extensive in the past than at present

epoch: a relative time unit and a subdivision of a period

glaciers: systems of moving ice that can occur at any time in high elevations and periodically will occur on the continents in the high latitudes during periods known popularly as ice ages

period: a relative time unit and a subdivision of an era

regression: a very slow fall in sea level that may result in the exposure of the continental shelves

tectonics: the general term for deep-earth geologic phenomena such as mountain building, volcanism, earthquakes, continental collisions, and seafloor spreading

transgression: a very slow rise in sea level that usually results in the flooding of the continents

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