CENOZOIC ERA

the third of the major eras of Earth history and the interval of time during which the continents assumed their modern configuration and geographic positions. It was also the time when the Earth's flora and fauna evolved toward those of the present. The Cenozoic, from the Greek for recent life, began about 66.4 million years ago and is divided by international agreement into two periods, the Tertiary (66.4 to 1.6 million years ago) and the Quaternary (1.6 million years ago to the present). The designations Tertiary and Quaternary have been carried over from earlier geologic nomenclature when they were used to distinguish more recent rocks from older ones, which were classified as Primary and Secondary. Some authorities prefer to subdivide the time interval encompassed by the Tertiary into the Paleogene and Neogene periods and dispense with the term Tertiary altogether. The Tertiary and Quaternary are subdivided into epochs, the Tertiary into the Paleocene (66.4 to 57.8 million years ago), Eocene (57.8 to 36.6 million years ago), Oligocene (36.6 to 23.7 million years ago), Miocene (23.7 to 5.3 million years ago), and Pliocene (5.3 to 1.6 million years ago); the Quaternary into the Pleistocene (1,600,000 to 10,000 years ago) and the Holocene (the last 10,000 years). The Tertiary Period was a time of mountain building. Major tectonic movements in the Earth's crust resulted in the great mountain ranges of today: the Andes, the Alps, the Himalayas, and the Cascade chain of volcanoes in western North America. The final phase of the Laramide orogeny in the Rocky Mountains also occurred during the Tertiary. The evolution of plant life in the Tertiary changed the face of the Earth. The angiosperms, flowering plants whose seeds are enclosed for protection and carry their own food supply in their fruit, overtook the gymnosperms, or naked seed plants, that were dominant during the Mesozoic Era (245 to 66.4 million years ago). The angiosperms apparently originated in what is now Southeast Asia. They spread rapidly throughout the world, reaching the Arctic and Antarctic regions, which were not ice-capped at the time. The distribution of angiosperms was affected to a degree by the orientation of major mountain ranges. In most of Eurasia the mountains run east-west; in the Americas and Southeast Asia they run north-south. After the great ice ages of the Pleistocene, many of the plants in Eurasia remained in the southern regions when the ice subsided because the mountains blocked their way. In America and Southeast Asia, however, they found their way back to the northern regions. The most important biological development of the Tertiary was the evolution of the mammals. Two major groups of mammals, the placentals and the marsupials, appeared during the Cretaceous Period (144 to 66.4 million years ago) and radiated rapidly during the early Tertiary. Continental drift was influential in mammalian evolution. Australia was isolated throughout much of the Tertiary and its marsupial population evolved without competition, diversifying into many forms and exploiting various ecological niches. South America had been reached by some specialized placental mammals as well as marsupials. The placentals evolved more successfully, though some marsupials also thrived. During the Paleocene Epoch mammalian evolution in both the Old and New World accelerated, and many increasingly differentiated forms developed. Among these were the rodents, carnivores, hoofed animals, and primates. During the Pliocene, the first hominoids, with upright posture and modern limb proportions, appeared. From the fossil record it appears that the earliest forms of the genus Homo evolved in East Africa, South Africa, and Indonesia. Early modern humans finally emerged during the Pleistocene. Throughout the Quaternary there occurred successive periods of glacial advance and retreat that considerably affected floral and faunal development in the Northern Hemisphere. Like many other species, the early humans were displaced by the glaciations, but unlike some other animals they adapted to the changing conditions and survived what has been called the Great Ice Age. Table 4: Geologic time scale. To see more information about a period, select one from the chart. third of the major eras of the Earth's history, beginning about 66.4 million years ago and extending to the present (see Table). It was the interval of time during which the continents assumed their modern configuration and geographic positions and during which the Earth's flora and fauna evolved toward those of the present. The term Cenozoic, originally spelled Kainozoic, was introduced by John Phillips in an 1840 Penny Cyclopaedia article to designate the most recent of the three major subdivisions of the Phanerozoic. Derived from the Greek for recent life, it reflects the sequential development and diversification of life on Earth from the Paleozoic (ancient life) through the Mesozoic (middle life). Today, the Cenozoic is internationally accepted as the youngest of the three subdivisions of the fossiliferous part of Earth history. The Cenozoic Era is generally divided into two periods, the Tertiary and the Quaternary. The designations Tertiary and Quaternary, however, are relics of early attempts in the late 18th century at formulating a stratigraphic classification that included the now wholly obsolete terms Primary and Secondary. In 1856 Moritz Hrnes introduced the terms Paleogene and Neogene, the latter encompassing rocks equivalent to those described by Charles Lyell as Miocene and older and newer Pliocene (which included what he later called the Pleistocene). Subsequent investigators have determined that the designation Neogene correctly applies to the rock systems and corresponding time intervals delineated by Lyell, though some authorities prefer to exclude the Pleistocene from the Neogene. The Paleogene encompasses the Paleocene, Eocene, and Oligocene. (The terms Paleocene and Oligocene were coined subsequent to Lyell's work and inserted in the lower part of the Cenozoic stratigraphic scheme.) William A. Berggren Additional reading The Cenozoic Era is treated in such general geologic histories as Donald R. Prothero, Interpreting the Stratigraphic Record (1989); N.J. Snelling (ed.), The Chronology of the Geological Record (1985); Steven M. Stanley, Earth and Life Through Time, 2nd ed. (1989); and Michael O. Woodburne (ed.), Cenozoic Mammals of North America: Geochronology and Biostratigraphy (1987). Other sources include Stephen Jay Gould, Time's Arrow, Time's Cycle: Myth and Metaphor in the Discovery of Geological Time (1987); Maurice Gignoux, Stratigraphic Geology (1955; originally published in French, 4th ed., 1950); M.J. Hambrey and W.B. Harland (eds.), Earth's Pre-Pleistocene Glacial Record (1981); A.M. Spencer (ed.), Mesozoic-Cenozoic Orogenic Belts (1974); and William J. Frazier and David R. Schwimmer, Regional Stratigraphy of North America (1987). Studies of the environment of this interval of Earth history include John M. Armentrout, Mark R. Cole, and Harry Terbest, Jr., Cenozoic Paleogeography of the Western United States (1979); and B.M. Funnell and W.R. Riedel (eds.), The Micropalaeontology of Oceans (1971). Flora and fauna of the period are studied in Charles B. Beck (ed.), Origin and Early Evolution of Angiosperms (1976); Donald E. Savage and Donald E. Russell, Mammalian Paleofaunas of the World (1983); and R.J.G. Savage, Mammal Evolution: An Illustrated Guide (1986). William A. Berggren