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Neogene

Neogene

The Neogene, a term introduced by Moritz Hörnes in the mid-19th century, is a period in the geochronological scale and a system in the chro- nostratigraphic scale. This dual procedure (time interval and corresponding rock record) is used by earth scientists to subdivide geologic time in deci­phering the history of the earth. The Neogene is the last period/system of the Cenozoic era/erathem and, accordingly, the most recent one of the earth’s history. It began 23 million years ago, at the Paleogene-Neogene boundary, and it ends at the present. Under the current proposal of the International Commission on Stratigraphy, it includes four epochs/series: Miocene, Pliocene, Pleistocene, and Holocene. The classification and interpretation of the last two epochs/series, as well as a Quaternary sub-era, have been, and still are, a matter of debate. An important consequence is that the scientific community inherited different notions for the Neogene. Thus, the end of the Neo­gene has been variously interpreted to be at the Pleistocene-Holocene, Pliocene-Pleistocene, and Tertiary-Quaternary boundaries.

Marine microfossils are the backbone of the sub­division of the Neogene into its constituent ages/ stages. Complex mammal evolution under the influence of major continental separations and climatic change and orbital forcing cyclicity in sediments and oxygen isotopes records in the Atlantic and Mediterranean (supported by the Australian-Antarctic marine magnetic polarity scale) provide a precise and highly accurate Neogene timescale.

The Neogene Earth looked much like our own. However, the relatively similar distribution of landmass between then and now masks some dra­matic changes. Some continental motion took place during the Neogene, the most significant event being the counterclockwise rotation of the Arabian Plate, connecting Africa and Eurasia and cutting off the remnants of the old Tethys. India collided with Asia, giving rise to the Himalayan Mountains and the connection between North and South America.

In the south, a continuous circumpolar current circled Antarctica, isolated from other landmasses. Thus both poles were thermally isolated from warm equatorial waters, and (perhaps for the first time since the Ordovician) both poles accumu­lated heavy coverings of ice. At the same time, the virtual closing of east-west circulation through the Mediterranean Sea and between the Americas changed the hot, circulating currents. Thus, dur­ing the Neogene the world became much drier and cooler, culminating in the Pleistocene ice ages and the harsh conditions of the present day.

The world dried out. Huge deserts developed in North Africa and Central Asia. Grasslands expanded and quickly replaced the thinning for­ests. During the Neogene, birds and mammals evolved considerably, and the dawn of the genus Homo occurred. However, most other animals not needing grasses were relatively unchanged. Grasses are poor fodder: tough, low in nutrients, high in tooth-destroying silicates. Consequently, herbivorous species were smashed or utterly changed. Some grazer species emerged and evolved high-crown teeth. Ruminants diversified, and cranial append­ages appeared. Nevertheless, their predators fol­lowed them into extinction or transformation. The later Neogene saw the creation of an entirely new type of hunter, the pursuit predator. The pursued developed their own responses: herd behaviors, seasonal migrations, and big bodies adapted for speed and endurance in open country.

Another line of adaptation led to small-bodied generalists (rodents, raccoons, rabbits, and opos­sums) and their predators, the foxes, cats, dogs, and snakes. These generalists were mainly unspe­cialized herbivores or omnivores with partially fossorial habits, strong territoriality, and high reproductive rates. Theirs was the ability to exploit many resources within small, locally, or temporarily favorable conditions.

The Miocene epoch (23 to 5.3 million years ago [mya]) or “less recent” is so called because it contains fewer modern animals than the follow­ing, Pliocene, epoch. The Miocene is the longest epoch of the Neogene. During the late Miocene the island continent of India slammed into Asia, pushing up the Himalayas. Elsewhere, the west­ern American cordilleras, the Alps, and the Caucasus rose as well. One of the best known events in the marine realm is the Messinian salin­ity crisis at the end of the Miocene. The rise of mountains in the western Mediterranean com­bined with the global fall of sea level due to for­mation of the Antarctic ice cap sealed the western end of the Mediterranean for about 600,000 years. During this time, the Mediterranean Sea virtually dried up, forming enormous evaporite deposits. When the present Strait of Gibraltar was ultimately opened, the Atlantic would have poured a vast volume of water into the Mediterranean drying basin, resulting in a giant waterfall, much higher than 1,000 meters and far more powerful than Niagara Falls. On the other side of the African continent, three major rifts opened in roughly an east to west sequence. These events were probably related to the counterclockwise rotation of the Arabian plate. The Miocene was a time of warmer global climates, but during the mid-Miocene (14 mya), a marked drop in tem­peratures occurred and further led to the buildup of the East Antarctic ice cap.

The Miocene was a time of huge transition, the end of the ancient world, and the birth of the more recent sort of world. Two major ecosystems first appeared during the Miocene: kelp forests and grasslands. It was also the high point of the age of mammals. Also, this period saw animals that had evolved on different continents during the Eocene and Oligocene spread via land bridges.

The Pliocene epoch (5.3 to 1.8 mya), compared to previous epochs, was a relatively brief period of only 3.5 million years. The name Pliocene means “more recent.” During this time, the earth approached its current form, with ice caps, rela­tively modern geography, modern mammals, and the evolution of hominids. Continents had taken up their present positions. Both North and South America were drifting northward. However, South America was moving somewhat faster, related to a shift in the Caribbean tectonic plate. Thus, a perma­nent land bridge between the Americas developed in the mid-Pliocene, allowing mammals to migrate across. The closing of the Isthmus of Panama iso­lated the waters of the Gulf of Mexico and sepa­rated the marine biota of the east and west coasts. This tectonic episode had major consequences for global temperatures, because warm equatorial ocean currents were cut off and the climate became cooler and drier. At the same time, the Himalayan uplift accelerated the unfolding cool­ing process.

The Pleistocene epoch (1.8 to .011 mya) is known as the ice age, because this short epoch witnessed a dramatic, continued cooling, culminat­ing in a series of advances and retreats of the ice as the climate fluctuated between cold (glacial) and warm (interglacial) periods at periodicities fitting Earth’s orbit cycles (Milankovitch cycles). The sea level rose during the melting of the glaciers; then land bridges, created during cooler periods when glaciers sequestered more water, enabled the migration of animals and humans across conti­nents. The term Pleistocene (“most recent”) was coined for strata with 90% to 100% present day species. Animals and plants were basically modern species, although distributions were unusual. The great mammalian megafauna flourished. Many giant mammals evolved and lived on all conti­nents. During the Pleistocene, the hominid ten­dency to increase brain size and hence intelligence continued, and finally modern man (Homo sapi­ens) emerged.

The Holocene epoch covers the last 11,500 years of the Neogene period. The term Holocene means “completely recent” and refers to the pres­ent geological epoch. The Holocene represents a marked climatic warming phase corresponding to the present interstadial (warm period between glaciations) phase. All other ages, epochs, and eras are represented by natural evolutionary and geo­logical phenomena. The Holocene in contrast is distinguished by being the epoch during which there has been an exponential growth in human population and knowledge. Human activities have had a marked, and for the most part extremely detrimental, effect on the rest of the biosphere.

Beatriz Azanza

See also Chronostratigraphy; Earth, Age of; Evolution, Organic; Fossil Record; Geologic Timescale; Geology; Glaciers; Ice Ages; Paleogene; Paleontology; Plate Tectonics; Stratigraphy

Further Readings

Gould, S. J. (Ed.). (1993). The book of life: An illustrated history of the evolution of life on Earth. New York: Norton.

Gradstein, F. M., Ogg, J. G., & Smith, A. G. (Eds.).

(2004). A geologic time scale. Cambridge, UK: Cambridge University Press.

Stanley, S. M. (2004). Earth system history (2nd ed.). New York: Freeman.

Wicander, R., & Monroe, J. S. (2003). Historical geology: Evolution of Earth and life through time (4th ed.). London: Brooks/Cole.

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