Tertiary Period

Interval of geologic time, 65–1.

8 million years ago. It constitutes the first of the two periods of the Cenozoic Era, the second being the Quaternary. The Tertiary has five subdivisions: (from oldest to youngest) the Paleocene, Eocene, Oligocene, Miocene, and Pliocene epochs. During most of the Tertiary, the spatial distribution of the major continents was largely similar to that of today. Emergence and submergence of land bridges between continents critically affected the distribution of both terrestrial and marine animals and plants. Virtually all the existing major mountain ranges were formed either partly or wholly during the Tertiary.

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▪ geology

Introduction

 interval of geologic time lasting from approximately 65.5 million to 2.6 million years ago. It is the traditional name for the first of two periods in the Cenozoic Era (65.5 million years ago to the present); the second is the Quaternary Period (Quaternary) (2.6 million years ago to the present). The Tertiary has five principal subdivisions, called epochs, which from oldest to youngest are the Paleocene (Paleocene Epoch) (65.5–55.8 million years ago), Eocene (Eocene Epoch) (55.8–33.9 million years ago), Oligocene (Oligocene Epoch) (33.9–23 million years ago), Miocene (Miocene Epoch) (23–5.3 million years ago), and Pliocene (Pliocene Epoch) (5.3–1.8 million years ago, extending into the Quaternary). Beginning in the late 20th century, a number of authorities preferred not to use the terms Tertiary and Quaternary, preferring instead to divide the time intervals encompassed by each into two different intervals known as the Paleogene Period (65.5–23 million years ago) and the Neogene Period (23 million years ago to the present). In 2005 the International Commission on Stratigraphy (ICS) decided to recommend keeping the Tertiary and Quaternary in their geologic time scale, but only as sub-eras within the Cenozoic.

 The Tertiary was an interval of enormous geologic, climatic, oceanographic, and biological change. It spanned the transition from a globally warm world containing relatively high sea levels and dominated by reptiles to a world of polar glaciation (glacier), sharply differentiated climate zones (climate), and mammalian dominance. It began in the aftermath of the mass extinction event that occurred at the very end of the Cretaceous Period (the so-called K-T boundary), when as much as 80 percent of species, including the dinosaurs (dinosaur), disappeared. The Tertiary witnessed the dramatic evolutionary expansion of not only mammals (mammal) but also flowering plants (angiosperm), insects (insect), birds (bird), corals (coral), deep-sea organisms, marine plankton, and mollusks (mollusk) (especially clams (clam) and snails (snail)), among many other groups. The Tertiary Period saw huge alterations in Earth's systems and the development of the ecological and climatic conditions that characterize the modern world. The end of the Tertiary is characterized by the growth of glaciers in the Northern Hemisphere and the emergence of primates (primate) that later gave rise to modern humans (human evolution) (Homo sapiens), chimpanzees (chimpanzee) (Pan troglodytes), and other living great apes (ape).

      The name Tertiary was introduced by Italian geologist Giovanni Arduino (Arduino, Giovanni) in 1760. Arduino devised a stratigraphic system in which sedimentary rocks (sedimentary rock) containing fossils (fossil) were called “tertiary” rocks to distinguish them from igneous (igneous rock) and metamorphic (metamorphic rock) rocks present in the cores of mountain ranges (“primary” rocks), the shales (shale) and limestones (limestone) of Europe (“secondary” rocks), and surficial gravel (“quaternary” rocks). Although by modern standards his system appears simplistic, it did provide the initial framework upon which modern stratigraphy is based.

Tertiary environment

paleogeography

  Major Paleogene paleogeographic events Major Paleogene paleogeographic eventsThe present-day configuration of the continents (continent) and oceans (ocean) on Earth is the result of a complex sequence of events involving the growth and rearrangement of Earth's tectonic plates (plate tectonics) that began almost 200 million years ago. By the beginning of the Tertiary, the supercontinent of Pangea had been fragmenting for more than 100 million years, and the geometry of the continents and oceans had assumed an essentially modern aspect with several notable exceptions. The fragmentation and dispersal of the Southern Hemisphere supercontinent known as Gondwana, which had begun in the early part of the Mesozoic Era (251–65.5 million years ago), continued into the Cenozoic (Cenozoic Era). Australia began to separate from Antarctica about 58 million years ago during the late Paleocene Epoch. The initial subsidence of the South Tasman Rise, which occurred about 35 million years ago during the late Eocene Epoch, resulted in a shallow but inexorably widening oceanic connection between the Indian and Pacific oceans. It was this progressive separation of the two continents that led to the development of the Antarctic Circumpolar Current, a current that sweeps around Antarctica and thermally isolates it from the effects of warmer waters and climates (climate) to the north. This current was strengthened further and assumed its modern form as Antarctica and South America separated and thus formed the Drake Passage. There is much debate over when this opening actually occurred. Some experts state that the Drake Passage opened as early as the Eocene about 41 million years ago, whereas others maintain that this event took place as late as the boundary between the Oligocene (Oligocene Epoch) and Miocene epochs (Miocene Epoch) about 23 million years ago.

 The collision of India and southern Asia began during the late Paleocene, approximately 55 million years ago, and continues today. The collision produced two main geologic results. First, it began to block the westward-flowing Tethys seaway (Tethys Sea) near the Equator, a process completed with the junction of Africa and Asia near present-day Iran about 18 million years ago. Second, the creation of the Himalayas and the Plateau of Tibet (Tibet, Plateau of), which resulted from the collision, altered global climates by changing patterns of weathering (and thus the transfer rate of carbon to the atmosphere) as well as wind circulation (wind). India's collision with southern Asia also altered patterns of oceanic productivity by increasing erosion and thus nutrient runoff to the Indian Ocean.

 The present-day Mediterranean Sea is a geologically recent descendant of a portion of the Tethys seaway. Between five and six million years ago during the Messinian Age (Messinian Stage), the western remnant of the Tethys seaway was subject to a brief paroxysm, known as the Messinian salinity crisis, that lasted approximately one million years and saw the entire basin virtually isolated from the world ocean. The basin experienced severe desiccation and the precipitation of vast deposits of evaporites (evaporite) (such as salt and gypsum) up to several kilometres in thickness. The basin was subsequently refilled from the west by the Atlantic Ocean through Gibraltar and has undergone significant geologic evolution during the most recent five million years. About one million years ago this part of the Tethys was transformed into the Mediterranean Sea by the elevation of the Gibraltar sill. Consequently, the Mediterranean basin became isolated from deep oceanic bottom waters, and the present-day pattern of circulation developed.

      In the Northern Hemisphere the fragmentation of the northern supercontinent of Laurasia, which occurred as the result of the separation of Eurasia from North America and Greenland, was accomplished with the opening of the Norwegian-Greenland Sea about 55 million years ago during the Eocene Epoch. Prior to this time, the Greenland-Scotland Ridge formed the Thulean land bridge, a continental connection that allowed the exchange of terrestrial mammals (mammal) between western Eurasia and eastern North America. The subsidence of this ridge during the early Eocene allowed the exchange of surface water between the Arctic and Atlantic oceans. The termination of the Thulean land connection led to the development of separate patterns of evolution among terrestrial vertebrates (vertebrate) in Europe and North America (see evolution: Geographic speciation (evolution)).

      On the Eurasian continent itself, the Ural Trough, a marine seaway that linked the Tethys with the Arctic region but also constituted a barrier to the east-west migration of terrestrial faunas, was terminated by regional uplift at the end of the Eocene. The resulting immigration of Eurasian land animals into western Europe, and the consequent changes that occurred in terrestrial vertebrates, is known among vertebrate paleontologists as the Grande Coupure (French: “Big Break”).

       Major Neogene paleogeographic events Major Neogene paleogeographic eventsThe Bering Land Bridge, which united Siberia and Alaska, served as a second connection between Eurasia and North America. This link seems to have been breached by the Arctic and Pacific oceans between five and seven million years ago, allowing the transit of cold water currents and marine faunas between the Pacific and Atlantic oceans. The Atlantic and Pacific were also linked by the Central American seaway in the area of present-day Costa Rica and Panama. This seaway, extant since the first half of the Cretaceous Period, prevented the interchange of terrestrial fauna between North and South America; however, for a brief interlude during the Paleocene, a land connection may have existed between North and South America across the volcanic archipelago of the Greater Antillean (Greater Antilles) arc. The seaway was closed by the elevation of the Central American isthmus between 5.5 and 3 million years ago. This event had two significant geologic results. First, the emergence of the isthmus permitted a major migration in land mammal faunas between North and South America—the so-called Great American Interchange—which allowed ground sloths (sloth) and other South American immigrants to move into North America as far as California, the Great Plains, and Florida. In addition, some North American mammals (such as cats, horses, elephants, and camels) migrated as far south as Patagonia. Second, the emergence of the isthmus deflected the westward-flowing North Equatorial Current toward the north and enhanced the northward-flowing Gulf Stream. This newly invigorated current carried warm, salty waters into high northern latitudes, which contributed to increased rates of evaporation (vaporization) over the oceans and greater precipitation over the region of eastern Canada and Greenland. This pattern eventually led to the formation and development of the polar ice cap in the Northern Hemisphere between 3.5 and 2.5 million years ago. Deflection of the Equatorial Current also changed circulation patterns throughout the Caribbean, Gulf of Mexico, and western North Atlantic, which may have altered patterns of oceanic productivity in the region, resulting in significant evolutionary changes (extinctions and originations) in marine faunas.

Paleoclimate (paleoclimatology)

 Climatic history is intimately linked to the dynamic evolution of ocean-continent geometry and the associated changes in oceanic circulation. It is also closely connected to the cycling of carbon through the chemical reservoirs of living and dead organic matter, oceans and atmosphere, and the sediments of Earth's crust. During the Tertiary Period the continued fragmentation of the world ocean due to changing positions of the main continental masses—principally a poleward shift in the Northern Hemisphere—led to less-efficient latitudinal (east-west) exchange of thermal energy. Paleobiogeographic and oxygen isotope studies support this view by providing evidence of a long-term global temperature decline, the formation and development of a thermally stratified ocean, with much warmer water at the surface and much cooler water at depth, and enhanced climatic differentiation during the Cenozoic. This long-term global temperature decline followed the “climatic optimum” at the Paleocene-Eocene boundary, called the Paleocene-Eocene Thermal Maximum (PETM), that occurred about 55.8 million years ago, which is also reflected in the oxygen isotope records. In general terms, Mesozoic oceanic circulation was latitudinal, and the longitudinal (meridional; north-south) transport of heat energy during that time was relatively inefficient. In contrast, Cenozoic circulation has been predominantly longitudinal, although longitudinal heat transport became increasingly less efficient during the Neogene as global temperatures decreased.

      During the Paleocene, warm equable climates extended from one polar region to the other; the mean temperature difference between each pole and the Equator was about 5 °C (9 °F) as compared with about 25 °C (45 °F) today. Even deep ocean waters were relatively warm during the Tertiary. The Paleocene-Eocene boundary was marked by a geologically brief episode (less than 100,000 years) of global warming involving elevated temperatures in high-latitude ocean waters, a decline in oceanic productivity, and a marked reduction in global wind intensity. There is considerable evidence that this event was caused by the dissolution of methane hydrates (Atlantic Ocean) on the ocean floor, which led to an abruptly increased greenhouse effect in the atmosphere.

      Fossil remains of tropical faunas such as mollusks (mollusk) and sharks (shark) in places such as Alaska and the island of Spitsbergen in the Norwegian Arctic and of reptiles (reptile) and mammals on Ellesmere Island in the Canadian Arctic Archipelago attest to the subtropical conditions that existed at high latitudes during the early Eocene. Global cooling began during the middle and late Eocene and accelerated rapidly across the Eocene-Oligocene boundary, thereby initiating the process of continental-scale glaciation in Antarctica. In addition, the cooler oceans of the early Oligocene may have been more productive than oceans of the late Eocene.

  ice sheets developed at sea level on West Antarctica during the early Oligocene and covered most of the continent by the middle of the Miocene Epoch about 13 million years ago. The virtually complete glaciation of Antarctica in the late Miocene about 5.5 million years ago has been associated with the isolation of the Mediterranean basin from the world ocean during the Messinian salinity crisis (see above Paleogeography (Tertiary Period)). The sequestration of significant volumes of salt in the Mediterranean basin changed the density of Atlantic deep water and reduced heat transfer from low latitudes to high latitudes. Mountain glaciers appeared in the Gulf of Alaska by the mid-Miocene and were followed by glaciers in Patagonian Argentina during the early Pliocene. The large ice sheets that eventually covered most of northern Europe, Greenland, and North America first formed about 3.5 million years ago, but a major expansion occurred 2.5 million years ago. Many authorities suggest that Earth may have passed over a thermal threshold that initiated an interval of clustered glacial periods, or ice ages (ice age), at this time, a mode in which Earth remains locked today. The repeated waxing and waning of the Northern Hemispheric glaciers over the past 2.5 million years has resulted in significant and repeated expansions of the high-latitude belts of westerly winds toward the Equator, changes in ocean circulation pattern, and, during cold phases, the southward displacement of cool, dry climatic belts to southern Europe, the Americas, and North Africa.

Tertiary life

 The end of the Mesozoic Era marked a major transition in Earth's biological history. A major extinction event took place that resulted in the loss of nearly 80 percent of marine and terrestrial animal species. plant life also suffered, but to a much lesser extent. Most authorities believe that the cause of this major extinction event was one or more impacts by a comet or a meteorite near Chicxulub, Mex., on the Yucatán Peninsula, although some authorities point to the massive volcanic eruptions of the Deccan Traps in India as an additional potential causal factor. In any case, the beginning of the Tertiary Period, which coincided with the onset of the Cenozoic Era, was marked by a reduction in biological diversity (biodiversity) both on land and in the oceans. This reduction was followed by a gradual recovery and an adaptive radiation, or rapid diversification, into new life-forms within a few hundred thousand to several million years. Present-day ecosystems (ecosystem) are for the most part populated by animals, plants, and single-celled organisms that survived and redeployed after the great extinction event at the end of the Mesozoic. A number of groups of organisms (e.g., insects (insect), flowering plants (angiosperm), marine snails (snail)) showed particularly rapid diversification after the Mesozoic, and life at the end of the Tertiary was more diverse than it had been at any time in the past.

Life on land

Flowering plants and grasses

      The Cretaceous-Tertiary transition was not marked by significant changes in terrestrial floras. Throughout the Cenozoic, angiosperms (angiosperm) (flowering plants) continued the remarkable radiation begun roughly 100 million years ago during the middle of the Cretaceous Period and quickly came to dominate most terrestrial habitats—today they account for approximately 80 percent of all known plant species. Of particular interest among flowering plants are the grasses (grass), which appeared by the late Paleocene Epoch. Simple grasslands (grassland), which bore grass but lacked the complex structural organization of sod, appeared in the Eocene, whereas short grasslands with sod appeared in the first half of the Miocene. The Miocene also saw the dramatic expansion of grazing mammals on several continents. Truly modern grasslands appeared in the late Miocene, five to eight million years ago, during a period of cooling and drying that may have been connected to the Messinian salinity crisis in the Mediterranean (see above Paleogeography (Tertiary Period)). The proportion of grasses utilizing the photosynthetic pathway (photosynthesis) also increased at this time, consistent with a decrease in atmospheric carbon dioxide at this time.

Birds (bird)

 The number of bird species increased significantly in the Tertiary and throughout the Cenozoic, with separate groups diversifying at different times and places. Among the more notable events in the evolution of birds was the emergence of large flightless birds ( Diatryma and related forms) during the Paleocene and Eocene epochs. These birds, which reached heights of more than 2 metres (6.5 feet), have generally been interpreted as running carnivores, inhabiting the ecological niche left vacant by the extinction of a group of dinosaurs (dinosaur) called the theropods (theropod) at the end of the Cretaceous. A similar interpretation has been given to the even-larger flightless birds of the Oligocene (Oligocene Epoch) of South America (such as Phorusrhacos and related forms), which evolved when South America was an island continent, isolated from advanced mammalian carnivores.

      The passerines (passeriform), the most diverse group of modern birds, have a poor fossil record and may have emerged as early as the Early Cretaceous or as late as the Oligocene. Passerines began an explosive period of diversification during the Miocene.

The rise of mammals (mammal)

 The most spectacular event in Cenozoic terrestrial environments has been the diversification and rise to dominance of the mammals. From only a few groups of small mammals in the late Cretaceous that lived in the undergrowth and hid from the dinosaurs, more than 20 orders of mammals evolved rapidly and were established by the early Eocene (Eocene Epoch). Although there is some evidence that this adaptive radiation event began well before the end of the Cretaceous, rates of speciation accelerated during the Paleocene and Eocene epochs. At the end of the Paleocene, a major episode of faunal turnover (extinction and origination) largely replaced many archaic groups (multituberculates (multituberculate), plesiadapids, and “condylarth” (Condylarthra) ungulates) with essentially modern groups such as the perissodactyls (perissodactyl) (which include primitive horses (horse), rhinoceroses (rhinoceros), and tapirs (tapir)), artiodactyls (artiodactyl) (which include camels (camel) and deer), rodents (rodent), rabbits (rabbit), bats (bat), proboscideans (proboscidean), and primates (primate).

 In the Eocene these groups dispersed widely, migrating via a northern route, probably from Eurasia to North America. In the late Eocene an episode of global cooling triggered changes in the vegetation that converted areas of thick rainforest to more open forest and grasslands (grassland), thereby causing another interval of evolutionary turnover that included the disappearance of the last of the primitive herbivores, such as the brontotheres (brontothere). From the Oligocene Epoch onward, land mammal communities were dominated by representatives of the mammalian groups living today, such as horses, rhinoceroses, antelopes (antelope), deer, camels, elephants (elephant), felines (feline), and canines (canine).

 These groups evolved significantly during the Miocene (Miocene Epoch) as the changes to climate and vegetation produced more open grassy habitat. Starting with primitive forms that had low-crowned teeth for browsing leafy vegetation, many herbivorous mammals evolved specialized teeth for grazing gritty grasses and long limbs for running and escaping from increasingly efficient predators. By the late Miocene, grassland communities analogous to those present in the modern savannas (savanna) of East Africa were established on most continents. Evolution within many groups of terrestrial mammals since the late Miocene has been strongly affected by the dramatic climate fluctuations of the late Cenozoic.

Mammalian migration from Eurasia to North America

 The rapid evolutionary diversification or radiation of mammals (mammal) in the early Tertiary was probably mostly a response to the removal of reptilian competitors by the mass extinction event occurring at the end of the Cretaceous Period. Later events in mammalian evolution, however, may have occurred in response to changes in geology, geography, and climatic conditions. In the middle of the Eocene Epoch, for example, the direct migration of land mammals between North America and Europe was interrupted by the severance of the Thulean land bridge, a connection that had existed prior to this time. Although Europe became cut off from North America, Asia (especially Siberia) remained in contact with Alaska during the late Eocene, and repeated migrations occurred throughout the Oligocene (Oligocene Epoch) and Miocene epochs (Miocene Epoch).

      During the early Miocene, a wave of mammalian immigration from Eurasia brought bear-dogs (early ancestors of modern canines of the genus Amphicyon), European rhinoceroses (rhinoceros), weasels (weasel), and a variety of deerlike mammals to North America. Also during this time, mastodons (mastodon) escaped from their isolation in Africa and reached North America by the middle of the Miocene. Horses (horse) and rodents evolved in the early Eocene, and anthropoid primates emerged during the middle Eocene. Immigration of African mammalian faunas, including proboscideans (mammoths (mammoth), mastodons, and other relatives of modern elephants), into Europe occurred about 18 million years ago (early Miocene). Climatic cooling and drying during the Miocene led to several episodes where grassland ecosystems (ecosystem) expanded and concomitant evolutionary diversifications of grazing mammals occurred.

Mammalian migration between North and South America

 During the late Pliocene, the land bridge formed by the Central American isthmus allowed opossums (opossum), porcupines (porcupine), armadillos (armadillo), and ground sloths (sloth) to migrate from South America and live in the southern United States. A much larger wave of typically Northern Hemispheric animals, however, moved south and may have contributed to the extinction of most of the mammals endemic to South America. These North American invaders included dogs (dog) and wolves (wolf), raccoons (raccoon), cats (feline), horses, tapirs (tapir), llamas (llama), peccaries (peccary), and mastodons.

Primates (primate)

      Amid the rapid diversification of mammals in the early Tertiary, primates evolved from animals similar to modern squirrels (squirrel) and tree shrews (tree shrew). Compared with other terrestrial mammals, primates possessed the largest brains (brain) relative to their body weight. This feature—along with limb extremities composed of flat nails rather than hooves or claws (claw), specialized nerve endings called Meissner's corpuscles that increased the tactile sensitivity in their hands and feet, and rounded molars and premolar cusps—allowed primates to adapt to and exploit arboreal environments and newly emergent grasslands. Although the first signs of primate dentition were present as early as the Paleocene Epoch, the first fully recognizable primate forms did not emerge until the Eocene. Members of the Tarsiidae (which include modern tarsiers (tarsier) and their ancestors) appeared in western Europe and North Africa, the Adapidae (which include lemurs (lemur), lorises (loris), and their ancestors) arose in North America and Europe, and the Omomyidae (which include the possible ancestors of monkeys (monkey) and apes (ape)) emerged in North America, Europe, Egypt, and Asia during the Eocene Epoch. In addition, fossil evidence indicates that the earliest monkeylike primates (Simiiformes) arose in China about 45 million years ago.

      The separation of the more primitive primates (lemurs, lorises, tarsiers, and their ancestors) from the more advanced forms (monkeys, apes, and humans) is thought to have occurred during the Oligocene Epoch. The skull of Rooneyia, an omomyid fossil discovered in Texas and dated to the Oligocene, possesses a mixture of primitive and advanced features and is thus considered to be a transitional primate form. Some primate groups abandoned the locomotor patterns of vertical clinging and leaping for quadrupedalism (walking on four limbs) during the Oligocene. Other developments include the emergence of the catarrhines (Old World monkeys, apes, and humans) in Africa and the platyrrhines (New World monkeys) in South America. The catarrhines are the only group to possess truly opposable thumbs. (Some lower primates possess nominally opposable thumbs but lack the precision grip of catarrhines.)

      By the Miocene, because of dramatic changes in Earth's geomorphology and climate and the emergence of vast grasslands, a new type of primate—the ground inhabitant—came into being. The benefit of a generalized body form and a larger brain assisted many primates in their transition to terrestrial lifestyles. During this time, Sivapithecus—a form considered to be the direct ancestor of orangutans (orangutan)—appeared in Eurasia, and Dryopithecus—the direct ancestor of gorillas (gorilla), chimpanzees (chimpanzee), and humans—appeared in parts of Africa and Eurasia. In addition, Morotopithecus bishopi, a species possessing the earliest traces of the modern hominoid skeletal features, appeared some 20 million years ago near Lake Victoria (Victoria, Lake) in Africa.

      The late Miocene-Pliocene primate fossil record is surprisingly sparse. No fossils traceable to the lineages of modern apes are known, and only meagre information exists for monkey families. Nevertheless, this interval is perhaps best known for the rise of the human lineage in central and eastern Africa, as evidenced by Sahelanthropus tchadensis from Chad (7 million years ago), Orrorin tugenensis from Kenya (6.1–5.8 million years ago), and Ardipithecus ramidus (4.4 million years ago). The foramen (the hole in the skull through which the spinal cord enters) of Ardipithecus is located centrally under the skull instead of at the rear of it. This feature is indicative of bipedalism, one of the characteristics that separate the human lineage from those of apes and chimpanzees. Other bipedal primates from the Pliocene include Kenyanthropus platyops and various species of Australopithecus. The precise evolutionary relationships among these forms remain controversial, but it is clear that they lie close to the evolutionary branching event that separates humans from apes.

Life in the oceans

Marine extinctions and recovery

      In the seas, several major Tertiary biotic events stand out. The major extinction event at the boundary between the Mesozoic (Mesozoic Era) and Cenozoic eras (Cenozoic Era), 65.5 million years ago, affected not only the dinosaurs of the terrestrial environments but also large marine reptiles (reptile), marine invertebrate faunas (rudists, belemnites (belemnoid), ammonites (ammonoid), bivalves (bivalve)), planktonic protozoans (protozoan) (foraminiferans (foraminiferan)), and phytoplankton. The recovery of biological diversity after this event took hundreds of thousands to millions of years, depending on the group. At the boundary between the Paleocene and the Eocene, between 30 and 50 percent of all species of deep-sea benthic foraminiferans became extinct in a sudden event associated with the warming of the deep oceans. The present-day fauna of the deep, cold oceans (the so-called psychrosphere) evolved in the latest part of the Eocene about 35 million years ago. This was concomitant with a significant cooling of oceanic deep waters of some 3–5 °C (5.4–9 °F). The transition between the Eocene and Oligocene was also marked by several extinction events among marine faunas. The closure of the Tethys seaway in the late Early Miocene about 15 million years ago resulted in the disappearance of many of the larger tropical foraminiferans called nummulitids (nummulite) (large lens-shaped foraminiferans) whose habitat ranged from Indonesia to Spain and as far north as Paris and London. Although the descendants of nummulitids can be found today in the Indo-Pacific region, they show much less diversity.

      The marine faunas of the eastern Pacific and western Atlantic region were similar throughout the Tertiary until about 3–5.5 million years ago. The elevation of the Central American isthmus at that time created a land barrier between the two regions that during the Tertiary resulted in the isolation of one fauna from another and differentiation (that is, “provincialization”) between the groups. In addition, the presence of the isthmus may have led to environmental changes in the western Atlantic that caused high rates of extinction in old species and the origination of new ones.

Radiation of invertebrates (invertebrate)

      In the oceans, patterns of evolution that had begun during the Cretaceous Period continued and in some cases accelerated during the Tertiary. These include the evolutionary radiation of crabs (crab), bony fish, snails, and clams (clam). An increase in predation may have been an important driving force of evolution in the sea during this time (see community ecology). Many groups of clams and snails, for example, show increased adaptations for resisting predators during the Tertiary. Episodes of rapid diversification also occurred in many groups of clams and snails during the Eocene Epoch and at the Miocene (Miocene Epoch)-Pliocene (Pliocene Epoch) boundary. Following the extinction of the reef-building rudists (large bivalve mollusks) at the end of the Cretaceous, reef-building corals (coral) had recovered by the Eocene, and their low-latitude continuous stratigraphic record is taken as an indicator of the persistence of the tropical realm.

Large marine animals

      Cetaceans (cetacean) (whales (whale) and their relatives) first appeared in the early Eocene, about 51 million years ago, and are thought to have evolved from early artiodactyls (artiodactyl) (a group of hoofed mammals possessing an even number of toes). Whale evolution accelerated during the Oligocene (Oligocene Epoch) and Miocene, and this is probably associated with an increase in oceanic productivity. Other new marine forms that emerged in late Paleogene seas were the penguins (penguin), a group of swimming birds (bird), and the pinnipeds (pinniped) (a group of mammals that includes seals (seal), sea lions (sea lion), and walruses (walrus)). The largest marine carnivore of the period was the shark (Carcharocles megalodon), which lived from the middle Miocene to the late Pliocene and reached lengths of at least 16 metres (about 50 feet).

Foraminiferans (foraminiferan)

      Foraminiferans (foraminiferan), especially those belonging to superfamily Globigerinacea, also evolved rapidly and dispersed widely during the Tertiary Period. Consequently, they have proved to be extremely useful as indicators in efforts to correlate oceanic sediments and uplifted marine strata at global and regional scales. Differential rates of evolution within separate groups of foraminiferans increase the utility of some forms in delineating stratigraphic zones, a step in the process of correlating rocks of similar age. For example, conical species of Morozovella and Globorotalia are often used to correlate rock strata across vast geographies because they have wide stratigraphic ranges that vary from one to five million years.

      The nummulitids (nummulite) were a group of large lens-shaped foraminiferans that inhabited the bottoms of shallow-water tropical marine realms. They had complex labyrinthine interiors and internal structural supports to strengthen their adaptation to life in high-energy environments. Nummulitids also received nourishment from single-celled symbiotic algae (tiny photosynthetic (photosynthesis) dinoflagellates (dinoflagellate)) they housed within their bodies. Nummulitids of the genus Nummulites grew to substantial size (up to 150 mm [6 inches] in diameter), and they occurred in massive numbers during a major transgression taking place during the middle of the Eocene Epoch. This transgression produced high sea levels and formed extensive limestone deposits in Egypt, which produced the blocks from which the pyramids (pyramid) were built. Nummulites lived throughout the Eurasian-Tethyan faunal province from the later part of the Paleocene Epoch to the early Oligocene, but it did not reach the Western Hemisphere. Following the extinction of Nummulites, other larger foraminiferans, the miogypsinids and lepidocyclinids, flourished.

Tertiary rocks

Major subdivisions of the Tertiary System

 Classically, the Cenozoic Era was divided into the Tertiary and Quaternary periods, separated at the boundary between the Pliocene (Pliocene Epoch) and Pleistocene epochs (Pleistocene Epoch) (1.8 million years ago); however, by the late 20th century many authorities considered the terms Tertiary and Quaternary to be obsolete. In 2005 the International Commission on Stratigraphy (ICS) decided to recommend keeping the Tertiary and Quaternary periods as units in the geologic time scale, but only as sub-eras within the Cenozoic Era. The ICS redivided the Cenozoic Era into the Paleogene Period (65.5–23 million years ago) and the Neogene Period (23 million years ago to the present). These divisions do not directly coincide with boundary years of the Tertiary and Quaternary periods. Under this paradigm, the Paleogene Period, the older of the two divisions, commences at the onset of the Cenozoic Era and includes the Paleocene Epoch (65.5–55.8 million years ago), Eocene Epoch (55.8–33.9 million years ago), and Oligocene Epoch (33.9–23 million years ago); the Neogene spans the interval between the beginning of the Miocene Epoch (23–5.3 million years ago) and the present. In the ICS scheme, the Tertiary sub-era extends from the onset of the Cenozoic Era through the Piacenzian Age (Piacenzian Stage) (3.6–2.6 million years ago) of the Pliocene Epoch (5.3–1.8 million years ago). This substitution of the terms Paleogene and Neogene for Tertiary and Quaternary, however, is still not universally accepted in the scientific community.

      Precise stratigraphic positions for the boundaries of the various traditional Tertiary series were not specified by early workers in the 19th century. It is only in more recent times that the international geologic community has formulated a philosophical framework for stratigraphy. By specifying the lower limits of rock units deposited during successive increments of geologic time at designated points in the rock record (called stratotypes), geologists have established a series of calibration points, called Global Boundary Stratotype Sections and Points (GSSPs), at which time and rock coincide. These boundary stratotypes are the linchpins of global chronostratigraphic units—essentially, the points of reference that mark time within the rock—and serve as the point of departure for global correlation.

      Several boundary stratotypes have been identified within Tertiary rocks. The Cretaceous (Cretaceous Period)-Tertiary, or K-T, boundary has been stratotypified in Tunisia in North Africa. (To many scholars, this boundary is also known as the Cretaceous-Paleogene, or K-P, boundary.) Its estimated age is 65.5 million years. The Paleocene-Eocene boundary has an estimated age of 55.8 million years; its GSSP is located near Luxor, Egypt. In the early 1990s the Eocene-Oligocene boundary was stratotypically established in southern Italy, with a currently estimated age of approximately 33.9 million years. The Oligocene-Miocene boundary (which also corresponds to the boundary between the Paleogene and Neogene systems) has been stratotyped in Carrosio, Italy; its age has been calculated at roughly 23 million years old. The GSSP associated with the Miocene-Pliocene boundary is located in Sicily and has been dated to about 5.3 million years ago, although the location of this boundary may be repositioned in the future. Although traditionally the boundary between the Tertiary and Quaternary systems was placed at the boundary between the Pleistocene and Pliocene epochs, more recently it has been placed within the later part of the Pliocene Series, at the base of the Gelasian Stage. This boundary has been stratotyped in Sicily near the town of Gela and dated to approximately 2.6 million years ago.

Occurrence and distribution of Tertiary deposits

      With the exception of the vast Tethys seaway (Tethys Sea), the basins of western Europe, and the extensive Mississippi Embayment of the Gulf Coast region in the United States, Tertiary marine deposits are located predominantly along continental margins and occur on all continents. Miocene deposits are found as far north as Alaska; Eocene deposits are found in eastern Canada; and Paleocene deposits are located in Greenland. Deposits of Paleogene age occur on Seymour Island near the Antarctic Peninsula, and Neogene deposits containing marine diatoms (diatom) (silica (silica mineral)-bearing marine phytoplankton) have recently been identified intercalated between glacial tills (till) on Antarctica itself.

      Global sea levels have fallen gradually by about 300 metres (about 1,000 feet) over the past 100 million years, but superimposed upon that trend is a higher-order series of globally fluctuating increases and decreases (that is, transgressions and regressions) in sea level. These fluctuations vary with a periodicity of several million years; where they have occurred along passive (that is, tectonically stable) continental margins, they have left a record of marginal marine, brackish accumulations that overlap with continental sedimentary (sedimentary rock) deposits in Europe, North Africa, the Middle East, southern Australia, and the Gulf and Atlantic coastal plains of North America. In most regions, Paleogene seas extended farther inland than did those of the Neogene. In fact, the most extensive transgression of the Tertiary is that of the Lutetian Age (Lutetian Stage) (Middle Eocene), about 49–40 million years ago. During that interval, the Tethys Sea expanded onto the continental margins of Africa and Eurasia and left extensive deposits of nummulitic rocks, which are made up of shallow-water carbonates (carbonate rock). Sediments of Tertiary age are widely developed on the deep ocean floor and on elevated seamounts (seamount) as well. In the shallower parts of the ocean (above depths of 4.5 km [about 3 miles]), sediments are calcareous (made of calcium carbonate), siliceous (derived from silica), or both, depending on local productivity. Below 4.5 km the sediments are principally siliceous or inorganic, as in the case of red clay, due to dissolution of calcium carbonate.

      Nonmarine Tertiary sedimentary and volcanic deposits are widespread in North America, particularly in the intermontane basins west of the Mississippi River. During the Neogene, volcanism and terrigenous deposition extended almost to the Pacific coast. In South America, thick nonmarine clastic sequences (conglomerates (conglomerate), sandstones (sandstone), and shales (shale)) occur in the mobile tectonic belt (tectonic landform) of the Andes Mountains and along their eastern front; these sequences extend eastward for a considerable distance into the Amazon basin. Tertiary marine deposits occur along the eastern margins of Brazil and Argentina, and they were already known to English naturalist Charles Darwin (Darwin, Charles) during his exploration of South America from 1832 to 1834.

volcanism and orogenesis (orogeny)

      Volcanism has continued throughout the Cenozoic on land and at the major oceanic ridges (oceanic ridge), such as the Mid-Atlantic Ridge and the East Pacific Rise, where new seafloor is continuously generated and carried away laterally by seafloor spreading (seafloor spreading hypothesis). Iceland, which was formed in the middle Miocene, is one of the few places where the processes that occur at the Mid-Atlantic Ridge can be observed today.

      Two of the most extensive volcanic outpourings recorded in the geologic record occurred during the Tertiary. About 67–66 million years ago, near the Cretaceous-Tertiary boundary, massive outpourings of basaltic lava formed the Deccan Traps (India) of India. About 55 million years ago, near the Paleocene-Eocene boundary, massive explosive volcanism took place in northwestern Scotland, northern Ireland, the Faeroe Islands, East Greenland, and along the rifted continental margins on both sides of the North Atlantic Ocean. Volcanic activity in the North Atlantic was associated with the rifting and separation of Eurasia from North America, which occurred on a line between Scandinavia and Greenland and left a stratigraphic record in the marine sedimentary basin of England and in ash deposits as far south as the Bay of Biscay. In both the Deccan and North Atlantic, comparable volumes of extensive basalts (basalt) in the amount of 10,000,000 cubic km (about 2,400,000 cubic miles) were erupted.

      The well-known volcanics of the Massif Central of south-central France, which figured so prominently in early (18th-century) investigations into the nature of igneous rocks (igneous rock), are of Oligocene age, as are those located in central Germany. The East African Rift System preserves a record of mid-to-late Tertiary rifting and the separation event that eventually led to the formation of a marine seaway linking the Indian Ocean with the Mediterranean.

      The circum-Pacific “Ring of Fire,” an active tectonic belt that extends from the Philippines through Japan and around the west coast of North and South America, was subject to seismic activity and andesitic (andesite) volcanism throughout much of the Tertiary. The extensive Columbia Plateau basalts were extruded over Washington and Oregon during the Miocene, and many of the volcanoes of Alaska, Oregon, southern Idaho, and northeastern California date to the Late Tertiary. Active volcanism occurred in the newly uplifted Rocky Mountains during the early part of the Tertiary, whereas in the southern Rocky Mountains and Mexico volcanic activity was more common in the mid- and late Tertiary. The linear volcanic trends, such as the Hawaiian, Emperor, and Line island (Line Islands) chains in the central and northwestern Pacific, are trails resulting from the movement of the Pacific Plate over volcanic “hot spots (plate tectonics)” (that is, magma-generating centres) that are probably fixed deep in Earth's mantle. The major hot spot island groups such as the Hawaiian (Hawaii) (which has been active over the past 30 million years), Galapagos (Galapagos Islands), and Society (Society Islands) (which were active during the Miocene) islands are volcanoes that rose from the seafloor. Central America, the Caribbean region, and northern South America were the sites of active volcanism throughout the Cenozoic.

      In contrast to the passive-margin sedimentation on the Atlantic and Gulf coastal plains, the Cordilleran (or Laramide (Laramide orogeny)) orogeny in the Late Cretaceous, Paleocene, and Eocene produced a series of upfolded and upthrusted mountains and deep intermontane basins in the area of the Rocky Mountains. Deeply downwarped basins accumulated as much as 2,400 metres (about 8,000 feet) of Paleocene and Eocene sediment in the Green River Basin of southwestern Wyoming and 4,300 metres (about 14,000 feet) of sediment in the Uinta Basin of northeastern Utah. Other basins ranging from Montana to New Mexico accumulated similar but thinner packages of nonmarine fluvial and lacustrine (lacustrine ecosystem) sediments rich in fossil mammals and fish. In the Oligocene and Miocene the influences of the cordilleras (cordillera), or mountain chains, on what is now the western United States had ceased, and the basins were gradually filled to the top by sediments and abundant volcanic ash deposits from eruptions in present-day Colorado, Nevada, and Utah. These basins were exhumed during the Pliocene-Pleistocene with renewed uplift of the long-buried Rocky Mountains, along with uplift of the Colorado Plateau, producing steep stream gradients that resulted in the cutting of the Grand Canyon to a depth of more than 1,800 metres (about 6,000 feet).

      Volcanism along the Cascade (Cascade Range) mountain chain has been active since the late Eocene, as evidenced by the major eruption of Mount St. Helens (Saint Helens, Mount) in 1980 and subsequent minor eruptions. This volcanism was gradually shut off in California as the movement of plate boundaries changed from one of subduction (subduction zone) to a sliding and transform motion (see plate tectonics: Principles of plate tectonics (plate tectonics)). With the development of the San Andreas Fault system, the western half of California started sliding northward. The Cascade– Sierra Nevada mountain chain began to swing clockwise, causing the extension of the Basin and Range Province in Nevada, Arizona, and western Utah. This crustal extension broke the Basin and Range into a series of north-south-trending fault-block mountains and downdropped basins, which filled with thousands of metres of upper Cenozoic sediment. These fault zones (particularly the Wasatch Fault in central Utah and the San Andreas zone in California) remain active today and are the source of most of the damaging earthquakes (earthquake) in North America. The Andean mountains were uplifted during the Neogene as a result of subduction of the South Pacific beneath the South American continent.

      Complex tectonic activity also occurred in Asia and Europe during the Tertiary. The main Alpine orogeny began during the late Eocene and Oligocene and continued throughout much of the Neogene. Major tectonic activity in the eastern North Atlantic (Bay of Biscay (Biscay, Bay of)) extended into southern France and culminated in the uplift of the Pyrenees in the late Eocene. On the south side of the Tethys, the coastal Atlas Mountains of North Africa experienced major uplift during this time, but the Betic region of southern Spain and the Atlas region of northern Morocco continued to display mirror-image histories of tectonic activity well into the late Neogene. In the Middle East the suturing of Africa and Asia occurred about 18 million years ago. Elsewhere, India had collided with the Asian continent about 45 million years ago, initiating the Himalayan uplift that was to intensify in the late Neogene (that is, Pliocene and Pleistocene) and culminate in the uplift of the great Plateau of Tibet (Tibet, Plateau of) and the Himalayan (Himalayas) mountain range. Major orogenic movement also occurred in the Indonesian-Malaysian-Japanese arc system during the Neogene. In New Zealand, which sits astride the Indian-Australian and Pacific plate boundary, the major tectonic uplift (the Kaikoura orogeny) of the Southern Alps began about 10 million years ago.

Sedimentary sequences

      Northwestern Europe contains a number of Tertiary marine basins that essentially rim the North Sea basin, itself the site of active subsidence during the Paleogene and infilling during the Neogene. The marine Hampshire and London basins, the Paris Basin, the Anglo-Belgian Basin, and the North German Basin have become the standard for comparative studies of the Paleogene part of the Cenozoic, whereas the Mediterranean region (Italy) has become the standard for the Neogene. The Tertiary record of the Paris Basin is essentially restricted to the Paleogene strata (namely, those of Paleocene–late Oligocene age), whereas scattered Pliocene-Pleistocene deposits occur in England and Belgium above the Paleogene. The strata are relatively thin, nearly horizontal, and often highly fossiliferous, particularly in the middle Eocene calcaire grossier (freshwater limestone) of the Paris Basin, from which a molluscan (mollusk) fauna of more than 500 species has been described. The Paris Basin is a roughly oval-shaped basin centred on Paris, whereas the Hampshire and London basins lie to the southwest and northeast of London, respectively. The London Basin and the Anglo-Belgian Basin were part of a single sedimentary basin across what is now the English Channel during the early part of the Paleogene.

 The total Paleogene stratigraphic succession in these basins is less than 300 metres (about 980 feet), and it is made up of clays, marls (marl), sands (sand), carbonates, lignites (lignite), and gypsum. These layers reflect alternations of marine, brackish, lacustrine, and terrestrial environments of deposition. The alternating transgressions and regressions of the sea have left a complex sedimentary record punctuated by numerous unconformities (interruptions in the deposition of sedimentary rock) and associated temporal hiatuses, and the correlation of these various units and events has challenged stratigraphers since the early 19th century. The integration of biostratigraphy, paleomagnetic stratigraphy, and tephrochronology (respectively, using fossils, magnetic properties, and ash layers to establish the age and succession of rocks) has resulted in a refined correlation of rock layers in these separate basins.

      In North America, by contrast, extensive Tertiary sediments occur on the Atlantic and Gulf coastal plains and extend around the margin of the Gulf of Mexico to the Yucatán Peninsula, a distance of more than 5,000 km (about 3,100 miles). Seaward these deposits can be traced from the Atlantic Coastal Plain to the continental margin and rise and in the Gulf Coastal Plain into the subsurface formations of this oil-bearing province of the Gulf of Mexico. During the Paleocene the Gulf Coast extended northward roughly 2,000 km (about 1,200 miles) in a feature called the Mississippi Embayment, which reached as far as southwestern North Dakota and Montana; there marine deposits known as the Cannonball Formation can be seen as outcrops of sandstone. Although eroded between northwestern South Dakota and southern Illinois, marine outcrops continue southward to the present coastline and continue in the subsurface of the Gulf of Mexico. Tertiary sediments with a thickness in excess of 6,000 metres (about 20,000 feet) are estimated to lie beneath the continental margin along the northern Gulf of Mexico. In the Tampico Embayment of eastern Mexico, thicknesses of more than 3,000 metres (about 10,000 feet) have been estimated for the Paleocene Velasco Formation alone, which developed under conditions of active subsidence and associated rapid deposition. Exposures in the Atlantic Coastal Plain and most of the Gulf Coastal Plain are of Paleogene age, but considerable thicknesses of Neogene sediment occur in offshore wells in front of the Mississippi delta, where thicknesses in excess of 10,000 metres (about 33,000 feet) have been recorded for the Neogene alone. Sediments are dominantly calcareous in the Florida region and become more marly and eventually sandy to the west, reflecting the input of terrigenous matter transported seasonally by the Mississippi River and its precursors. Because of general faunal and floral similarities, it is possible to make relatively precise stratigraphic correlations in the Paleogene between the Gulf and Atlantic coastal plain region and the basins in northwestern Europe.

Establishing Tertiary boundaries

      The name Tertiary was introduced by Italian geologist Giovanni Arduino (Arduino, Giovanni) in 1760 as the second youngest division of Earth's rocks. The oldest rocks were the primitive, or “primary,” igneous and metamorphic rocks (metamorphic rock) (composed of schists (schist), granites (granite), and basalts) that formed the core of the high mountains in Europe. Arduino designated rocks composed predominantly of shales and limestones in northern Italy as elements of the fossiliferous “secondary,” or Mesozoic (Mesozoic Era), group. He considered younger groups of fossiliferous sedimentary rocks, found chiefly at lower elevations, as “tertiary” rocks and the smaller pebbles and gravel that covered them as “quaternary” rocks. Although originally intended as a descriptive generalization of rock types, many of Arduino's contemporaries and successors gave these categories a temporal connotation and equated them with rocks formed prior to, during, and after the Noachian deluge. In 1810 French mineralogist, geologist, and naturalist Alexandre Brongniart (Brongniart, Alexandre) included all the sedimentary deposits of the Paris Basin in his terrains tertiares, or Tertiary. Soon thereafter all rocks younger than Mesozoic in western Europe were called Tertiary.

 The subdivision of the Tertiary into smaller units was originally based on fossil faunas of western Europe that were known to 19th-century natural scientists. These faunas primarily contained mollusks exhibiting varying degrees of similarity with modern types. At the same time, the science of stratigraphy was in its infancy, and the primary focus of its earliest practitioners was to use the newly discovered sequential progression of fossils in layered sedimentary rocks to establish a global sequence of temporally ordered stages. Scottish geologist Charles Lyell (Lyell, Sir Charles, Baronet) employed a simple statistical measure based on the relative percentages of living species of mollusks to fossil mollusks found in different layers of Tertiary rocks. These percentages had been compiled by Lyell's colleague and friend Gérard-Paul Deshayes, a French geologist who had amassed a collection of more than 40,000 mollusks and was preparing a monograph on the mollusks of the Paris Basin.

      In 1833 Lyell divided the Tertiary into four subdivisions (from older to younger): Eocene (Eocene Epoch), Miocene (Miocene Epoch), the “older Pliocene (Pliocene Epoch),” and the “newer Pliocene.” (The latter was renamed Pleistocene (Pleistocene Epoch) in 1839.) The Eocene contained about 3 percent of the living mollusk species, the Miocene about 20 percent, the older Pliocene more than one-third and often over 50 percent, and the newer Pliocene about 90 percent. Lyell traveled extensively and had a broad and comprehensive understanding of the regional geology for his day. He understood, for example, that rocks of the Tertiary were unevenly distributed over Europe and that there were no rocks of the younger part of the period in the Paris Basin. He used the deposits in the Paris, Hampshire, and London basins as typical for the Eocene. For the Miocene he used the sediments of the Loire Basin near Touraine, the deposits in the Aquitaine Basin near Bordeaux in southwestern France, and the Bormida River valley and Superga near Turin, Italy. The sub-Apennine formations of northern Italy were used for the older Pliocene, and the marine strata in the Gulf of Noto, on the Island of Ischia (Ischia, Island of) (also in Italy), and near Uddevalla (in Sweden) were used for the newer Pliocene.

      The limits between Lyell's Tertiary subdivisions were not rigidly specified, and Lyell himself recognized the approximate and imperfect nature of his scheme. Indeed, in their original form, Lyell's subdivisions would today be termed biostratigraphic units (bodies of rocks characterized by particular fossil assemblages) rather than chronostratigraphic units (bodies of rocks deposited during a specific interval of time).

      Subsequent stratigraphic studies in northern Europe showed that deposits were included variously in the upper Eocene or lower Miocene by different geologists of the day. This situation led German geologist H.E. Beyrich, in 1854, to create the term Oligocene (Oligocene Epoch) for rocks in the North German Basin and Mainz Basin and to insert it between the Eocene and the Miocene in the stratigraphic scheme. As originally proposed, the Oligocene included the Tongrian and Rupelian stages (Rupelian Stage) as well as strata that subsequently formed the basis for the Chattian Stage. The Tongrian is no longer used as a standard unit, its place being taken by the Rupelian.

      The term Paleocene (Paleocene Epoch) was proposed by German paleobotanist Wilhelm P. Schimper in 1874 on the basis of fossil floras in the Paris Basin that he considered intermediate between Cretaceous and Eocene forms. Typical strata include the sands of Bracheux, the travertines (travertine) of Sézanne, and the lignites and sandstones of Soissons. The problem of the Paleocene is that, of all the chronostratigraphic units of the Tertiary, it alone is defined on the basis of nonmarine strata, making recognition of its upper limit and general correlation difficult elsewhere. Acceptance of the term Paleocene into the general system of stratigraphic names was irregular, and only in 1939 did the United States Geological Survey, general arbiter of standard stratigraphic nomenclature in North America, formally accept it. The Danian Stage was proposed by the Swiss geologist Pierre Jean Édouard Desor in 1846 for chalk deposits in Denmark. It was assigned to the Cretaceous by virtue of the similarity of its invertebrate megafossils to those of the latest Cretaceous elsewhere. However, since the late 1950s, micropaleontologists have recognized that calcareous marine plankton (foraminiferans (foraminiferan) and coccolith-bearing nannoplankton) exhibit a major taxonomic change at the boundary between the Maastrichtian (Maastrichtian Stage) (uppermost Cretaceous) Stage and the Danian (Danian Stage) (lowermost Tertiary) Stage. The Danian is now widely regarded as being the oldest stage of the Cenozoic.

      In 1948 the 18th International Geological Congress placed the base of the Pleistocene at the base of the marine strata of the Calabrian Stage of southern Italy, using the initial appearance of northern or cool-water invertebrate faunas in Mediterranean marine strata as the marker. Subsequent studies showed that the type section was ill-chosen and that the base of the Calabrian Stage was equivalent to much younger levels within the Pleistocene. A newly designated stratotype section was chosen at Vrica in Calabria, and the base of the Pleistocene was found comparable to a level dated to nearly 1.8 million years ago. Officially, the oldest interval of the Pleistocene remains the Calabrian; however, members of the International Union of Geological Sciences (IUGS) and the International Commission on Stratigraphy (ICS), prompted by the International Union for Quaternary Research (INQUA), were giving serious consideration at the beginning of the 21st century to setting the base of the Pleistocene (and thus the top of the Tertiary System) coincident with the base of the Gelasian Stage.

Correlation of Tertiary strata

 The boundaries of the Tertiary were originally only qualitatively estimated on the basis of the percentages of living species of (primarily) mollusks in the succession of marine strata in the western European basins (see above). The need for more precise correlations of Mesozoic and Cenozoic marine strata in Europe led to the concept of stages, which was introduced in 1842 by French paleontologist Alcide d'Orbigny (Orbigny, Alcide Dessalines d'). These stages were originally defined as rock sequences composed of distinctive assemblages of fossils that were believed to change abruptly as a result of major transgressions and regressions of the sea. This methodology has since been improved and refined, but it forms the basis for modern biostratigraphic correlation. Early attempts at global correlations of strata were made by direct comparisons with the faunas in the type areas in Europe; however, it was soon realized that faunal provincialization led to spurious correlations. In 1919 an independent set of percentages for the Indonesian region was proposed, which was subsequently modified into the so-called East India Letter Stage classification system based on the occurrence of taxa of larger foraminiferans.

      Since about the mid-1900s, increasing efforts have been made to apply radioisotopic dating (dating) techniques to the development of a geochronologic scale, particularly for the Cenozoic Era. The decay of potassium-40 to argon-40 (see potassium-argon dating) has proved very useful in this respect, and refinements in mass spectroscopy (spectroscopy) and the development of laser-fusion dating involving the decay of argon-40 to argon-39 have resulted in the ability to date volcanic mineral samples in amounts as small as single crystals with a margin of error of less than 1 percent over the span of the entire Cenozoic Era.

      Also, since the mid-1960s, investigators have demonstrated that Earth's magnetic field (geomagnetic field) has undergone numerous reversals in the past. It is known that most rocks pick up and retain the magnetic orientation of the field at the time they are formed through either sedimentary or igneous processes. With the development of techniques for measuring the rock's original orientation of magnetization, a sequence of polarity reversals has been dated for the late Neogene. In addition, a paleomagnetic chronology has been built for the entire Cenozoic. This work is based on the recognition that the magnetic lineations detected in rocks on the ocean floor were formed when basaltic magma had been extruded from the oceanic ridges. Earth's magnetic polarity undergoes a reversal roughly every 500,000 years, and newly formed rocks assume the ambient magnetic polarity of the time. As a result, strips of normal and reversed polarity that reflect these magnetic reversals can be observed in deep-sea cores. The calibration of the composite geomagnetic polarity succession to time and the relation of this chronology to the isotopic time scale, however, have proved to be the greatest source of disagreement over various current versions of the geologic time scale. Calibrations of a time scale must ultimately be based on the application of meaningful isotopic ages to the succession of polarity intervals and geologic stages. A geochronologic scheme is thus an integration of several methodologies; it makes use of the best attributes of seafloor-spreading history (that is, the pattern of seafloor magnetic anomalies), magnetostratigraphy, and biostratigraphy in the application of relevant isotopic ages to derive a high-resolution and internally consistent time scale. The recent application of cyclical components driven by astronomical phenomena into the stratigraphic record, such as lithological couplets of marl and chalks (chalk) and fluctuations in the ratios and percentages of fossil taxa, has resulted in fine-tuning the geologic time scale to a resolution of about 5,000 years in the late Neogene.

      Micropaleontologists have created a number of zones based on the regional distribution of calcareous plankton (foraminiferans and nannoplankton) and those of the siliceous variety (radiolarians (radiolarian) and diatoms), making it possible to correlate sediments from the high northern to high southern latitudes by way of the equatorial region. The resulting high-resolution zonal biostratigraphy and its calibration to an integrated geochronology provide the framework in which a true historical geology has become feasible.

Warren D. Allmon

Additional Reading

The paleontology of the Tertiary is discussed in Donald R. Prothero, Linda C. Ivany, and Elizabeth A. Nesbitt (eds.), From Greenhouse to Icehouse: The Marine Eocene-Oligocene Transition (2003); Alan Feduccia, The Origin and Evolution of Birds, 2nd ed. (1999); Marie-Pierre Aubry, Spencer G. Lucas, and William A. Berggren (eds.), Late Paleocene-Early Eocene Climatic and Biotic Events in the Marine and Terrestrial Records (1998); Donald R. Prothero, The Eocene-Oligocene Transition: Paradise Lost (1994); Donald R. Prothero and William A. Berggren (eds.), Eocene-Oligocene Climatic and Biotic Evolution (1992); Stephen Jay Gould (ed.), The Book of Life (1993, reissued 2001); Else Marie Friis, William G. Chaloner, and Peter R. Crane (eds.), The Origins of Angiosperms and Their Biological Consequences (1987); and R.J.G. Savage, Mammal Evolution: An Illustrated Guide (1986).Felix M. Gradstein, James G. Ogg, and Alan G. Smith (eds.), A Geologic Time Scale 2004 (2004), provides an excellent account of Tertiary stratigraphy (with special emphasis on the Paleogene and Neogene systems), the evolution of scientific thought with regard to the Tertiary, and early 21st-century changes to nomenclature. The geology of the Tertiary Period is also described in Steven M. Stanley, Earth System History, 2nd. ed. (2004); and James P. Kennett, Marine Geology (1982). Other discussions regarding the nomenclature of subdivisions within the Tertiary are provided in Derek J. Blundell and Andrew C. Scott (eds.), Lyell: The Past Is the Key to the Present (1998); William A. Berggren et al. (eds.), Geochronology, Time Scales, and Global Stratigraphic Correlation (1995); and W. Brian Harland et al., A Geologic Time Scale 1989 (1990).Warren D. Allmon

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