The Paleogene is a geochronological and chrono- stratigraphic unit of the Cenozoic era/erathem. It is a period in the geochronological scale and a system in the chronostratigraphic scale, and it is placed between the Cretaceous and Neogene periods. It began 65 million years ago, at the Cretaceous- Paleogene (K-Pg or K-T) boundary, and ended 23 million years ago, at the Oligocene-Miocene (O-M) boundary; thus, the Paleogene lasted 42 million years. It consists of three epochs and/or series: Paleocene, Eocene, and Oligocene.

The Paleogene followed the Cretaceous period and began with the Cretaceous-Paleogene mass extinction event. There is paleontological evidence of abrupt changes in flora and fauna in this event (most often referred to as the K-T boundary mass extinc­tion), including the total extinction of dinosaurs, ammonites, belemnites, cephalopods, and rudist mol­luscs; and the catastrophic mass extinction of plank- tic foraminifers, calcareous nannofossils, corals, bivalves, brachiopods, fishes, mammals, and other reptile groups. The Paleogene is most notable as being the period in which mammals and birds were diversified, exploiting ecological niches untouched by the previously extinct dinosaurs. Both groups evolved and came to dominate the land. Mammals evolved considerably into large forms in terrestrial and marine environments; birds evolved into roughly modern forms in an airborne environment.

During the Paleogene, global tectonic processes continued that had begun during the Mesozoic era, with the continents drifting toward their present positions. Although these were gradual processes, the drifting of the continents caused significant paleoclimatic and paleoceanographic turnovers during the Paleogene. The former components of the old supercontinent Gondwana continued to split apart, with South America, Africa, and Antarctica-Australia pulling away from each other. Africa moved north toward Europe, slowly closing the occidental Tethys Ocean until it disappeared during the Eocene, and uplifting the Alps during the Oligocene. Similarly, India initiated its rapid migration toward the north, until it collided with Asia, narrowing the oriental Tethys Ocean, folding the Himalayas, and forming the Indian Ocean. The Tethys Ocean vanished during the Paleogene, becoming today’s Mediterranean Sea, the remnant of that old ocean. The northern supercontinent Laurasia began to break up during the Eocene, with Europe, Greenland, and North America drift­ing apart. The tectonic splitting of the Greenland and Norwegian seas increased the submarine vol­canic and hydrothermal activity (North Atlantic flood basalts) during the Paleocene-Eocene transi­tion. Antarctica and Australia began to split in the late Eocene, and South America and Antarctica in the Oligocene, which allowed the formation of the circumantarctic current.

The climate of the earliest Paleogene was slightly cooler than that of the preceding Cretaceous. Nevertheless, the temperature rose again in the late Paleocene, reaching its highest point at the Paleocene-Eocene (P-E) boundary. A sudden and extreme global warming event occurred in the P-E boundary, 55.8 million years ago, called the Paleocene-Eocene Thermal Maximum (PETM). It was an episode that lasted less than 100,000 years, very rapid in geologic terms, and it caused an intense warming of the high latitudes (up to 7° C) and a mass extinction in the benthonic fauna of the bathyal and abyssal oceanic environments (mainly benthic foraminifera). It is hypothesized that PETM was caused by runaway greenhouse effect due to a sudden release of methane from oceanic hydrates. This methane flux and its oxidation product car­bon dioxide could be of a magnitude similar to that from present-day anthropogenic sources, creating the sudden increase of greenhouse warming. The main cause of this short-term change may be related to the reorganization of tectonic plates that produced an increase of volcanic activity (mainly in the North Atlantic) as well as significant paleogeo­graphic and paleoceanographic turnovers. Among these last, the most important was the closing of the Tethys Ocean with the formation of vast areas of shallow epicontinental seas. This may have been responsible for the shift in the locus of ongoing deep-water formation from cold and nutrient- depleted deep waters produced in the polar (Artic and Antarctic) regions to warm, saline, and oxygen-deficient deep waters formed in Tethyan evaporative basins. The stability of these methane hydrates depends on temperature, and, therefore, it is possible that the abrupt deep sea warming induced a shift in sediment geotherms.

The climate continued to be warm and humid worldwide during the early and middle Eocene, with tropical-subtropical deciduous forest cover­ing nearly the entire globe (even in Greenland and Patagonia) and ice-free polar regions covered with coniferous and deciduous trees in a temperate environment. The equatorial areas, including the Tethys Ocean region, were characterized by a tropical, hot, and arid climate. The Eocene global climate was the warmest and most homogeneous of the Cenozoic, but the climatic conditions began to change in the late Eocene. A global cooling, initiated toward the end of the Eocene, occurred during the Oligocene. This cooling caused gradual extinctions along the Eocene-Oligocene (E-O) transition, between 39 and 33 million years ago, that drastically affected land mammals and vegeta­tion. Tropical areas, such as jungles and rainfor­ests, were replaced by more temperate savannahs and grasslands. The E-O cooling episode was the most recent transition from a greenhouse (Cretaceous to Eocene) to an icehouse (Oligocene to present-day) climate mode. The cause of this climatic cooling was the establishment of the cir- cumantarctic current that isolated the Antarctic. Feedback mechanisms, such as the formation of the Antarctic icecap, a substantial drop in sea level, and an increase in the earth’s albedo, drove rapid climate change, which eventually led to the Pleistocene glaciations.

During the first epoch of the Paleogene, the Paleocene, the flora are marked by the develop­ment of modern plant species, including the appearance of cacti and palms. The flowering plants or angiosperms, which first appeared in the beginning of the Cretaceous, continued their devel­opment and proliferation. Along with them evolved the insects that fed on these plants and pollinated them. During the PETM and Eocene, the high temperatures and warm oceans created a tropical, humid environment, with forests spreading throughout the globe from pole to pole. By the time of the climatic cooling of the late Eocene and Oligocene, deciduous forests covered large parts of the northern continents (North America and Eurasia, including the Arctic areas), and tropical rainforests held on only in equatorial South America, Africa, India, and Australia. The tundra stretched out over vast areas of Antarctica, and open plains and deserts became more common.

Because of the dinosaur extinction at the K-T boundary, the reptiles were reduced to palaeophid snakes, soft-shelled turtles, varanid lizards, and crocodilia. With the extinction of marine plesio­saurs and ichthyosaurs, sharks became the chief ocean predators. Birds began to diversify during the Paleocene, but the most modern bird groups appeared during the Eocene and Oligocene, includ­ing hawks, owls, pelicans, loons, and pigeons, among others. The fossil mammal evidence from the Paleocene is scarce, but it is characterized by an evolutionary radiation of small mammals, mainly insectivorous species, including monotremes, mar­supials, multituberculates, and primitive placen- tals, such as the mesonychid. The most important radiation of mammals occurred during the climatic optimum of the Eocene, when there appeared new and modern groups such as artidactyls and peris- sodactyls. Early forms of many other mammalian orders also appeared, including primates, bats, proboscidians, rodents, and cetaceans. Several mammal groups were extinguished during the cooling episode of the E-O transition, including mesonychids and creodonts.

Ignacio Arenillas

See also Cretaceous; Fossil Record; Geologic Timescale; Geology; K-T Boundary; Neogene; Paleontology

Further Readings

De Graciansky, P. C., Hardenbol, J., Jacquin, T., & Vail, P. R. (Eds.). (1998). Mesozoic and Cenozoic sequence stratigraphy of European basins. Tulsa, OK: Society for Sedimentary Geology (SEPM).

Gradstein, F. M., Ogg, J. G., & Smith, A. G. (Eds.). (2004). A geologic time scale 2004. Cambridge, UK: Cambridge University Press.

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