Paleogene

The Paleogene Period (also spelled Palaeogene or Palæogene) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period Ma (million years ago) to the beginning of the Neogene Period  Ma. It is the first period of the Cenozoic Era and is divided into the Paleocene, Eocene, and Oligocene epochs. The earlier term Tertiary Period was used to define the time now covered by the Paleogene Period and subsequent Neogene Period; despite no longer being recognized as a formal stratigraphic term, "Tertiary" still sometimes remains in informal use. Paleogene is often abbreviated "Pg", although the United States Geological Survey uses the abbreviation "" for the Paleogene on the Survey's geologic maps.

During the Paleogene period, mammals continued to diversify from relatively small, simple forms into a large group of diverse animals in the wake of the Cretaceous–Paleogene extinction event that ended the preceding Cretaceous Period.

The Period is marked by considerable changes in climate from the Paleocene–Eocene Thermal Maximum, through global cooling during the Eocene to the first appearance of permanent ice sheets in the Antarctic at the beginning of the Oligocene.

Stratigraphy
The Paleogene is divided into three series/epochs: the Paleocene, Eocene, and Oligocene. These stratigraphic units can be defined globally or regionally. For global stratigraphic correlation, the International Commission on Stratigraphy (ICS) ratify global stages based on a Global Boundary Stratotype Section and Point (GSSP) from a single formation (a stratotype) identifying the lower boundary of the stage.

Paleocene
The Paleocene is the first series/epoch of the Paleogene and lasted from 66.0 Ma to 56.0 Ma. It is divided into three stages: the Danian 66.0 - 61.6 Ma; Selandian 61.6 - 59.2 Ma; and, Thanetian 59.2 - 56.0 Ma. The GSSP for the base of the Cenozoic, Paleogene and Paleocene is at Oued Djerfane, west of El Kef, Tunisia. It is marked by an iridium anomaly produced by an asteroid impact, and is associated with the Cretaceous–Paleogene extinction event. The boundary is defined as the rusty colored base of a 50 cm thick clay, which would have been deposited over only a few days. Similar layers are seen in marine and continental deposits worldwide. These layers include the iridium anomaly, microtektites, nickel-rich spinel crystals and shocked quartz, all indicators of a major extraterrestrial impact. The remains of the crater are found at Chicxulub on the Yucatan Peninsula in Mexico. The extinction of the non-avian dinosaurs, ammonites and dramatic changes in marine plankton and many other groups of organisms, are also used for correlation purposes.

Eocene
The Eocene is the second series/epoch of the Paleogene, and lasted from 56.0 Ma to 33.9 Ma. It is divided into four stages: the Ypresian 56.0 Ma to 47.8 Ma; Lutetian 47.8 Ma to 41.2 Ma; Bartonian 41.2 Ma to 37.71 Ma; and, Priabonian 37.71 Ma to 33.9 Ma. The GSSP for the base of the Eocene is at Dababiya, near Luxor, Egypt and is marked by the start of a significant variation in global carbon isotope ratios, produced by a major period of global warming. The change in climate was due to a rapid release of frozen methane clathrates from seafloor sediments at the beginning of the Paleocene-Eocene thermal maximum (PETM).

Oligocene
The Oligocene is the third and youngest series/epoch of the Paleogene, and lasted from 33.9 Ma to 23.03 Ma. It is divided into two stages: the Rupelian 33.9 Ma to 27.82 Ma; and, Chattian 27.82 - 23.03 Ma. The GSSP for the base of the Oligocene is at Massignano, near Ancona, Italy. The extinction the hantkeninid planktonic foraminifera is the key marker for the Eocene-Oligocene boundary, which was a time of climate cooling that led to widespread changes in fauna and flora.

Climate
The global climate of the Paleogene began with the brief but intense "impact winter" caused by the Chicxulub impact. This cold period was terminated by an abrupt warming. After temperatures stabilised, the steady cooling and drying of the Late Cretaceous-Early Paleogene Cool Interval (LKEPCI) that had spanned the last two ages of the Late Cretaceous continued. About 62.2 Mya, the Latest Danian Event, a hyperthermal event, took place. About 59 Ma, the LKEPCI was brought to an end by the Thanetian Thermal Event, a change from the relative cool of the Early and Middle Palaeocene and the beginning of an intense supergreenhouse effect.

According to a study published in 2018, from about 56 to 48 Mya, annual air temperatures over land and at mid-latitude averaged about 23–29 °C (± 4.7 °C), which is 5–10 °C warmer than most previous estimates. For comparison, this was 10 to 15 °C greater than the current annual mean temperatures in these areas. At the Palaeocene-Eocene boundary, the Paleocene–Eocene Thermal Maximum (PETM) occurred, one of the warmest times of the Phanerozoic eon, during which global mean surface temperatures increased to 31.6. It was followed by the less severe Eocene Thermal Maximum 2 (ETM2) about 53.69 Ma. Eocene Thermal Maximum 3 (ETM3) occurred about 53 Ma. The Early Eocene Climatic Optimum was brought to an end by the Azolla event, a change of climate about 48.5 Mya, believed to have been caused by a proliferation of aquatic ferns from the genus Azolla, resulting in the sequestering of large amounts of carbon dioxide by those plants. From this time until about 34 Mya, there was a slow cooling trend known as the Middle-Late Eocene Cooling (MLEC). Approximately 41.5 Mya, this cooling was interrupted temporarily by the Middle Eocene Climatic Optimum (MECO). Then, about 39.4 Mya, a temperature decrease termed the Late Eocene Cool Event (LECE) is detected in the oxygen isotope record. A rapid decrease of global temperatures and formation of continental glaciers on Antarctica marked the end of the Eocene. This sudden cooling was caused partly by the formation of the Antarctic Circumpolar Current, which significantly lowered oceanic water temperatures.

During the earliest Oligocene occurred the Early Oligocene Glacial Maximum (Oi1), which lasted for about 200,000 years. After Oi1, global mean surface temperature continued to decrease gradually during the Rupelian Age. Another major cooling event occurred at the end of the Rupelian; its most likely cause was extreme biological productivity in the Southern Ocean fostered by tectonic reorganisation of ocean currents and an influx of nutrients from Antarctica. In the Late Oligocene, global temperatures began to warm slightly, though they continued to be significantly lower than during the previous epochs of the Paleogene and polar ice remained.

Palaeogeography
During the Paleogene, the continents continued to drift closer to their current positions. India was in the process of colliding with Asia, forming the Himalayas. The Atlantic Ocean continued to widen by a few centimeters each year. Africa was moving north to collide with Europe and form the Mediterranean Sea, while South America was moving closer to North America (they would later connect at the Isthmus of Panama). Inland seas retreated from North America early in the period. Australia had also separated from Antarctica and was drifting toward Southeast Asia. The 1.2 million year cycle of obliquity amplitude modulation governed eustatic sea level changes on shorter timescales, with periods of low amplitude coinciding with intervals of low sea levels and vice versa.

Flora and fauna
Tropical taxa diversified faster than those at higher latitudes after the Cretaceous–Paleogene extinction event, resulting in the development of a significant latitudinal diversity gradient. Mammals began a rapid diversification during this period. After the Cretaceous–Paleogene extinction event, which saw the demise of the non-avian dinosaurs, mammals began to evolve from a few small and generalized forms into most of the modern varieties we see presently. Some of these mammals evolved into large forms that dominated the land, while others became capable of living in marine, specialized terrestrial, and airborne environments. Those that adapted to the oceans became modern cetaceans, while those that adapted to trees became primates, the group to which humans belong. Birds, extant dinosaurs which were already well established by the end of the Cretaceous, also experienced adaptive radiation as they took over the skies left empty by the now extinct pterosaurs. Some flightless birds such as penguins, ratites, and terror birds also filled niches left by the hesperornithes and other extinct dinosaurs.

Pronounced cooling in the Oligocene resulted in a massive floral shift, and many extant modern plants arose during this time. Grasses and herbs, such as Artemisia, began to proliferate, at the expense of tropical plants, which began to decrease. Conifer forests developed in mountainous areas. This cooling trend continued, with major fluctuation, until the end of the Pleistocene period. This evidence for this floral shift is found in the palynological record. Some important flora were, grasses, beeches, monocots, pines and kauri.