66–23.03 million years ago
|Mean atmospheric O2 content over period duration||ca. 26 Vol %
(130 % of modern level)
|Mean atmospheric CO2 content over period duration||ca. 500 ppm
(2 times pre-industrial level)
|Mean surface temperature over period duration||ca. 18 °C
(4 °C above modern level)
The Paleogene (alternatively British English Palaeogene or Palæogene, informally Lower Tertiary) is a geologic period and system that began ended and comprises the first part of the Cenozoic Era. Lasting 43 million years, the Paleogene is most notable as being the time in which mammals evolved 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. Birds also evolved considerably during this period, changing into roughly modern forms.
This period consists of the Paleocene, Eocene, and Oligocene Epochs. The end of the Paleocene (55.5/54.8 Mya) was marked by one of the most significant periods of global change during the Cenozoic, the Paleocene-Eocene Thermal Maximum, which upset oceanic and atmospheric circulation and led to the extinction of numerous deep-sea benthic foraminifera and on land, a major turnover in mammals. The Paleogene follows the Cretaceous Period and is followed by the Miocene Epoch of the Neogene Period. The terms 'Paleogene System' (formal) and 'lower Tertiary System' (informal) are applied to the rocks deposited during the 'Paleogene Period'. The somewhat confusing terminology seems to be due to attempts to deal with the comparatively fine subdivisions of time possible in the relatively recent geologic past, when more information is preserved. By dividing the Tertiary Period into two periods instead of five epochs, the periods are more closely comparable to the duration of 'periods' in the Mesozoic and Paleozoic Eras.
Climate and geography 
The global climate during the Paleogene departed from the hot and humid conditions of the late Mesozoic era and began a cooling and drying trend which, although having been periodically disrupted by warm periods such as the Paleocene–Eocene Thermal Maximum, persists today. The trend was partly caused by the formation of the Antarctic Circumpolar Current, which significantly cooled oceanic water temperatures.
The continents during the Paleogene continued to drift closer to their current positions. India was in the process of colliding with Asia, subsequently forming the Himalayas. The Atlantic Ocean continued to widen by a few centimeters each year. Africa was moving north to meet with Europe and form the Mediterranean, while South America was moving closer to North America (they would later connect via the Isthmus of Panama). Inland seas retreated from North America early in the period. Australia had also separated from Antarctica and was drifting towards Southeast Asia.
Flora and fauna 
Mammals began a rapid diversification during this period. After the Cretaceous–Paleogene extinction event, which saw the demise of the non-avian dinosaurs, they transformed from a few small and generalized forms and began to evolve into most of the modern varieties we see today. Some of these mammals would evolve into large forms that would dominate the land, while others would become capable of living in marine, specialized terrestrial, and airborne environments. Some mammals took to the oceans and became modern cetaceans, while others took to the trees and became primates, the group to which humans belong. Birds, which were already well established by the end of the Cretaceous, also experienced an adaptive radiation as they took over the skies left empty by the now extinct Pterosaurs. Most other branches of life remained relatively unchanged in comparison to birds and mammals during this period.
As the Earth began to cool, tropical plants were less numerous and were now restricted to equatorial regions. Deciduous plants became more common, which could survive through the seasonal climate the world was now experiencing. One of the most notable floral developments during this period was the evolution of the first grass species. This new plant type expanded and formed new ecological environments we know today as savannas and prairies. These grasslands also began to replace many forests because they could survive better in the drier climate typical in many regions of the world during this period.
Oil industry relevance 
The Paleogene is notable in the context of offshore oil drilling, and especially in Gulf of Mexico oil exploration, where it is usually referred to as the "Lower Tertiary". These rock formations represent the current cutting edge of deep-water oil discovery.
Lower Tertiary rock formations encountered in the Gulf of Mexico oil industry tend to be comparatively high temperature and high pressure reservoirs, often with high sand content (70%+) or under very thick salt sediment layers.
Lower Tertiary explorations to date include (partial list):
- Image:Phanerozoic Carbon Dioxide.png
- Image:All palaeotemps.png
- Formerly the period covered by the Paleogene was called the first part of the Tertiary, whose usage is no longer official. "Whatever happened to the Tertiary and Quaternary?"
- Retallack, G.J. (1997). "Neogene Expansion of the North American Prairie". PALAIOS 12 (4): 380–390. doi:10.2307/3515337. JSTOR 3515337.
- Zachos, J.C.; Kump, L.R. (2005). "Carbon cycle feedbacks and the initiation of Antarctic glaciation in the earliest Oligocene". Global and Planetary Change 47 (1): 51–66. Bibcode:2005GPC....47...51Z. doi:10.1016/j.gloplacha.2005.01.001.
- Krijgsman, W.; Garcés, M.; Langereis, C.G.; Daams, R.; Van Dam, J.; Van Der Meulen, A.J.; Agustí, J.; Cabrera, L. (1996). "A new chronology for the middle to late Miocene continental record in Spain". Earth and Planetary Science Letters 142 (3-4): 367–380. Bibcode:1996E&PSL.142..367K. doi:10.1016/0012-821X(96)00109-4.
- "Lower Tertiary". Halliburton. Retrieved 2011-07-13.
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|Paleocene Epoch||Eocene Epoch||Oligocene Epoch|
|Danian | Selandian
|Ypresian | Lutetian
|Rupelian | Chattian|
|Preceded by Proterozoic Eon||542 Ma - Phanerozoic Eon- Present|
|542 Ma - Paleozoic Era -251 Ma||251 Ma - Mesozoic Era - 65 Ma||65 Ma - Cenozoic Era - Present|