temp=18The Paleogene (alternatively Palaeogene) is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the
Cenozoicera. [Formerly the period covered by the Paleogene was called the first part of the Tertiary, which usage is no longer official. [http://www.stratigraphy.org/geowhen/TQ.html "Whatever happened to the Tertiary and Quaternary?"] ] Lasting 42 million years, the Paleogene is most notable as being the time in which it is theorized that mammals evolved from relatively small, simple forms into a plethora of diverse animals in the wake of the mass extinctionthat ended the preceding CretaceousPeriod. 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 even airborne environments. Birds also evolved considerably during this period changing into roughly-modern forms. Most other branches of lifeon earthremained relatively unchanged in comparison to birds and mammals during this period. Some continental motion took place. Climates cooled somewhat over the duration of the Paleogene and inland seas retreated from North America early in the Period.
This period consists of the
Paleocene, Eocene, and OligoceneEpochs. The end of the Paleocene (55.5/54.8 Ma) was marked by one of the most significant periods of global change during the Cenozoic, a sudden global change, 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 MioceneEpoch of the NeogenePeriod. 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 TertiaryPeriod into two periods instead of five epochs, the periods are more closely comparable to the duration of 'periods' in the Mesozoic and Paleozoic Eras.
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