This article needs attention from an expert in Geology or Palaeontology. The specific problem is: Too technical: See talk page.WikiProject GeologyorWikiProject Palaeontology may be able to help recruit an expert.(November 2023)
This period consists of the Paleocene, Eocene, and Oligocene epochs. The end of the Paleocene (56 Mya) was marked by the Paleocene–Eocene Thermal Maximum, one of the most significant periods of global change during the Cenozoic, which changed oceanic and atmospheric circulation and resulted in the extinction of numerous deep-sea benthic foraminifera and on land, a major extinction of mammals. The term "Paleogene System" applies to the rocks deposited during the Paleogene Period.
The global climate of the Palaeogene 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 Palaeogene Cool Interval (LKEPCI) that had spanned the last two stages of the Late Cretaceous continued.[9] About 62.2 Ma, the Latest Danian Event, a hyperthermal event, took place.[10][11][12] 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.[9]
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.[13][14][15] For comparison, this was 10 to 15 °C greater than the current annual mean temperatures in these areas.[15] At the Palaeocene-Eocene boundary occurred the Paleocene–Eocene Thermal Maximum (PETM),[16] one of the warmest times of the Phanerozoic eon, during which global mean surface temperatures increased to 31.6.[17] It was followed by the less severe Eocene Thermal Maximum 2 (ETM2) about 53.69 Ma.[18] 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).[9] Approximately 41.5 Ma, this cooling was interrupted temporarily by the Middle Eocene Climatic Optimum (MECO).[19] Then, about 39.4 Mya, a temperature decrease termed the Late Eocene Cool Event (LECE) is detected in the oxygen isotope record.[9] A rapid decrease of global temperatures and formation of continental glaciers on Antarctica marked the end of the Eocene.[20] This sudden cooling was caused partly by the formation of the Antarctic Circumpolar Current,[21] which significantly lowered oceanic water temperatures.[22]
During the earliest Oligocene occurred the Early Oligocene Glacial Maximum (Oi1), which lasted for about 200 thousand years.[23] After Oi1, global mean surface temperature continued to decrease gradually during the Rupelian Age. [9] 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.[24] In the Late Oligocene, global temperatures began to warm slightly, though they continued to be significantly lower than during the previous epochs of the Palaeogene and polar ice remained.[9]
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 Myear 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.[25]
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.[26]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.[27] This evidence for this floral shift is found in the palynological record.[28]
^Meredith, R. W.; Janecka, J. E.; Gatesy, J.; Ryder, O. A.; Fisher, C. A.; Teeling, E. C.; Goodbla, A.; Eizirik, E.; Simao, T. L. L.; Stadler, T.; Rabosky, D. L.; Honeycutt, R. L.; Flynn, J. J.; Ingram, C. M.; Steiner, C.; Williams, T. L.; Robinson, T. J.; Burk-Herrick, A.; Westerman, M.; Ayoub, N. A.; Springer, M. S.; Murphy, W. J. (28 October 2011). "Impacts of the Cretaceous Terrestrial Revolution and KPg Extinction on Mammal Diversification". Science. 334 (6055): 521–524. Bibcode:2011Sci...334..521M. doi:10.1126/science.1211028. PMID21940861. S2CID38120449.
