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| A map of the world
as it appeared during the Oligocene epoch (33
ma). |
Paleogene
The Paleogene (/ˈpeɪl.i.əˌdʒiːn, -i.oʊ-, ˈpæ.li-, -li.oʊ-/
PAL-ee-ə-jeen, -ee-oh-, PAY-lee-, -lee-oh-; also
spelled Palaeogene or Palæogene; informally Lower
Tertiary or Early Tertiary) is a geologic period and
system that spans 43 million years from the end of the
Cretaceous Period 66 million years ago (Mya) to the
beginning of the Neogene Period 23.03 Mya. It is the
beginning of the Cenozoic Era of the present Phanerozoic
Eon. The earlier term Tertiary Period was used to define
the span of time now covered by the Paleogene and
subsequent Neogene periods; despite no longer being
recognised as a formal stratigraphic term, 'Tertiary' is
still widely found in earth science literature and
remains in informal use. The Paleogene is most notable
for being the time during which mammals diversified 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 United States Geological Survey uses the
abbreviation PE for the Paleogene, but the more commonly
used abbreviation is PG with the PE being used for
Paleocene.
This period consists of the Paleocene, Eocene, and
Oligocene epochs. The end of the Paleocene (55.5/54.8
Mya) was marked by the Paleocene–Eocene Thermal Maximum,
one of the most significant periods of global change
during the Cenozoic, 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 term 'Paleogene System' is
applied to the rocks deposited during the 'Paleogene
Period'. |
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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. Though
periodically disrupted by warm periods, such as the
Paleocene–Eocene Thermal Maximum, this trend persisted
until the end of the most recent glacial period of the
current ice age, when temperatures began to rise again.
The trend was partly caused by the formation of the
Antarctic Circumpolar Current, which significantly
lowered oceanic water temperatures. A 2018 study
estimated that during the early Palaeogene about 56-48
million years ago, annual air temperatures, over land
and at mid-latitude, averaged about 23–29 °C (± 4.7 °C),
which is 5–10 °C higher than most previous estimates.
For comparison, this was 10 to 15 °C higher than the
current annual mean temperatures in these areas. The
authors suggest that the current atmospheric carbon
dioxide trajectory, if it continues, could establish
these temperatures again.
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 meet
with Europe and form the Mediterranean Sea, 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 toward Southeast Asia. |
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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, mammals
began to evolve from a few small and generalized forms
into most of the modern varieties we see today. 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 took to the oceans became
modern cetaceans, while those that took to the trees
became primates, the group to which humans belong.
Birds, 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. |
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Pronounced cooling in the Oligocene led to 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 decline. Conifer forests developed in
mountainous areas. This cooling trend continued, with
major fluctuation, until the end of the Pleistocene.
This evidence for this floral shift is found in the
palynological record. |
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 Kiddle: Paleogene
Wikipedia: Paleogene |
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