The
Oligocene is a geologic
epoch of the
Paleogene period
and extends from about 33.9 million to 23 million years before the
present. As with other older geologic periods, the rock beds that
define the period are well identified but the exact dates of the
start and end of the period are slightly uncertain. The name
Oligocene comes from the Greek (
oligos, few) and
(
kainos, new), and refers to the sparsity of additional
modern
mammalian faunas after a burst of
evolution during the
Eocene. The Oligocene follows the Eocene epoch and is
followed by the
Miocene epoch. The Oligocene
is the third and final epoch of the
Paleogene period.
The Oligocene is often considered an important time of transition,
a link between "[the] archaic world of the tropical Eocene and the
more modern-looking
ecosystems of the
Miocene." Major changes during the Oligocene included a global
expansion of grasslands, and a regression of tropical broad leaf
forests to the equatorial belt.
The start of the Oligocene is marked by a major
extinction event, a faunal replacement of
European with Asian fauna except for the endemic rodent and
marsupial families called the
Grande Coupure. The Oligocene-Miocene
boundary is not set at an easily identified worldwide event but
rather at regional boundaries between the warmer late Oligocene
[26,23 Ma] and the relatively cooler Miocene.
Subdivisions
Oligocene
faunal stages from youngest
to oldest are:
| Chattian or Late Oligocene |
(28.4 ± 0.1 – 23.03 mya) |
| Rupelian or Early Oligocene |
(33.9 ± 0.1 – 28.4 ± 0.1 mya) |
Climate
The
Paleogene Period general temperature
decline is interrupted by a Oligocene 7M-year stepwise climate
change. A deeper 8.2
oC 0.4M-year temperature depression
leads the 2
oC 7M-year stepwise climate change 33.5Ma.
The stepwise climate spanned 7M-years 25.5Ma thru 32.5Ma as
depicted in the PaleoTemps chart. The Oligocene climate change was
a global Lorraine E. Lisiecki Nov 2004;
A Pliocene-Pleistocene
stack of 57 globally distributed benthic δ18O
records Brown University, PALEOCEANOGRAPHY, VOL. 20 increase
in ice volume and a 55m decrease in sea level (35.7-33.5Ma) with a
closely related (25.5-32.5Ma) temperature depression. The 7M-year
depression abruptly terminated within 1-2M-year of the La Garita
Caldera volcanism event 28-26 Ma. A deep 400 k-year glaciating
Oligocene Miocene boundary event is recorded at the McMurdo Sound
and King George Island.
Paleogeography
During this period, the continents continued to
drift toward their present positions.
Antarctica
continued to become more isolated and finally
developed a permanent ice
cap.(Haines)
Mountain building in western
North America continued, and the
Alps started to rise in
Europe as
the
African plate continued to push
north into the
Eurasian plate,
isolating the remnants of the
Tethys Sea.
A brief marine incursion marks the early Oligocene in Europe.
Oligocene marine exposures are rare in North America. There appears
to have been a land bridge in the early Oligocene between
North America and
Europe
since the
faunas of the two regions
are very similar.
During sometime in the Oligocene, South America was finally detached from
Antarctica and drifted north towards North America. It also allowed the
Antarctic Circumpolar
Current to flow, rapidly cooling the continent.
Flora
Angiosperms continued their expansion
throughout the world; tropical and sub-tropical forests were
replaced by temperate deciduous woodlands. Open plains and deserts
became more common.
Grasses expanded from the
water-bank habitat in the Eocene and moved out into open tracts;
however even at the end of the period it was not quite common
enough for modern
savanna.(Haines)
In North America, subtropical species dominated with
cashews and
lychee trees
present, and temperate trees such as
roses,
beech and
pine were
common. The
legumes of the pea and bean
family spread, and
sedge,
bulrushes and
ferns continued
their ascent.
Fauna
Important Oligocene land faunas are found on all continents except
Australia. Even more open landscapes
allowed animals to grow to larger sizes than they had earlier in
the
Paleogene. Marine faunas became fairly
modern, as did terrestrial
vertebrate
faunas in the northern continents. This was probably more as a
result of older forms dying out than as a result of more modern
forms evolving. Many groups during this time, such as
horses,
entelodonts,
rhinoceroses,
oreodonts, and
camels, became
more cursorial during this time, adapting to the plains that were
spreading as the Eocene rainforests receded.
South America was isolated from the
other continents and evolved a quite distinct fauna during the
Oligocene, home to strange animals such as
pyrothere and
astrapothere, as well as
litopterns and
notoungulates.
Sebecosuchian crocodiles,
terror
birds, and carnivorous
marsupials
like the
borhyaenids remained the
dominant predators.
Brontotheres died
out in the Earliest Oligocene, and
creodonts died out outside Africa and the Middle
East at the end of the period.
Multituberculates, an ancient lineage of
primitive mammals, also went extinct in the Oligocene. The
Oligocene was home to a wide variety of strange mammals. A good
example of this would be in the
White River Badlands of the United
States, which were formerly a semi-arid prairie home to many
different types of endemic mammals, including entelodonts like
Archaeotherium,
camels (such as
Poebrotherium), running
rhino,
three-toed
horses (such as
Mesohippus),
nimravids,
protoceratid, and early dogs like
Hesperocyon.
Oreodonts, an endemic American group, were very
diverse during this time. In Asia during the Oligocene, a group of
running rhinos gave rise to the
indricotheres, like
Indricotherium, which were the largest
land mammals ever to walk the Earth.
The marine animals of Oligocene oceans resembled today's fauna,
such as the
bivalves. The fossil record of
marine mammals is a little spotty during this time, and not as well
known as the Eocene or Miocene, but some fossils have been found.
The baleen and toothed
cetaceans (whales)
just appeared, and their ancestors, the
archaeocete cetaceans began to decrease in
diversity due to their lack of echolocation, which was very useful
as the water became colder and cloudier. Other factors to their
decline could include climate changes and competition with today's
modern cetaceans and the
carcharhinid
sharks, which also appeared in this epoch.
Early
desmostylians, like
Behemotops, are known from the Oligocene.
Pinnipeds probably appeared near the end
of the epoch from a
bear-like or
otter-like ancestor.
Oceans
Oceans continued to cool, particularly around Antarctica.
Impact Events
Recorded extraterrestrial impacts:
- Nunavut, Canada (23 Ma, crater 24 km diameter,)
Supervolcanic explosions
La Garita
Caldera
(28 thru 26 million years ago, VEI=9.2)
See also
References
- Ogg, Jim; June, 2004, Overview of Global Boundary
Stratotype Sections and Points (GSSP's)
http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.
External links
Top topics
Wikis
Quiz
Map
Search
