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Cretaceous

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   The Cretaceous Period is one of the major divisions of the geologic
   timescale, reaching from the end of the Jurassic Period (i.e. from
   about 146 to 136 million years ago ( Ma)) to the beginning of the
   Paleocene epoch of the Tertiary Period (about 65.5 to 64 Ma). As the
   longest geological Period, the Cretaceous constitutes nearly half of
   the Mesozoic. The end of the Cretaceous defines the boundary between
   the Mesozoic and Cenozoic eras.

   The Cretaceous (from Latin creta meaning ' chalk') as a separate period
   was first defined by a Belgian geologist Jean d'Omalius d'Halloy in
   1822, using strata in the Paris basin and named for the extensive beds
   of chalk ( calcium carbonate deposited by the shells of marine
   invertebrates), found in the upper Cretaceous of the continental Europe
   and Great Britain (including the White Cliffs of Dover).
          Mesozoic era
   Triassic Jurassic Cretaceous

Dating

   As with other older geologic periods, the rock beds that define the
   Cretaceous are well identified but the exact dates of the period's
   start and end are uncertain by a few million years. No great extinction
   or burst of diversity separated the Cretaceous from the Jurassic.
   However, the end of the period is most sharply defined, being placed at
   an iridium-rich layer found worldwide that is believed to be associated
   with the Chicxulub impact crater in Yucatan and the Gulf of Mexico.
   This layer has been tightly dated at 65.5 Ma. This bolide collision is
   probably responsible for the major, extensively-studied
   Cretaceous-Tertiary extinction event.

Divisions

   The Cretaceous is usually separated into Early and Late Cretaceous
   Epochs. The faunal stages from youngest to oldest are listed below;
   time is referred to as early or late, and the corresponding rocks are
   referred to as lower or upper:
   Upper/Late Cretaceous
   Maastrichtian         (70.6 ± 0.6 – 65.5 ± 0.3 Ma)
   Campanian             (83.5 ± 0.7 – 70.6 ± 0.6 Ma)
   Santonian             (85.8 ± 0.7 – 83.5 ± 0.7 Ma)
   Coniacian             (89.3 ± 1.0 – 85.8 ± 0.7 Ma)
   Turonian              (93.5 ± 0.8 – 89.3 ± 1.0 Ma)
   Cenomanian            (99.6 ± 0.9 – 93.5 ± 0.8 Ma)

   Lower/Early Cretaceous
   Albian                (112.0 ± 1.0 – 99.6 ± 0.9 Ma)
   Aptian                (125.0 ± 1.0 – 112.0 ± 1.0 Ma)
   Barremian             (130.0 ± 1.5 – 125.0 ± 1.0 Ma)
   Hauterivian           (136.4 ± 2.0 – 130.0 ± 1.5 Ma)
   Valanginian           (140.2 ± 3.0 – 136.4 ± 2.0 Ma)
   Berriasian            (145.5 ± 4.0 – 140.2 ± 3.0 Ma)

Paleogeography

   During the Cretaceous, the late Paleozoic - early Mesozoic
   supercontinent of Pangea completed its breakup into present day
   continents, although their positions were substantially different at
   the time. As the Atlantic Ocean widened, the convergent-margin
   orogenies that had begun during the Jurassic continued in the North
   American Cordillera, as the Nevadan orogeny was followed by the Sevier
   and Laramide orogenies.

   Though Gondwana was still intact in the beginning of the Cretaceous,
   Gondwana itself broke up as South America, Antarctica and Australia
   rifted away from Africa (though India and Madagascar remained attached
   to each other). The Indian Ocean was newly formed. Such active rifting
   lifted great undersea mountain chains along the welts, raising eustatic
   sea levels worldwide. To the north of Africa the Tethys Sea continued
   to narrow. Broad shallow seas advanced across central North America
   (the Western Interior Seaway) and Europe, and then started to recede,
   leaving thick marine deposits sandwiched between coal beds.

   The Cretaceous is justly famous for its chalk; indeed, more chalk
   formed in the Cretaceous than in any other Period in the Phanerozoic.
   Laid down by calcareous nannoplankton, they reflect seas that were
   enriched with calcium relative to today's (and hence, more saturated),
   due to increased mid-ocean ridge activity. These widespread carbonates
   and other sedimentary deposits make the Cretaceous rock record
   especially fine. Famous formations from North America include the rich
   marine fossils of Kansas's Smoky Hill Chalk, and the terrestrial fauna
   of the late Cretaceous Hell Creek Formation. Other important Cretaceous
   exposures occur in Europe and China. In the area that is now India,
   massive lava beds called the Deccan Traps were laid down in the very
   late Cretaceous and early Paleocene.

Climate

   The climate was very warm during the Cretaceous; there was no ice at
   the poles. Sea level was much higher than today, and large areas of the
   continental crust were covered with shallow seas; sediment cores also
   show that temperatures in the deep ocean were 15-20° C higher than
   today. The Tethys Seaway connected the tropical oceans east to west,
   which also helped equalize the global climate. Warm-adapted plant
   fossils are known from localities as far north as Alaska and Greenland,
   while dinosaur fossils have been found within 15 degrees of the
   Cretaceous south pole.

   The planet was probably not much warmer on average than it had been
   during the Triassic or Jurassic periods, but instead had a gentler
   temperature gradient from the equator to the poles; a side effect of
   this may have been weaker global winds, contributing to less upwelling
   and more stagnant oceans than today's.

Flora

   Flowering plants, also known as angiosperms, spread during this period,
   although they did not become predominant until near the end of the
   Period ( Campanian age). Their evolution was aided by the appearance of
   bees; in fact angiosperms and insects are a good example of
   coevolution. The first representatives of many modern trees, including
   figs, planes and magnolias for example, appear in the Cretaceous. At
   the same time, some earlier Mesozoic gymnosperms, like Conifers
   continued to thrive, although other taxa like Bennettitales died out
   before the end of the period.

Fauna

Land animals

   On land, mammals were a small and still relatively minor component of
   the fauna. The fauna was dominated by archosaurian reptiles, especially
   dinosaurs, which were at their most diverse. Pterosaurs were common in
   the early and middle Cretaceous, but as the Cretaceous proceeded faced
   growing competition from the adaptive radiation of birds, and by the
   end of the period only two highly specialised families remained.

   A fascinating glimpse of life in the Early Cretaceous is provided by
   the Liaoning lagerstätte ( Chaomidianzi formation) in China, where the
   beautifully preserved remains of a number of types of small dinosaurs,
   birds, and mammals have been found. The coelurosaur dinosaurs found
   there represent a number of types of the group maniraptora, which is
   transitional between dinosaurs and birds, and are remarkable for the
   presence of hair-like feathers.

   During the Cretaceous the insects began to diversify, and the oldest
   known ants, termites and some lepidopterans appeared. Aphids,
   grasshoppers, and gall wasps appeared. Another important insect to
   evolve was the eusocial bee, which was integral to the ecology and
   evolution of flowering plants.

   Numerous exceptionally preserved insects have been found in the Lower
   Cretaceous Siberian lagerstätte of Baissa.

Marine animals

   In the seas, rays, modern sharks and teleosts became common. Marine
   reptiles included ichthyosaurs in the early and middle of the
   Cretaceous, plesiosaurs throughout the entire period, and mosasaurs in
   the Late Cretaceous.

   Baculites, a straight-shelled form of ammonite, flourished in the seas.
   The Hesperornithiformes were flightless, marine diving birds that swam
   like grebes. Globotruncanid Foraminifera thrived. The Cretaceous also
   saw the first radiation of the diatoms in the oceans (freshwater
   diatoms did not appear until the Miocene).

Extinction

   In the extinction event that defines the end of the Cretaceous, a
   significant number of species (~50%) and known families (~25%)
   disappeared. Plants were nearly unscathed, while marine organisms were
   hit the hardest. These include a large number (~95%) of types of
   planktic foraminifers (excepting the Globigerinida), an even larger
   number of Coccolithophores, all the ammonite and belemnite cephalopods,
   and all reef-forming rudist molluscs and inoceramid clams), as well as
   all marine reptiles except turtles and crocodiles. Dinosaurs are the
   most famous victims of the Cretaceous extinction. Dinosaurs that were
   unique to the very end of the period (such as Tyrannosaurus rex,
   Triceratops, and Ankylosaurus) were wiped out. The last of the
   pterosaurs became extinct and the vast majority of birds did as well,
   including the Enantiornithes and Hesperornithiformes.

   The intensive mid-Cretaceous insect extinction began during the Albian.

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