The 'Cretaceous'
Period is one of the major divisions of the
geologic timescale, reaching from the end of the
Jurassic Period (i.e. from 145.5 ± 4.0
million years ago (
Ma)) to the beginning of the
Paleocene epoch of the
Tertiary Period (about 65.5 ± 0.3 Ma). The youngest and longest geological period of the
Mesozoic, the Cretaceous constitutes about 80 million years. The end of the Cretaceous defines the boundary between the Mesozoic and
Cenozoic eras.
The Cretaceous (from
Latin ''creta'' meaning '
chalk'
[1]) as a separate period was first defined by a Belgian geologist
Jean d'Omalius d'Halloy in 1822, using
strata in the
Paris Basin[2] and named for the extensive beds of chalk (
calcium carbonate deposited by the shells of marine
invertebrates, principally
coccoliths), found in the upper Cretaceous of continental
Europe and the
British Isles (including the
White Cliffs of Dover).
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:
Paleogeography
During the Cretaceous, the late
Paleozoic - early Mesozoic
supercontinent of
Pangaea 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.
Geography of the US in the Late Cretaceous Period
Though
Gondwana was still intact in the beginning of the Cretaceous, it broke up as
South America,
Antarctica and
Australia rifted away from
Africa (though
India and
Madagascar remained attached to each other); thus, the South Atlantic and
Indian Oceans were 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, then receded late in the period, leaving thick marine deposits sandwiched between
coal beds. At the peak of the Cretaceous
transgression, one-third of Earth's present land area was submerged.
[3]
The Cretaceous is justly famous for its
chalk; indeed, more chalk formed in the Cretaceous than in any other period in the
Phanerozoic.
[4] Mid-ocean ridge activity--or rather, the circulation of seawater through the enlarged ridges--enriched the oceans in calcium; this made the oceans more saturated, as well as increased the bioavailability of the element for
calcareous nanoplankton.
[5] 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 Member 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 erupted in the very late Cretaceous and early Paleocene.
Climate
The Berrasian epoch showed a cooling trend that had been seen in the last epoch of the Jurassic. There is evidence that snowfalls were common in the higher latitudes and the tropics became wetter than during the Triassic and Jurassic
[6]. Glaciation was however restricted to alpine glaciers on some high-latitude mountains, though seasonal snow may have existed further south.
After the end of the Berrasian, however, temperatures increased again, and these conditions were almost constant until the end of the period
[7]. This trend was due to intense volcanic activity which produced large quantities of carbon dioxide. The development of a number of
mantle plumes across the widening mid-ocean ridges further pushed sea levels up, so that large areas of the continental crust were covered with shallow seas. The
Tethys Sea connected the tropical oceans east to west also helped warm 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.
[8]
A very gentle
temperature gradient from the
equator to the poles meant weaker global winds, contributing to less
upwelling and more stagnant
oceans than today. This evidenced by widespread black
shale deposition and frequent
anoxic events.
[9] Sediment cores show that tropical sea surface temperatures may have briefly been as warm as 42°C (107°F), 17°C warmer than at present, and that they averaged around 37°C. Meanwhile deep ocean temperatures were as much as 15-20°C higher than today's.
[10],
[11]
Life
Plants
Flowering plants (
angiosperms) spread during this period, although they did not become predominant until the
Campanian stage near the end of the epoch. 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 leafy trees, including
figs,
planes and
magnolias, appeared in the Cretaceous. At the same time, some earlier Mesozoic
gymnosperms like
Conifers continued to thrive; pehuéns (Monkey Puzzle trees, ''
Araucaria'' and other
conifers being notably plentiful and widespread, although other gymnosperm taxa like
Bennettitales died out before the end of the period.
Terrestrial fauna
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 they faced growing competition from the
adaptive radiation of
birds, and by the end of the period only two highly specialised
families remained.
The
Liaoning lagerstätte (
Chaomidianzi formation) in China provides a glimpse of life in the Early Cretaceous, where preserved remains of numerous types of small dinosaurs, birds, and mammals have been found. The
coelurosaur dinosaurs found there represent types of the group
maniraptora, which is transitional between dinosaurs and birds, and are notable for the presence of hair-like
feathers.
During the Cretaceous,
insects began to diversify, and the oldest known
ants,
termites and some
lepidopterans appeared.
Aphids,
grasshoppers, and
gall wasps appeared.
Numerous exceptionally preserved
insects have been found in the Lower Cretaceous
Siberian lagerstätte of
Baissa.
Marine fauna
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 genus of straight-shelled form of
ammonite, flourished in the seas. The
Hesperornithiformes were flightless, marine diving birds that swam like
grebes. Globotruncanid
Foraminifera and
echinoderms such as sea urchins and
starfish (sea stars) thrived. The first radiation of the
diatoms (generally
siliceous, rather than
calcareous) in the oceans occurred during the Cretaceous; freshwater diatoms did not appear until the
Miocene. The Cretaceous was also an important interval in the evolution of
bioerosion, the production of borings and scrapings in rocks and shells (Taylor and Wilson, 2003).
Extinction
Main articles: Cretaceous-Tertiary extinction event
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.
See also
★
Chalk Formation
★
List of fossil sites ''(with link directory)''
★
Western Interior Seaway
References
★ Neal L Larson, Steven D Jorgensen, Robert A Farrar and Peter L Larson. ''Ammonites and the other Cephalopods of the Pierre Seaway''. Geoscience Press, 1997.
★ Ogg, Jim; June, 2004, ''Overview of Global Boundary Stratotype Sections and Points (GSSP's)'' http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.
★ Ovechkina, M.N. and Alekseev, A.S. 2005. Quantitative changes of calcareous nannoflora in the Saratov region (Russian Platform) during the late Maastrichtian warming event. ''Journal of Iberian Geology'' '31' (1): 149-165.
PDF
★
History of Insects, Rasnitsyn, A.P. and Quicke, D.L.J., , , Kluwer Academic Publishers, 2002, ISBN 1-4020-0026-X — detailed coverage of various aspects of the evolutionary history of the insects.
★ Skinner, Brian J., and Stephen C. Porter. ''The Dynamic Earth: An Introduction to Physical Geology.'' 3rd ed. New York: John Wiley & Sons, Inc., 1995. ISBN 0-471-60618-9}
★ Stanley, Steven M. ''Earth System History.'' New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6
★ Taylor, P.D. and Wilson, M.A., 2003. Palaeoecology and evolution of marine hard substrate communities. Earth-Science Reviews 62: 1-103.
[1]
Notes
1. Glossary of Geology, , , , American Geological Institute, 1972,
2. Great Soviet Encyclopedia, , , , Sovetskaya Enciklopediya, 1974,
3. Dougal Dixon et al., ''Atlas of Life on Earth'', (New York: Barnes & Noble Books, 2001), p. 215.
4. Stanley, Steven M. ''Earth System History.'' New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6 p. 280
5. Stanley, pp. 279-81
6. The Berrasian Age
7. Ibid.
8. Stanley, pp. 480-2
9. Stanley, pp. 481-2
10. than a Hot Tub: Atlantic Ocean Temperatures Much Higher in the Past" PhysOrg.com. Retrieved 12/3/06.
11. Skinner, Brian J., and Stephen C. Porter. ''The Dynamic Earth: An Introduction to Physical Geology.'' 3rd ed. New York: John Wiley & Sons, Inc., 1995. ISBN 0-471-59549-7. p. 557
External links
★
UCMP Berkeley Cretaceous page
★
Bioerosion website at The College of Wooster
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