CRETACEOUS

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).

Contents

Dating

Divisions

Paleogeography

Climate

Life

Plants

Terrestrial fauna

Marine fauna

Extinction

See also

References

Notes

External links

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 – 65.8 ± 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 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.

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