Cretaceous ( /krɪˈteɪʃəs/, kri-TAY-shəs) is a geologic
period and system that spans 79 million years from the end of the
Jurassic Period 145 million years ago (mya) to the beginning of the
Paleogene Period 66 mya. It is the last period of the
Cretaceous Period is usually abbreviated K, for its German
translation Kreide (chalk).
Cretaceous was a period with a relatively warm climate, resulting
in high eustatic sea levels that created numerous shallow inland seas.
These oceans and seas were populated with now-extinct marine reptiles,
ammonites and rudists, while dinosaurs continued to dominate on land.
During this time, new groups of mammals and birds, as well as
flowering plants, appeared. The
Cretaceous ended with a large mass
extinction, the Cretaceous–
Paleogene extinction event, in which many
groups, including non-avian dinosaurs, pterosaurs and large marine
reptiles died out. The end of the
Cretaceous is defined by the abrupt
Paleogene boundary (K–Pg boundary), a geologic
signature associated with the mass extinction which lies between the
1.1 Research history
1.2 Stratigraphic subdivisions
1.3 Rock formations
4.2 Terrestrial fauna
4.3 Marine fauna
Cretaceous extinction event
5 See also
8 External links
Cretaceous as a separate period was first defined by 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, principally
coccoliths), found in the upper
Cretaceous of Western Europe. The
Cretaceous was derived from
Latin creta, meaning chalk.
Cretaceous is divided into Early and
Late Cretaceous epochs, or
Lower and Upper
Cretaceous series. In older literature the Cretaceous
is sometimes divided into three series:
Gallic (middle) and Senonian (upper/late). A subdivision in eleven
stages, all originating from European stratigraphy, is now used
worldwide. In many parts of the world, alternative local subdivisions
are still in use.
As with other older geologic periods, the rock beds of the Cretaceous
are well identified but the exact age of the system's base is
uncertain by a few million years. No great extinction or burst of
diversity separates the
Cretaceous from the Jurassic. However, the top
of the system is sharply defined, being placed at an iridium-rich
layer found worldwide that is believed to be associated with the
Chicxulub impact crater, with its boundaries circumscribing parts of
Yucatán Peninsula and into the Gulf of Mexico. This layer has
been dated at 66.043 Ma.
A 140 Ma age for the Jurassic-
Cretaceous boundary instead of the
usually accepted 145 Ma was proposed in 2014 based on a stratigraphic
Vaca Muerta Formation in Neuquén Basin, Argentina.
Víctor Ramos, one of the authors of the study proposing the 140 Ma
boundary age sees the study as a "first step" toward formally changing
the age in the International Union of Geological Sciences.
From youngest to oldest, the subdivisions of the
Maastrichtian – (66-72.1 MYA)
Campanian – (72.1-83.6 MYA)
Santonian – (83.6-86.3 MYA)
Coniacian – (86.3-89.8 MYA)
Turonian – (89.8-93.9 MYA)
Cenomanian – (93.9-100.5 MYA)
Albian – (100.5-113.0 MYA)
Aptian – (113.0-125.0 MYA)
Barremian – (125.0-129.4 MYA)
Hauterivian – (129.4-132.9 MYA)
Valanginian – (132.9-139.8 MYA)
Berriasian – (139.8-145.0 MYA)
Drawing of fossil jaws of
Mosasaurus hoffmanni, from the Maastrichtian
of Dutch Limburg, by Dutch geologist
Pieter Harting (1866).
The high sea level and warm climate of the
Cretaceous meant large
areas of the continents were covered by warm, shallow seas, providing
habitat for many marine organisms. The
Cretaceous was named for the
extensive chalk deposits of this age in Europe, but in many parts of
the world, the deposits from the
Cretaceous are of marine limestone, a
rock type that is formed under warm, shallow marine circumstances. Due
to the high sea level there was extensive space for such
sedimentation. Because of the relatively young age and great thickness
of the system,
Cretaceous rocks are evident in many areas worldwide.
Chalk is a rock type characteristic for (but not restricted to) the
Cretaceous. It consists of coccoliths, microscopically small calcite
skeletons of coccolithophores, a type of algae that prospered in the
In northwestern Europe, chalk deposits from the Upper
characteristic for the
Chalk Group, which forms the white cliffs of
Dover on the south coast of
England and similar cliffs on the French
Normandian coast. The group is found in England, northern France, the
low countries, northern Germany,
Denmark and in the subsurface of the
southern part of the North Sea.
Chalk is not easily consolidated and
Chalk Group still consists of loose sediments in many places. The
group also has other limestones and arenites. Among the fossils it
contains are sea urchins, belemnites, ammonites and sea reptiles such
In southern Europe, the
Cretaceous is usually a marine system
consisting of competent limestone beds or incompetent marls. Because
the Alpine mountain chains did not yet exist in the Cretaceous, these
deposits formed on the southern edge of the European continental
shelf, at the margin of the Tethys Ocean.
Stagnation of deep sea currents in middle
Cretaceous times caused
anoxic conditions in the sea water leaving the deposited organic
matter undecomposed. Half the worlds petroleum reserves were laid down
at this time in the anoxic conditions of what would become the Persian
Gulf and Gulf of Mexico. In many places around the world, dark anoxic
shales were formed during this interval. These shales are an
important source rock for oil and gas, for example in the subsurface
of the North Sea.
During the Cretaceous, the late-Paleozoic-to-early-Mesozoic
Pangaea completed its tectonic breakup into the
present-day continents, although their positions were substantially
different at the time. As the
Atlantic Ocean widened, the
convergent-margin mountain building (orogenies) that had begun during
Jurassic continued in the North American Cordillera, as the
Nevadan orogeny was followed by the Sevier and Laramide orogenies.
Geography of the
Contiguous United States
Contiguous United States in the late Cretaceous
Gondwana was still intact in the beginning of the Cretaceous,
it broke up as South America,
Australia rifted away
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
Tethys Sea continued to narrow. Broad shallow seas advanced across
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
one-third of Earth's present land area was submerged.
Cretaceous is justly famous for its chalk; indeed, more chalk
formed in the
Cretaceous than in any other period in the
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. 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
the Weald) and
China (the Yixian Formation). In the area that is now
India, massive lava beds called the
Deccan Traps were erupted in the
Cretaceous and early Paleocene.
The cooling trend of last epoch of the
Jurassic continued into the
first age of the Cretaceous. There is evidence that snowfalls were
common in the higher latitudes and the tropics became wetter than
Triassic and Jurassic. Glaciation was however
restricted to high-latitude mountains, though seasonal snow may have
existed farther from the poles. Rafting by ice of stones into marine
environments occurred during much of the
Cretaceous but evidence of
deposition directly from glaciers is limited to the Early Cretaceous
of the Eromanga Basin in southern Australia.
After the end of the first age, however, temperatures increased again,
and these conditions were almost constant until the end of the
period. The warming may have been due to intense volcanic activity
which produced large quantities of carbon dioxide. Between 70–69 Ma
and 66–65 Ma, isotopic ratios indicate elevated atmospheric CO2
pressures with levels of 1000–1400 ppmV and mean annual temperatures
Texas between 21 and 23 °C (70-73 °F). Atmospheric
CO2 and temperature relations indicate a doubling of pCO2 was
accompanied by a ~0.6 °C increase in temperature. The
production of large quantities of magma, variously attributed to
mantle plumes or to extensional tectonics, further pushed sea
levels up, so that large areas of the continental crust were covered
with shallow seas. The
Tethys Sea connecting the tropical oceans east
to west also helped to warm the global climate. Warm-adapted plant
fossils are known from localities as far north as
Greenland, while dinosaur fossils have been found within 15 degrees of
Cretaceous south pole.
Nonetheless, there is evidence of Antarctic marine glaciation in the
A very gentle temperature gradient from the equator to the poles meant
weaker global winds, which drive the ocean currents, resulted in less
upwelling and more stagnant oceans than today. This is evidenced by
widespread black shale deposition and frequent anoxic events.
Sediment cores show that tropical sea surface temperatures may have
briefly been as warm as 42 °C (108 °F), 17 °C
(31 °F) warmer than at present, and that they averaged around
37 °C (99 °F). Meanwhile, deep ocean temperatures were as
much as 15 to 20 °C (27 to 36 °F) warmer than
Further information: Cool tropics paradox
Although the first representatives of leafy trees and true grasses
emerged in the Cretaceous, the flora was still dominated by conifers
Araucaria (Here: Modern
Araucaria araucana in Chile).
Flowering plants (angiosperms) spread during this period, although
they did not become predominant until the
Campanian Age near the end
of the period. 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 continued to thrive; pehuéns (monkey puzzle
trees, Araucaria) and other conifers being notably plentiful and
widespread. Some fern orders such as Gleicheniales appeared as
early in the fossil record as the Cretaceous, and achieved an early
Gymnosperm taxa like
Bennettitales and hirmerellan
conifers died out before the end of the period.
On land, mammals were generally small sized, but a very relevant
component of the fauna, with cimolodont multituberculates outnumbering
dinosaurs in some sites. Neither true marsupials nor placentals
existed until the very end, but a variety of non-marsupial
metatherians and non-placental eutherians had already begun to
diversify greatly, ranging as carnivores (Deltatheroida), aquatic
foragers (Stagodontidae) and herbivores (Schowalteria, Zhelestidae).
Various "archaic" groups like eutriconodonts were common in the Early
Cretaceous, but by the
Late Cretaceous northern mammalian faunas were
dominated by multituberculates and therians, with dryolestoids
dominating South America.
The apex predators were archosaurian reptiles, especially dinosaurs,
which were at their most diverse stage. Pterosaurs were common in the
early and middle Cretaceous, but as the
Cretaceous proceeded they
declined for poorly understood reasons (once thought to be due to
competition with early birds, but now it is understood avian adaptive
radiation is not consistent with pterosaur decline), and by the
end of the period only two highly specialized families remained.
Liaoning lagerstätte (Chaomidianzi formation) in
China is a
treasure chest of preserved remains of numerous types of small
dinosaurs, birds and mammals, that provides a glimpse of life in the
Early Cretaceous. 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
Insects diversified during the Cretaceous, and the oldest known ants,
termites and some lepidopterans, akin to butterflies and moths,
appeared. Aphids, grasshoppers and gall wasps appeared.
Tyrannosaurus rex, one of the largest land predators of all time,
lived during the late Cretaceous.
Up to 2 m long and 0.5 m high at the hip,
Velociraptor was feathered
and roamed the late Cretaceous.
Triceratops, one of the most recognizable genera of the Cretaceous
A pterosaur, Anhanguera piscator
Confuciusornis, a genus of crow-sized birds from the Early Cretaceous
Ichthyornis was a toothed seabird-like ornithuran from the late
In the seas, rays, modern sharks and teleosts became common.
Marine reptiles included ichthyosaurs in the early and mid-Cretaceous
(becoming extinct during the late
anoxic event), plesiosaurs throughout the entire period, and mosasaurs
appearing in the Late Cretaceous.
Baculites, an ammonite genus with a straight shell, flourished in the
seas along with reef-building rudist clams. The Hesperornithiformes
were flightless, marine diving birds that swam like grebes.
Foraminifera and echinoderms such as sea urchins and
starfish (sea stars) thrived. The first radiation of the diatoms
(generally siliceous shelled, 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, hardgrounds and shells.
A scene from the early Cretaceous: a
Woolungasaurus is attacked by a
Tylosaurus was a large mosasaur, carnivorous marine reptiles that
emerged in the late Cretaceous.
Strong-swimming and toothed predatory waterbird
late Cretacean oceans.
Discoscaphites iris, Owl Creek Formation (Upper
Cretaceous), Ripley, Mississippi
A plate with
Nematonotus sp., Pseudostacus sp. and a partial Dercetis
triqueter, found in Hakel, Lebanon
Cretaceous extinction event
The impact of a meteorite or comet is today widely accepted as the
main reason for the Cretaceous–
Paleogene extinction event.
Main article: Cretaceous–
Paleogene extinction event
The impact of a large body with the Earth may have been the
punctuation mark at the end of a progressive decline in biodiversity
Maastrichtian Age of the
Cretaceous Period. The result was
the extinction of three-quarters of Earth's plant and animal species.
The impact created the sharp break known as
K–Pg boundary (formerly
known as the K–T boundary). Earth's biodiversity required
substantial time to recover from this event, despite the probable
existence of an abundance of vacant ecological niches.
Despite the severity of K-Pg extinction event, there was significant
variability in the rate of extinction between and within different
clades. Species which depended on photosynthesis declined or became
extinct as atmospheric particles blocked solar energy. As is the case
today, photosynthesizing organisms, such as phytoplankton and land
plants, formed the primary part of the food chain in the late
Cretaceous, and all else that depended on them suffered as well.
Herbivorous animals, which depended on plants and plankton as their
food, died out as their food sources became scarce; consequently, the
top predators such as
Tyrannosaurus rex also perished. Yet only
three major groups of tetrapods disappeared completely; the non-avian
dinosaurs, the plesiosaurs and the pterosaurs. The other Cretaceous
groups that did not survive into the
Cenozoic era, the ichthyosaurs
and last remaining temnospondyls and non-mammalian cynodonts were
already extinct millions of years before the event occurred.[citation
Coccolithophorids and molluscs, including ammonites, rudists,
freshwater snails and mussels, as well as organisms whose food chain
included these shell builders, became extinct or suffered heavy
losses. For example, it is thought that ammonites were the principal
food of mosasaurs, a group of giant marine reptiles that became
extinct at the boundary.
Omnivores, insectivores and carrion-eaters survived the extinction
event, perhaps because of the increased availability of their food
sources. At the end of the
Cretaceous there seem to have been no
purely herbivorous or carnivorous mammals. Mammals and birds which
survived the extinction fed on insects, larvae, worms and snails,
which in turn fed on dead plant and animal matter. Scientists theorise
that these organisms survived the collapse of plant-based food chains
because they fed on detritus.
In stream communities, few groups of animals became extinct. Stream
communities rely less on food from living plants and more on detritus
that washes in from land. This particular ecological niche buffered
them from extinction. Similar, but more complex patterns have been
found in the oceans.
Extinction was more severe among animals living
in the water column, than among animals living on or in the sea floor.
Animals in the water column are almost entirely dependent on primary
production from living phytoplankton, while animals living on or in
the ocean floor feed on detritus or can switch to detritus
The largest air-breathing survivors of the event, crocodilians and
champsosaurs, were semi-aquatic and had access to detritus. Modern
crocodilians can live as scavengers and can survive for months without
food and go into hibernation when conditions are unfavourable, and
their young are small, grow slowly, and feed largely on invertebrates
and dead organisms or fragments of organisms for their first few
years. These characteristics have been linked to crocodilian survival
at the end of the Cretaceous.
Numerous borings in a
Cretaceous cobble, Faringdon, England; these are
excellent examples of fossil bioerosion.
Cretaceous hardground from
Texas with encrusting oysters and borings.
The scale bar is 10 mm.
Rudist bivalves from the
Cretaceous of the Omani Mountains, United
Arab Emirates. Scale bar is 10 mm.
Inoceramus from the
Cretaceous of South Dakota.
Cretaceous Thermal Maximum
List of fossil sites
List of fossil sites (with link directory)
South Polar dinosaurs
Western Interior Seaway
Phanerozoic Carbon Dioxide.png
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Geologic history of Earth
Quaternary (present–2.588 Mya)
Holocene (present–11.784 kya)
Pleistocene (11.784 kya–2.588 Mya)
Neogene (2.588–23.03 Mya)
Pliocene (2.588–5.333 Mya)
Miocene (5.333–23.03 Mya)
Paleogene (23.03–66.0 Mya)
Oligocene (23.03–33.9 Mya)
Eocene (33.9–56.0 Mya)
Paleocene (56.0–66.0 Mya)
Cretaceous (66.0–145.0 Mya)
Late (66.0–100.5 Mya)
Early (100.5–145.0 Mya)
Jurassic (145.0–201.3 Mya)
Late (145.0–163.5 Mya)
Middle (163.5–174.1 Mya)
Early (174.1–201.3 Mya)
Triassic (201.3–251.902 Mya)
Late (201.3–237 Mya)
Middle (237–247.2 Mya)
Early (247.2–251.902 Mya)
Permian (251.902–298.9 Mya)
Lopingian (251.902–259.8 Mya)
Guadalupian (259.8–272.3 Mya)
Cisuralian (272.3–298.9 Mya)
Carboniferous (298.9–358.9 Mya)
Pennsylvanian (298.9–323.2 Mya)
Mississippian (323.2–358.9 Mya)
Devonian (358.9–419.2 Mya)
Late (358.9–382.7 Mya)
Middle (382.7–393.3 Mya)
Early (393.3–419.2 Mya)
Silurian (419.2–443.8 Mya)
Pridoli (419.2–423.0 Mya)
Ludlow (423.0–427.4 Mya)
Wenlock (427.4–433.4 Mya)
Llandovery (433.4–443.8 Mya)
Ordovician (443.8–485.4 Mya)
Late (443.8–458.4 Mya)
Middle (458.4–470.0 Mya)
Early (470.0–485.4 Mya)
Cambrian (485.4–541.0 Mya)
Furongian (485.4–497 Mya)
Series 3 (497–509 Mya)
Series 2 (509–521 Mya)
Terreneuvian (521–541.0 Mya)
(541.0 Mya–2.5 Gya)
Neoproterozoic era (541.0 Mya–1 Gya)
Ediacaran (541.0-~635 Mya)
Cryogenian (~635-~720 Mya)
Tonian (~720 Mya-1 Gya)
Mesoproterozoic era (1–1.6 Gya)
Stenian (1-1.2 Gya)
Ectasian (1.2-1.4 Gya)
Calymmian (1.4-1.6 Gya)
Paleoproterozoic era (1.6–2.5 Gya)
Statherian (1.6-1.8 Gya)
Orosirian (1.8-2.05 Gya)
Rhyacian (2.05-2.3 Gya)
Siderian (2.3-2.5 Gya)
Archean eon² (2.5–4 Gya)
Neoarchean (2.5–2.8 Gya)
Mesoarchean (2.8–3.2 Gya)
Paleoarchean (3.2–3.6 Gya)
Eoarchean (3.6–4 Gya)
Hadean eon² (4–4.6 Gya)
kya = thousands years ago. Mya = millions years ago.
Gya = billions
years ago.¹ =
Phanerozoic eon. ² =
Source: (2017/02). International Commission on Stratigraphy. Retrieved
13 July 2015. Divisions of Geologic Time—Major Chronostratigraphic
and Geochronologic Units USGS Retrieved 10 March 2013.