The Cretaceous, derived from the Latin "creta", usually abbreviated K for its German translation
Kreide, is a geologic period and system from circa 145 ± 4 to 66 million years ago. In
the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the
Paleogene period of the Cenozoic era. It is the last period of the Mesozoic Era, and,
spanning 79 million years, the longest period of the Phanerozoic Eon.
The Cretaceous was a period with a relatively warm climate, resulting in high eustatic sea
levels and creating 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. At the same 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 K–Pg boundary,
a geologic signature associated with the mass extinction which lies between the Mesozoic
and Cenozoic eras.
Geology
Research history The 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, found in the upper Cretaceous of western
Europe. The name Cretaceous was derived from Latin creta, meaning chalk.
Stratigraphic subdivisions The 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: Neocomian, Gallic and Senonian. 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 ages 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 the Yucatán Peninsula and into the Gulf of Mexico. This
layer has been dated at 66.043 Ma. Rock formations
The high eustatic sea level and warm climate of the Cretaceous meant a large area of the
continents was covered by warm shallow seas. The Cretaceous was named for the extensive
chalk deposits of this age in Europe, but in many parts of the world, the Cretaceous
system consists for a major part of marine limestone, a rock type that is formed under
warm, shallow marine circumstances. Due to the high sea level there was extensive accommodation
space for sedimentation so that thick deposits could form. Because of the relatively young
age and great thickness of the system, Cretaceous rocks crop out in many areas worldwide.
Chalk is a rock type characteristic for the Cretaceous. It consists of coccoliths, microscopically
small calcite skeletons of coccolithophores, a type of algae that prospered in the Cretaceous
seas. In northwestern Europe, chalk deposits from
the Upper Cretaceous are 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 the 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 as Mosasaurus. 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. 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. Paleogeography
During the Cretaceous, the late-Paleozoic-to-early-Mesozoic supercontinent of Pangaea completed its tectonic
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; 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 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.
The Cretaceous is justly famous for its chalk; indeed, more chalk formed in the Cretaceous
than in any other period in the Phanerozoic. 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 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 Berriasian 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. Glaciation was however restricted to alpine glaciers on 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 Berriasian, however,
temperatures increased again, and these conditions were almost constant until the end of the
period. This trend was due to intense volcanic activity which produced large quantities of
carbon dioxide. 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 in warming 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.
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 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, 17 °C warmer than
at present, and that they averaged around 37 °C. Meanwhile deep ocean temperatures
were as much as 15 to 20 °C higher than today's.
Life
Flora Flowering plants 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
continued to thrive; pehuéns 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 broad distribution. 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. Early marsupial
mammals evolved in the Early Cretaceous, with true placentals emerging in the Late Cretaceous
period. The fauna was dominated by 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, and by the
end of the period only two highly specialized families remained.
The Liaoning lagerstätte 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. 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.
Marine fauna In the seas, rays, modern sharks and teleosts
became common. Marine reptiles included ichthyosaurs in the early and mid-Cretaceous, 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. Globotruncanid Foraminifera and echinoderms such as sea urchins and starfish
thrived. The first radiation of the diatoms 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.
End-Cretaceous extinction event
There was a progressive decline in biodiversity during the Maastrichtian stage of the Cretaceous
period prior to the suggested ecological crisis induced by events at the K–Pg boundary.
Furthermore, biodiversity required a substantial amount of time to recover from the K–T event,
despite the probable existence of an abundance of vacant ecological niches.
Despite the severity of this boundary event, there was significant variability in the rate
of extinction between and within different clades. Species which depended on photosynthesis
declined or became extinct because of the reduction in solar energy reaching the Earth's
surface due to atmospheric particles blocking the sunlight. 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. Evidence suggests that herbivorous animals, which depended on
plants and plankton as their food, died out as their food sources became scarce; consequently,
top predators such as Tyrannosaurus rex also perished.
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 feeding.
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.
See also
Chalk Formation Cretaceous Thermal Maximum
List of fossil sites South Polar dinosaurs
Western Interior Seaway
References
Bibliography Yuichiro Kashiyama; Nanako O. Ogawa; Junichiro
Kuroda; Motoo Shiro; Shinya Nomoto; Ryuji Tada; Hiroshi Kitazato; Naohiko Ohkouchi.
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Neal L Larson, Steven D Jorgensen, Robert A Farrar and Peter L Larson. Ammonites and
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External links UCMP Berkeley Cretaceous page
Bioerosion website at The College of Wooster Cretaceous Microfossils: 180+ images of Foraminifera