Cretaceous

Source: Wikipedia, the free encyclopedia.
Cretaceous
~145.0 – 66.0 Ma
Period
Stratigraphic unitSystem
Time span formalityFormal
Lower boundary definitionNot formally defined
Lower boundary definition candidates
Lower boundary GSSP candidate section(s)None
Upper boundary definition
K-Pg extinction event
Upper boundary GSSPEl Kef Section, El Kef, Tunisia
36°09′13″N 8°38′55″E / 36.1537°N 8.6486°E / 36.1537; 8.6486
Upper GSSP ratified1991

The Cretaceous (

Era, as well as the longest. At around 79 million years, it is the ninth and longest geological period of the entire Phanerozoic. The name is derived from the Latin creta, 'chalk
', which is abundant in the latter half of the period. It is usually abbreviated K, for its German translation Kreide.

The Cretaceous was a period with a relatively warm

ammonites, and rudists, while dinosaurs
continued to dominate on land. The world was largely ice-free, although there is some evidence of brief periods of glaciation during the cooler first half, and forests extended to the poles.

Many of the dominant taxonomic groups present in modern times can be ultimately traced back to origins in the Cretaceous. During this time, new groups of

birds appeared, including the earliest relatives of placentals & marsupials (Eutheria and Metatheria respectively), with the earliest crown group birds appearing towards to the end of the Cretaceous. Teleost fish, the most diverse group of modern vertebrates continued to diversify during the Cretaceous with the appearence of their most diverse subgroup Acanthomorpha during this period. During the Early Cretaceous, flowering plants appeared and began to rapidly diversify, becoming the dominant group of plants across the Earth by the end of the Cretaceous, coincident with the decline and extinction of previously widespread gymnosperm
groups.

The Cretaceous (along with the Mesozoic) ended with the

Eras
.

Etymology and history

The Cretaceous as a separate period was first defined by Belgian geologist

coccoliths), found in the upper Cretaceous of Western Europe. The name Cretaceous was derived from the Latin creta, meaning chalk.[5] The twofold division of the Cretaceous was implemented by Conybeare and Phillips in 1822. Alcide d'Orbigny in 1840 divided the French Cretaceous into five étages (stages): the Neocomian, Aptian, Albian, Turonian, and Senonian, later adding the Urgonian between Neocomian and Aptian and the Cenomanian between the Albian and Turonian.[6]

Geology

Subdivisions

The Cretaceous is divided into

Senonian (upper/late). A subdivision into 12 stages
, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use.

From youngest to oldest, the subdivisions of the Cretaceous period are:

Subdivisions of the Cretaceous
Epoch Stage Start
(base)		 End
(top)		 Definition Etymology
(
Mya
)
Late Cretaceous Maastrichtian 72.1 ± 0.2 66.0 top: iridium anomaly at the Cretaceous–Paleogene boundary
base:first occurrence of Pachydiscus neubergicus 		Maastricht Formation, Maastricht, Netherlands
Campanian 83.6 ± 0.2 72.1 ± 0.2 base: last occurrence of Marsupites testudinarius Champagne, France
Santonian 86.3 ± 0.5 83.6 ± 0.2 base: first occurrence of Cladoceramus undulatoplicatus Saintes, France
Coniacian 89.8 ± 0.3 86.3 ± 0.5 base: first occurrence of Cremnoceramus rotundatus Cognac, France
Turonian 93.9 ± 0.8 89.8 ± 0.3 base: first occurrence of Watinoceras devonense Tours, France
Cenomanian 100.5 ± 0.9 93.9 ± 0.8 base: first occurrence of Rotalipora globotruncanoides Cenomanum; Le Mans, France
Early Cretaceous Albian 113.0 ± 1.0 100.5 ± 0.9 base: first occurrence of Praediscosphaera columnata Aube, France
Aptian 121.4 ± 1.0 113.0 ± 1.0 base: magnetic anomaly M0r Apt, France
Barremian 125.77 ± 1.5 121.4 ± 1.0 base: first occurrence of Spitidiscus hugii and S. vandeckii Barrême, France
Hauterivian 132.6 ± 2.0 125.77 ± 1.5 base: first occurrence of Acanthodiscus Hauterive, Switzerland
Valanginian 139.8 ± 3.0 132.6 ± 2.0 base: first occurrence of Calpionellites darderi Valangin, Switzerland
Berriasian 145.0 ± 4.0 139.8 ± 3.0 base: first occurrence of Berriasella jacobi (traditionally);
first occurrence of Calpionella alpina (since 2016)		 Berrias, France

Boundaries

The impact of a meteorite or comet is today widely accepted as the main reason for the Cretaceous–Paleogene extinction event.
See also: Cretaceous–Paleogene extinction event

The lower boundary of the Cretaceous is currently undefined, and the Jurassic–Cretaceous boundary is currently the only system boundary to lack a defined

Strambergella jacobi, formerly placed in the genus Berriasella, but its use as a stratigraphic indicator has been questioned, as its first appearance does not correlate with that of C. alpina.[9] The boundary is officially considered by the International Commission on Stratigraphy to be approximately 145 million years ago,[10] but other estimates have been proposed based on U-Pb geochronology, ranging as young as 140 million years ago.[11][12]

The upper boundary of the Cretaceous is sharply defined, being placed at an

Chicxulub impact crater, with its boundaries circumscribing parts of the Yucatán Peninsula and extending into the Gulf of Mexico. This layer has been dated at 66.043 Mya.[13]

At the end of the Cretaceous, the impact of a large

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.[14]

Despite the severity of the K-Pg extinction event, there were significant variations in the rate of extinction between and within different

cynodonts (Tritylodontidae)  —  were already extinct millions of years before the event occurred.[citation needed
]

rudists, freshwater snails, and mussels, as well as organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example, ammonites are thought to have been the principal food of mosasaurs, a group of giant marine lizards related to snakes that became extinct at the boundary.[16]

detritus.[17][14][18]

In

ocean floor feed on detritus or can switch to detritus feeding.[14]

The largest air-breathing survivors of the event,

crocodilians and champsosaurs, were semiaquatic 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 unfavorable, 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.[17]

Geologic formations

Drawing of fossil jaws of Mosasaurus hoffmanni, from the Maastrichtian of Dutch Limburg, by Dutch geologist Pieter Harting (1866)
theropod
dinosaur from the Early Cretaceous of Italy

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 conditions. 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 Cretaceous seas.

Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water leaving the deposited organic matter undecomposed. Half of the world's petroleum reserves were laid down at this time in the anoxic conditions of what would become the Persian Gulf and the Gulf of Mexico. In many places around the world, dark anoxic shales were formed during this interval,[20] such as the Mancos Shale of western North America.[21] These shales are an important source rock for oil and gas, for example in the subsurface of the North Sea.

Europe

See also: Category:Cretaceous System of Europe

In northwestern Europe, chalk deposits from the Upper Cretaceous are characteristic for the

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.

North America

See also: Category:Cretaceous System of North America
Map of North America During the Late Cretaceous

During the Cretaceous, the present North American continent was isolated from the other continents. In the Jurassic, the North Atlantic already opened, leaving a proto-ocean between Europe and North America. From north to south across the continent, the Western Interior Seaway started forming. This inland sea separated the elevated areas of Laramidia in the west and Appalachia in the east. Three dinosaur clades found in Laramidia (troodontids, therizinosaurids and oviraptorosaurs) are absent from Appalachia from the Coniacian through the Maastrichtian.[22]

Paleogeography

During the Cretaceous, the late-

North American Cordillera, as the Nevadan orogeny was followed by the Sevier and Laramide orogenies
.

transgression, one-third of Earth's present land area was submerged.[25]

The Cretaceous is justly famous for its

lava beds called the Deccan Traps
were erupted in the very late Cretaceous and early Paleocene.

Climate

Further information: Cool tropics paradox

Palynological evidence indicates the Cretaceous climate had three broad phases: a Berriasian–Barremian warm-dry phase, an Aptian–Santonian warm-wet phase, and a Campanian–Maastrichtian cool-dry phase.[28] As in the Cenozoic, the 400,000 year eccentricity cycle was the dominant orbital cycle governing carbon flux between different reservoirs and influencing global climate.[29] The location of the Intertropical Convergence Zone (ITCZ) was roughly the same as in the present.[30]

The cooling trend of the last epoch of the Jurassic, the Tithonian, continued into the Berriasian, the first age of the Cretaceous.[31] The North Atlantic seaway opened and enabled the flow of cool water from the Boreal Ocean into the Tethys.[32] There is evidence that snowfalls were common in the higher latitudes during this age, and the tropics became wetter than during the Triassic and Jurassic. Glaciation was restricted to high-latitude mountains, though seasonal snow may have existed farther from the poles.[31] After the end of the first age, however, temperatures began to increase again, with a number of thermal excursions, such as the middle Valanginian Weissert Thermal Excursion (WTX),[33] which was caused by the Paraná-Etendeka Large Igneous Province's activity.[34] It was followed by the middle Hauterivian Faraoni Thermal Excursion (FTX) and the early Barremian Hauptblatterton Thermal Event (HTE). The HTE marked the ultimate end of the Tithonian-early Barremian Cool Interval (TEBCI).[33] During this interval, precession was the dominant orbital driver of environmental changes in the Vocontian Basin.[35] For much of the TEBCI, northern Gondwana experienced a monsoonal climate.[36] A shallow thermocline existed in the mid-latitude Tethys.[37] The TEBCI was followed by the Barremian-Aptian Warm Interval (BAWI).[33] This hot climatic interval coincides with Manihiki and Ontong Java Plateau volcanism and with the Selli Event.[38] Early Aptian tropical sea surface temperatures (SSTs) were 27–32 °C, based on TEX86 measurements from the equatorial Pacific.[39] During the Aptian, Milankovitch cycles governed the occurrence of anoxic events by modulating the intensity of the hydrological cycle and terrestrial runoff.[40] The early Aptian was also notable for its millennial scale hyperarid events in the mid-latitudes of Asia.[41] The BAWI itself was followed by the Aptian-Albian Cold Snap (AACS) that began about 118 Ma.[33] A short, relatively minor ice age may have occurred during this so-called "cold snap", as evidenced by glacial dropstones in the western parts of the Tethys Ocean[42] and the expansion of calcareous nannofossils that dwelt in cold water into lower latitudes.[43] The AACS is associated with an arid period in the Iberian Peninsula.[44]

Temperatures increased drastically after the end of the AACS,

ectothermic reptiles were able to inhabit them.[64]

Beginning in the Santonian, near the end of the MKH, the global climate began to cool, with this cooling trend continuing across the Campanian.

Late Palaeocene, when it gave way to another supergreenhouse interval.[33]

isotherms

The production of large quantities of magma, variously attributed to

south pole.[76] It was suggested that there was Antarctic marine glaciation in the Turonian Age, based on isotopic evidence.[77] However, this has subsequently been suggested to be the result of inconsistent isotopic proxies,[78] with evidence of polar rainforests during this time interval at 82° S.[79] 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.[80][81]

Flora

Facsimile of a fossil of Archaefructus from the Yixian Formation, China
Jaguariba wiersemana specimen in the collection of the Natural History Museum, Berlin
, Germany
Main article: Cretaceous Terrestrial Revolution

ginkgophytes, gnetophytes and close relatives, as well as the extinct Bennettitales. Other groups of plants included pteridosperms or "seed ferns", a collective term that refers to disparate groups of extinct seed plants with fern-like foliage, including groups such as Corystospermaceae and Caytoniales. The exact origins of angiosperms are uncertain, although molecular evidence suggests that they are not closely related to any living group of gymnosperms.[82]

The earliest widely accepted evidence of flowering plants are monosulcate (single-grooved)

monocots are known from the Aptian.[82] Flowering plants underwent a rapid radiation beginning during the middle Cretaceous, becoming the dominant group of land plants by the end of the period, coincident with the decline of previously dominant groups such as conifers.[85] The oldest known fossils of grasses are from the Albian,[86] with the family having diversified into modern groups by the end of the Cretaceous.[87] The oldest large angiosperm trees are known from the Turonian (c. 90 Mya) of New Jersey, with the trunk having a preserved diameter of 1.8 metres (5.9 ft) and an estimated height of 50 metres (160 ft).[88]

During the Cretaceous, ferns in the order Polypodiales, which make up 80% of living fern species, would also begin to diversify.[89]

Terrestrial fauna

On land,

dryolestoids dominating South America
.

The

birds also diversified. They inhabited every continent, and were even found in cold polar latitudes. 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[92]). By the end of the period only three highly specialized families remained; Pteranodontidae, Nyctosauridae, and Azhdarchidae.[93]

The

avialans. Fossils of these dinosaurs from the Liaoning lagerstätte 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.[94]

Rhynchocephalians

Skeleton of Prosphenodon avelasi a large herbivorous rhynchocephalian known from the mid-Cretaceous of South America

Rhynchocephalians (which today only includes the tuatara) disappeared from North America and Europe after the Early Cretaceous,[95] and were absent from North Africa[96] and northern South America[97] by the early Late Cretaceous. The cause of the decline of Rhynchocephalia remains unclear, but has often been suggested to be due to competition with advanced lizards and mammals.[98] They appear to have remained diverse in high-latitude southern South America during the Late Cretaceous, where lizards remained rare, with their remains outnumbering terrestrial lizards 200:1.[96]

Choristodera

Philydrosaurus, a choristodere from the Early Cretaceous of China

Choristoderes, a group of freshwater aquatic reptiles that first appeared during the preceding Jurassic, underwent a major evolutionary radiation in Asia during the Early Cretaceous, which represents the high point of choristoderan diversity, including long necked forms such as Hyphalosaurus and the first records of the gharial-like Neochoristodera, which appear to have evolved in the regional absence of aquatic neosuchian crocodyliformes. During the Late Cretaceous the neochoristodere Champsosaurus was widely distributed across western North America.[99] Due to the extreme climatic warmth in the Arctic, choristoderans were able to colonise it too during the Late Cretaceous.[64]

Marine fauna

In the seas,

Hesperornithiformes were flightless, marine diving birds that swam like grebes
.

Ostracods were abundant in Cretaceous marine settings; ostracod species characterised by high male sexual investment had the highest rates of extinction and turnover.[105] Thylacocephala, a class of crustaceans, went extinct in the Late Cretaceous. 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.[94] Calcareous nannoplankton were important components of the marine microbiota and important as biostratigraphic markers and recorders of environmental change.[106]

The Cretaceous was also an important interval in the evolution of

hardgrounds
and shells.

See also

References

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Bibliography
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