TheCretaceous (IPA:/krɪˈteɪʃəs/krih-TAY-shəss)[2] is ageological period that lasted from about 143.1 to 66million years ago (Mya). It is the third and final period of theMesozoicEra, as well as the longest. At around 77.1 million years, it is the ninth and longest geological period of the entirePhanerozoic. The name is derived from the Latincreta, 'chalk', which is abundant in the latter half of the period. It is usually abbreviatedK, for its German translationKreide.
The Cretaceous was a period with a relatively warmclimate, resulting in higheustatic sea levels that created numerous shallowinland seas. These oceans and seas were populated with now-extinctmarine reptiles,ammonites, andrudists, whiledinosaurs 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 ofmammals andbirds appeared, including the earliest relatives ofplacentals &marsupials (Eutheria andMetatheria respectively), with the earliestcrown 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 appearance of their most diverse subgroupAcanthomorpha during this period. During the Early Cretaceous,flowering plants appeared and began to rapidly diversify, becoming the dominant group ofplants across the Earth by the end of the Cretaceous, coincident with the decline andextinction of previously widespreadgymnosperm groups.
The Cretaceous (along with the Mesozoic) ended with theCretaceous–Paleogene extinction event, a largemass extinction in which many groups, including non-avian dinosaurs,pterosaurs, and largemarine reptiles, died out, widely thought to have been caused by the impact of a large asteroid that formed theChicxulub crater in the Gulf of Mexico. The end of the Cretaceous is defined by the abruptCretaceous–Paleogene boundary (K–Pg boundary), a geologic signature associated with the mass extinction that lies between the Mesozoic andCenozoicEras.
The Cretaceous as a separate period was first defined by Belgian geologistJean d'Omalius d'Halloy in 1822 as theTerrain Crétacé,[3] usingstrata in theParis Basin[4] and named for the extensive beds ofchalk (calcium carbonate deposited by the shells of marineinvertebrates, principallycoccoliths), found in the upper Cretaceous ofWestern Europe. The name Cretaceous was derived from theLatincreta, meaningchalk.[5] The twofold division of the Cretaceous was implemented byConybeare and Phillips in 1822.Alcide d'Orbigny in 1840 divided the French Cretaceous into fiveétages (stages): theNeocomian, Aptian, Albian, Turonian, and Senonian, later adding theUrgonian between Neocomian and Aptian and the Cenomanian between the Albian and Turonian.[6]
The Cretaceous is divided intoEarly andLate Cretaceousepochs, or Lower and Upper Cretaceousseries. In older literature, the Cretaceous is sometimes divided into three series:Neocomian (lower/early),Gallic (middle) andSenonian (upper/late). A subdivision into 12stages, 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:[7]
The lower boundary of the Cretaceous is currently undefined, and the Jurassic–Cretaceous boundary is currently the only system boundary to lack a definedGlobal Boundary Stratotype Section and Point (GSSP). Placing a GSSP for this boundary has been difficult because of the strong regionality of most biostratigraphic markers, and the lack of anychemostratigraphic events, such asisotope excursions (large sudden changes inratios of isotopes) that could be used to define or correlate a boundary.Calpionellids, an enigmatic group ofplanktonicprotists with urn-shaped calcitictests briefly abundant during the latest Jurassic to earliest Cretaceous, have been suggested as the most promising candidates for fixing the Jurassic–Cretaceous boundary.[8] In particular, the first appearanceCalpionella alpina, coinciding with the base of the eponymous Alpina subzone, has been proposed as the definition of the base of the Cretaceous.[9] The working definition for the boundary has often been placed as the first appearance of the ammoniteStrambergella jacobi, formerly placed in the genusBerriasella, but its use as a stratigraphic indicator has been questioned, as its first appearance does not correlate with that ofC. alpina.[10] The boundary is officially considered by theInternational Commission on Stratigraphy to be approximately 145 million years ago,[11] but other estimates have been proposed based on U-Pb geochronology, ranging as young as 140 million years ago.[12][13]
The upper boundary of the Cretaceous is sharply defined, being placed at aniridium-rich layer found worldwide that is believed to be associated with theChicxulub impact crater, with its boundaries circumscribing parts of theYucatán Peninsula and extending into theGulf of Mexico. This layer has been dated at 66.043 Mya.[14]
At the end of the Cretaceous, the impact of a largebody with the Earth may have been the punctuation mark at the end of a progressive decline inbiodiversity during the Maastrichtian age. The result was the extinction of three-quarters of Earth's plant and animal species. The impact created the sharp break known as theK–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 vacantecological niches.[15]
Despite the severity of the K-Pg extinction event, there were significant variations in the rate of extinction between and within differentclades. Species that depended onphotosynthesis declined or became extinct as atmospheric particles blockedsolar energy. As is the case today, photosynthesizing organisms, such asphytoplankton and landplants, formed the primary part of thefood 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 toppredators, such asTyrannosaurus rex, also perished.[16] Yet only three major groups oftetrapods disappeared completely; the nonaviandinosaurs, theplesiosaurs and thepterosaurs. The other Cretaceous groups that did not survive into the CenozoicEra—theichthyosaurs, last remainingtemnospondyls (Koolasuchus), and nonmammaliancynodonts (Tritylodontidae)— were already extinct millions of years before the event occurred.[citation needed]
Omnivores,insectivores, andcarrion-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 orcarnivorousmammals. Mammals and birds that survived the extinction fed oninsects,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 ondetritus.[18][15][19]
Instreamcommunities, 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.[20] Similar, but more complex patterns have been found in the oceans. Extinction was more severe among animals living in thewater column than among animals living on or in the seafloor. Animals in the water column are almost entirely dependent onprimary production from living phytoplankton, while animals living on or in theocean floor feed on detritus or can switch to detritus feeding.[15]
The largest air-breathing survivors of the event,crocodilians andchampsosaurs, 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.[18]
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 ofmarinelimestone, a rock type that is formed under warm, shallow marine conditions. Due to the high sea level, there was extensivespace for suchsedimentation. 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 ofcoccoliths, microscopically smallcalcite skeletons ofcoccolithophores, a type ofalgae 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 anoxicshales were formed during this interval,[21] such as theMancos Shale of western North America.[22] These shales are an importantsource rock foroil and gas, for example in the subsurface of the North Sea.
In southern Europe, the Cretaceous is usually a marine system consisting ofcompetent limestone beds or incompetentmarls. Because theAlpine mountain chains did not yet exist in the Cretaceous, these deposits formed on the southern edge of the Europeancontinental shelf, at the margin of theTethys Ocean.
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, theWestern Interior Seaway started forming. This inland sea separated the elevated areas ofLaramidia in the west andAppalachia in the east. Three dinosaur clades found in Laramidia (troodontids, therizinosaurids and oviraptorosaurs) are absent from Appalachia from the Coniacian through the Maastrichtian.[23]
Gondwana had begun to break up during the Jurassic Period, but its fragmentation accelerated during the Cretaceous and was largely complete by the end of the period.South America,Antarctica, andAustralia rifted away fromAfrica (thoughIndia andMadagascar remained attached to each other until around 80 million years ago); thus, the South Atlantic andIndian Oceans were newly formed. Such active rifting lifted great undersea mountain chains along the welts, raisingeustatic sea levels worldwide. To the north of Africa theTethys Sea continued to narrow. During most of the Late Cretaceous, North America would be divided in two by theWestern Interior Seaway, a large interior sea, separatingLaramidia to the west andAppalachia to the east, then receded late in the period, leaving thick marine deposits sandwiched betweencoal beds. Bivalve palaeobiogeography also indicates that Africa was split in half by a shallow sea during the Coniacian and Santonian, connecting the Tethys with the South Atlantic by way of the central Sahara and Central Africa, which were then underwater.[24] Yet another shallow seaway ran between what is now Norway and Greenland, connecting the Tethys to the Arctic Ocean and enabling biotic exchange between the two oceans.[25] At the peak of the Cretaceoustransgression, one-third of Earth's present land area was submerged.[26]
The Cretaceous is justly famous for itschalk; indeed, more chalk formed in the Cretaceous than in any other period in thePhanerozoic.[27]Mid-ocean ridge activity—or rather, the circulation of seawater through the enlarged ridges—enriched the oceans incalcium; this made the oceans more saturated, as well as increased thebioavailability of the element forcalcareous nanoplankton.[28] These widespreadcarbonates and othersedimentary deposits make the Cretaceous rock record especially fine. Famousformations from North America include the rich marine fossils ofKansas'sSmoky Hill Chalk Member and the terrestrial fauna of the late CretaceousHell Creek Formation. Other important Cretaceous exposures occur inEurope (e.g., theWeald) andChina (theYixian Formation). In the area that is now India, massivelava beds called theDeccan Traps were erupted in the very late Cretaceous and early Paleocene.
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.[29] 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.[30] The location of the Intertropical Convergence Zone (ITCZ) was roughly the same as in the present.[31]
The cooling trend of the last epoch of the Jurassic, the Tithonian, continued into the Berriasian, the first age of the Cretaceous.[32] The North Atlantic seaway opened and enabled the flow of cool water from the Boreal Ocean into the Tethys.[33] 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.[32] After the end of the first age, however, temperatures began to increase again, with a number of thermal excursions, such as the middleValanginianWeissert Thermal Excursion (WTX),[34] which was caused by the Paraná-Etendeka Large Igneous Province's activity.[35] It was followed by the middleHauterivian Faraoni Thermal Excursion (FTX) and the earlyBarremian Hauptblatterton Thermal Event (HTE). The HTE marked the ultimate end of the Tithonian-early Barremian Cool Interval (TEBCI).[34] During this interval, precession was the dominant orbital driver of environmental changes in the Vocontian Basin.[36] For much of the TEBCI, northern Gondwana experienced a monsoonal climate.[37] A shallow thermocline existed in the mid-latitude Tethys.[38] The TEBCI was followed by the Barremian-Aptian Warm Interval (BAWI).[34] This hot climatic interval coincides withManihiki andOntong Java Plateau volcanism and with theSelli Event.[39] Early Aptian tropicalsea surface temperatures (SSTs) were 27–32 °C, based onTEX86 measurements from the equatorial Pacific.[40] During the Aptian, Milankovitch cycles governed the occurrence of anoxic events by modulating the intensity of the hydrological cycle and terrestrial runoff.[41] The early Aptian was also notable for its millennial scale hyperarid events in the mid-latitudes of Asia.[42] The BAWI itself was followed by the Aptian-Albian Cold Snap (AACS) that began about 118 Ma.[34] A short, relatively minor ice age may have occurred during this so-called "cold snap", as evidenced by glacialdropstones in the western parts of the Tethys Ocean[43] and the expansion of calcareous nannofossils that dwelt in cold water into lower latitudes.[44] The AACS is associated with an arid period in theIberian Peninsula.[45]
Temperatures increased drastically after the end of the AACS,[46] which ended around 111 Ma with the Paquier/Urbino Thermal Maximum, giving way to the Mid-Cretaceous Hothouse (MKH), which lasted from the earlyAlbian until the early Campanian.[34] Faster rates of seafloor spreading and entry of carbon dioxide into the atmosphere are believed to have initiated this period of extreme warmth,[47] along with high flood basalt activity.[48] The MKH was punctuated by multiple thermal maxima of extreme warmth. The Leenhardt Thermal Event (LTE) occurred around 110 Ma, followed shortly by the l’Arboudeyesse Thermal Event (ATE) a million years later. Following these two hyperthermals was theAmadeus Thermal Maximum around 106 Ma, during the middle Albian. Then, around a million years after that, occurred the Petite Verol Thermal Event (PVTE). Afterwards, around 102.5 Ma, the Event 6 Thermal Event (EV6) took place; this event was itself followed by the Breistroffer Thermal Maximum around 101 Ma, during the latest Albian. Approximately 94 Ma, the Cenomanian-Turonian Thermal Maximum occurred,[34] with this hyperthermal being the most extreme hothouse interval of the Cretaceous[49][50][51] and being associated with a sea level highstand.[52] Temperatures cooled down slightly over the next few million years, but then another thermal maximum, the Coniacian Thermal Maximum, happened, with this thermal event being dated to around 87 Ma.[34] Atmospheric CO2 levels may have varied by thousands of ppm throughout the MKH.[53] Mean annual temperatures at the poles during the MKH exceeded 14 °C.[54] Such hot temperatures during the MKH resulted in a very gentletemperature gradient from theequator to the poles; the latitudinal temperature gradient during the Cenomanian-Turonian Thermal Maximum was 0.54 °C per ° latitude for the Southern Hemisphere and 0.49 °C per ° latitude for the Northern Hemisphere, in contrast to present day values of 1.07 and 0.69 °C per ° latitude for the Southern and Northern hemispheres, respectively.[55] This meant weaker global winds, which drive the ocean currents, and resulted in lessupwelling and more stagnantoceans than today.[56] This is evidenced by widespread blackshale deposition and frequentanoxic events.[21] Tropical SSTs during the late Albian most likely averaged around 30 °C. Despite this high SST, seawater was not hypersaline at this time, as this would have required significantly higher temperatures still.[57] On land, arid zones in the Albian regularly expanded northward in tandem with expansions of subtropical high pressure belts.[58] The Cedar Mountain Formation's Soap Wash flora indicates a mean annual temperature of between 19 and 26 °C in Utah at the Albian-Cenomanian boundary.[59] Tropical SSTs during the Cenomanian-Turonian Thermal Maximum were at least 30 °C,[60] though one study estimated them as high as between 33 and 42 °C.[61] An intermediate estimate of ~33-34 °C has also been given.[62] Meanwhile, deep ocean temperatures were as much as 15 to 20 °C (27 to 36 °F) warmer than today's;[63] one study estimated that deep ocean temperatures were between 12 and 20 °C during the MKH.[64] The poles were so warm thatectothermic reptiles were able to inhabit them.[65]
Beginning in the Santonian, near the end of the MKH, the global climate began to cool, with this cooling trend continuing across the Campanian.[66] This period of cooling, driven by falling levels of atmospheric carbon dioxide,[64] caused the end of the MKH and the transition into a cooler climatic interval, known formally as the Late Cretaceous-Early Palaeogene Cool Interval (LKEPCI).[34] Tropical SSTs declined from around 35 °C in the early Campanian to around 28 °C in the Maastrichtian.[67] Deep ocean temperatures declined to 9 to 12 °C,[64] though the shallow temperature gradient between tropical and polar seas remained.[68] Regional conditions in theWestern Interior Seaway changed little between the MKH and the LKEPCI.[69] During this period of relatively cool temperatures, the ITCZ became narrower,[70] while the strength of both summer and winter monsoons in East Asia was directly correlated to atmosphericCO2 concentrations.[71] Laramidia likewise had a seasonal, monsoonal climate.[72] The Maastrichtian was a time of chaotic, highly variable climate.[73] Two upticks in global temperatures are known to have occurred during the Maastrichtian, bucking the trend of overall cooler temperatures during the LKEPCI. Between 70 and 69 Ma and 66–65 Ma, isotopic ratios indicate elevated atmospheric CO2 pressures with levels of 1000–1400 ppmV and mean annual temperatures inwest Texas between 21 and 23 °C (70 and 73 °F). Atmospheric CO2 and temperature relations indicate a doubling of pCO2 was accompanied by a ~0.6 °C increase in temperature.[74] The latter warming interval, occurring at the very end of the Cretaceous, was triggered by the activity of the Deccan Traps.[75] The LKEPCI lasted into theLate Palaeocene, when it gave way to another supergreenhouse interval.[34]
A computer-simulated model of surface conditions in Middle Cretaceous, 100 mya, displaying the approximate shoreline and calculatedisotherms
The production of large quantities of magma, variously attributed tomantle plumes or toextensional tectonics,[76] further pushed sea levels up, so that large areas of the continental crust were covered with shallow seas. TheTethys Sea connecting the tropical oceans east to west also helped to warm the global climate. Warm-adaptedplant fossils are known from localities as far north asAlaska andGreenland, whiledinosaur fossils have been found within 15 degrees of the Cretaceoussouth pole.[77] It was suggested that there wasAntarctic marine glaciation in theTuronian Age, based on isotopic evidence.[78] However, this has subsequently been suggested to be the result of inconsistent isotopic proxies,[79] with evidence of polar rainforests during this time interval at 82° S.[80] 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 theEromanga Basin in southernAustralia.[81][82]
Flowering plants (angiosperms) make up around 90% of living plant species today. Prior to the rise of angiosperms, during the Jurassic and the Early Cretaceous, the higher flora was dominated bygymnosperm groups, includingcycads,conifers,ginkgophytes,gnetophytes and close relatives, as well as the extinctBennettitales. Other groups of plants includedpteridosperms or "seed ferns", a collective term that refers to disparate groups of extinct seed plants with fern-like foliage, including groups such asCorystospermaceae andCaytoniales. The exact origins of angiosperms are uncertain, although molecular evidence suggests that they are not closely related to any living group of gymnosperms.[83]
The earliest widely accepted evidence of flowering plants are monosulcate (single-grooved)pollen grains from the lateValanginian (~ 134 million years ago) found in Israel[84] and Italy,[85] initially at low abundance.Molecular clock estimates conflict with fossil estimates, suggesting the diversification ofcrown-group angiosperms during the Late Triassic or the Jurassic, but such estimates are difficult to reconcile with the heavily sampled pollen record and the distinctive tricolpate to tricolporoidate (triple grooved) pollen ofeudicot angiosperms.[83] Among the oldest records of Angiospermmacrofossils areMontsechia from theBarremian agedLas Hoyas beds of Spain andArchaefructus from the Barremian-Aptian boundaryYixian Formation in China. Tricolpate pollen distinctive of eudicots first appears in the Late Barremian, while the earliest remains ofmonocots are known from theAptian.[83] 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.[86] The oldest known fossils ofgrasses are from theAlbian,[87] with the family having diversified into modern groups by the end of the Cretaceous.[88] The oldest large angiosperm trees are known from theTuronian (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).[89]
During the Cretaceous,ferns in the orderPolypodiales, which make up 80% of living fern species, would also begin to diversify.[90]
Theapex predators werearchosaurianreptiles, especiallydinosaurs, which were at their most diverse stage. Avians such as the ancestors of modern-daybirds 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 earlybirds, but now it is understood avianadaptive radiation is not consistent with pterosaur decline[93]). By the end of the period only three highly specializedfamilies remained;Pteranodontidae,Nyctosauridae, andAzhdarchidae.[94]
Skeleton ofPriosphenodon avelasi a large herbivorous rhynchocephalian known from the mid-Cretaceous of South America
Rhynchocephalians (which today only includes thetuatara) disappeared from North America and Europe after theEarly Cretaceous,[96] and were absent from North Africa[97] and northern South America[98] by the earlyLate 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.[99] 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.[97]
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 majorevolutionary radiation in Asia during the Early Cretaceous, which represents the high point of choristoderan diversity, including long necked forms such asHyphalosaurus and the first records of the gharial-likeNeochoristodera, which appear to have evolved in the regional absence of aquaticneosuchian crocodyliformes. During the Late Cretaceous the neochoristodereChampsosaurus was widely distributed across western North America.[100] Due to the extreme climatic warmth in the Arctic, choristoderans were able to colonise it too during the Late Cretaceous.[65]
Baculites, anammonite genus with a straight shell, flourished in the seas along with reef-buildingrudist clams.Inoceramids were also particularly notable among Cretaceous bivalves,[102] and they have been used to identify major biotic turnovers such as at the Turonian-Coniacian boundary.[103][104] Predatory gastropods with drilling habits were widespread.[105] Globotruncanidforaminifera andechinoderms such as sea urchins andstarfish (sea stars) thrived.Ostracods were abundant in Cretaceous marine settings; ostracod species characterised by high male sexual investment had the highest rates of extinction and turnover.[106]Thylacocephala, a class of crustaceans, went extinct in the Late Cretaceous. The first radiation of thediatoms (generallysiliceous shelled, rather thancalcareous) in the oceans occurred during the Cretaceous; freshwater diatoms did not appear until theMiocene.[95] Calcareous nannoplankton were important components of the marine microbiota and important as biostratigraphic markers and recorders of environmental change.[107]
The Cretaceous was also an important interval in the evolution ofbioerosion, the production of borings and scrapings in rocks,hardgrounds and shells.
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