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The Cretaceous ( ) is the geological period that lasted from about 143.1 to 66 Ma (million years ago). It is the third and final period of the Mesozoic Era, as well as the longest. At around 77.1 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 deposits from 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 climate, resulting in high eustatic sea levels that created numerous shallow inland seas. These oceans and seas were populated with now-extinct , ammonites and rudists; while on land, continued to dominate. 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 and birds appeared, including the earliest relatives of Placentalia and (Eutheria and Metatheria respectively), with the earliest crown group birds appearing towards 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 subgroup Acanthomorpha during this period. During the Early Cretaceous, appeared and began to rapidly diversify, becoming the dominant group of 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 Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, , and large , died out; it is widely thought to have been caused by the impact of a large asteroid that formed the Chicxulub crater in the Gulf of Mexico. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary (K–Pg boundary), a geologic signature associated with the mass extinction that lies between the Mesozoic and Cenozoic Eras.
From youngest to oldest, the subdivisions of the Cretaceous Period are:Cohen, K.M., Finney, S.C., Gibbard, P.L. & Fan, J.-X. (2013; updated) The ICS International Chronostratigraphic Chart. Episodes 36: 199–204.
| +Subdivisions of the Cretaceous | ||||
| Paleocene | Danian | 66 | ||
| top: iridium anomaly at the Cretaceous–Paleogene boundary base:first occurrence of Pachydiscus | Maastricht Formation, Maastricht, Netherlands | |||
| Champagne, France | ||||
| Saintes, France | ||||
| Cognac, France | ||||
| Tours, France | ||||
| Cenomanum; Le Mans, France | ||||
| Aube, France | ||||
| Apt, France | ||||
| Barrême, France | ||||
| Hauterive, Switzerland | ||||
| Valangin, Switzerland | ||||
| Berrias, France | ||||
The upper boundary of the Cretaceous is sharply defined, being placed at an iridium-rich layer found worldwide that is believed to be associated with the Chicxulub 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 Ma.
At the end of the Cretaceous, the impact of a large body with the Earth may have arrived at the end of a progressive decline in biodiversity during the Maastrichtian age. The results of the impact were the extinction of three-quarters of Earth's plant and animal species, and the creation of a sharp break known as the 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 .
Despite the severity of the K-Pg extinction event, there were significant variations in the rate of extinction between and within different . Species that 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 , 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 , such as Tyrannosaurus, also perished. Yet only three major groups of disappeared completely; the non-avian , the and the . The other Cretaceous groups that did not survive into the Cenozoic the Ichthyosauria, last remaining Temnospondyli (Koolasuchus), and nonmammalian were already extinct millions of years before the event occurred.
Coccolithophorids and , including , , , and , as well as organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example, Ammonoidea are thought to have been the principal food of , a group of giant marine related to snakes 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 carnivore . Mammals and birds that survived the extinction fed on , , , 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 Biocoenosis, 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 Pelagic zone than among animals living on or in the seafloor. 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, and Choristodera, 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.
Chalk is a rock type characteristic for (but not restricted to) the Cretaceous. It consists of , microscopically small calcite skeletons of , 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 were formed during this interval, such as the Mancos Shale of western North America. These shales are an important source rock for Fossil fuel, for example in the subsurface of the North Sea.
In southern Europe, the Cretaceous is usually a marine system consisting of competent limestone beds or incompetent . Because the Alpine orogeny 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.
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, and Australia rifted away from Africa (though India and Madagascar remained attached to each other until around 80 million years ago); thus, the South Atlantic and 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. During most of the Late Cretaceous, North America would be divided in two by the Western Interior Seaway, a large interior sea, separating Laramidia to the west and Appalachia to the east, then receded late in the period, leaving thick marine deposits sandwiched between coal 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. 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. At the peak of the Cretaceous transgression, one-third of Earth's present land area was submerged. (2026). 9780760719572, Barnes & Noble Books. ISBN 9780760719572
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 Coccolithophores. These widespread and other sedimentary rock 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 (e.g., the Weald) and China (the Yixian Formation). In the area that is now India, massive called the Deccan Traps erupted in the very late Cretaceous and early Paleocene.
The cooling trend of the last epoch of the Jurassic, the Tithonian, continued into the Berriasian, the first age of the Cretaceous. The North Atlantic seaway opened and enabled the flow of cool water from the Boreal Ocean into the Tethys. 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. 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 Event (WTX), which was caused by the Paraná-Etendeka Large Igneous Province's activity. 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). During this interval, precession was the dominant orbital driver of environmental changes in the Vocontian Basin. For much of the TEBCI, northern Gondwana experienced a monsoonal climate. A shallow thermocline existed in the mid-latitude Tethys. The TEBCI was followed by the Barremian-Aptian Warm Interval (BAWI). This hot climatic interval coincides with Manihiki Plateau and Ontong Java Plateau volcanism and with the Selli Event. Early Aptian tropical sea surface temperatures (SSTs) were 27–32 °C, based on TEX86 measurements from the equatorial Pacific. During the Aptian, Milankovitch cycles governed the occurrence of anoxic events by modulating the intensity of the hydrological cycle and terrestrial runoff. The early Aptian was also notable for its millennial scale hyperarid events in the mid-latitudes of Asia. The BAWI itself was followed by the Aptian-Albian Cold Snap (AACS) that began about 118 Ma. A short, relatively minor ice age may have occurred during this so-called "cold snap", as evidenced by glacial in the western parts of the Tethys Ocean and the expansion of calcareous nannofossils that dwelt in cold water into lower latitudes. The AACS is associated with an arid period in the Iberian Peninsula.
Temperatures increased drastically after the end of the AACS, which ended around 111 Ma with the Paquier/Urbino Thermal Maximum, giving way to the Mid-Cretaceous Hothouse (MKH), which lasted from the early Albian until the early Campanian. Faster rates of seafloor spreading and entry of carbon dioxide into the atmosphere are believed to have initiated this period of extreme warmth, along with high flood basalt activity. 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 the Amadeus Event 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, with this hyperthermal being the most extreme hothouse interval of the Cretaceous and being associated with a sea level highstand. 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. Atmospheric CO2 levels may have varied by thousands of ppm throughout the MKH. Mean annual temperatures at the poles during the MKH exceeded 14 °C. Such hot temperatures during the MKH resulted in a very gentle temperature gradient from the equator 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. This meant weaker global winds, which drive the ocean currents, and resulted in less upwelling and more stagnant than today. This is evidenced by widespread black shale deposition and frequent . 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. On land, arid zones in the Albian regularly expanded northward in tandem with expansions of subtropical high pressure belts. 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. Tropical SSTs during the Cenomanian-Turonian Thermal Maximum were at least 30 °C, though one study estimated them as high as between 33 and 42 °C. An intermediate estimate of ~33-34 °C has also been given. Meanwhile, deep ocean temperatures were as much as warmer than today's; one study estimated that deep ocean temperatures were between 12 and 20 °C during the MKH. The poles were so warm that ectothermic reptiles were able to inhabit them.
Beginning in the Santonian, near the end of the MKH, the global climate began to cool, with this cooling trend continuing across the Campanian. This period of cooling, driven by falling levels of atmospheric carbon dioxide, 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). Tropical SSTs declined from around 35 °C in the early Campanian to around 28 °C in the Maastrichtian. Deep ocean temperatures declined to 9 to 12 °C, though the shallow temperature gradient between tropical and polar seas remained. Regional conditions in the Western Interior Seaway changed little between the MKH and the LKEPCI. During this period of relatively cool temperatures, the ITCZ became narrower, while the strength of both summer and winter monsoons in East Asia was directly correlated to atmospheric Carbon dioxide concentrations. Laramidia likewise had a seasonal, monsoonal climate. The Maastrichtian was a time of chaotic, highly variable climate. 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 in west Texas between . Atmospheric CO2 and temperature relations indicate a doubling of pCO2 was accompanied by a ~0.6 °C increase in temperature. The latter warming interval, occurring at the very end of the Cretaceous, was triggered by the activity of the Deccan Traps. The LKEPCI lasted into the Late Paleocene, when it gave way to another supergreenhouse interval.
The production of large quantities of magma, variously attributed to or to extensional tectonics, (2026). 9781405161480, Wiley-Blackwell. ISBN 9781405161480 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 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. It was suggested that there was Antarctica marine glaciation in the Turonian Age, based on isotopic evidence. However, this has subsequently been suggested to be the result of inconsistent isotopic proxies, with evidence of polar rainforests during this time interval at 82° S. 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.
High temperatures fueling massive storms, hurricanes, and wildfires caused damages to trees. This along with arthropod damage caused trees during this time to produce large amounts of resin leading to the Cretaceous Resinous Interval lasting from 125 to 75 million years ago.
The earliest widely accepted evidence of flowering plants are monosulcate (single-grooved) pollen grains from the late Valanginian (~ 134 Ma) found in Israel (1996). 9780585230955, Chapman & Hall. ISBN 9780585230955 and Italy,Trevisan L. 1988. Angiospermous pollen (monosulcate–trichotomosulcate phase) from the very early Lower Cretaceous of southern Tuscany (Italy): some aspects. 7th International Palynological Congress Abstracts Volume. Brisbane, Australia: University of Queensland, 165. initially at low abundance. Molecular clock estimates conflict with fossil estimates, suggesting the diversification of Crown 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 of Eudicots angiosperms. Among the oldest records of Angiosperm are Montsechia from the Barremian aged Las Hoyas beds of Spain and Archaefructus from the Barremian-Aptian boundary Yixian Formation in China. Tricolpate pollen distinctive of eudicots first appears in the Late Barremian, while the earliest remains of monocots are known from the Aptian. 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. The oldest known fossils of Poaceae are from the Albian, with the family having diversified into modern groups by the end of the Cretaceous. The oldest large angiosperm trees are known from the Turonian (c. 90 Ma) of New Jersey, with the trunk having a preserved diameter of and an estimated height of .
During the Cretaceous, in the order Polypodiales, which make up 80% of living fern species, would also begin to diversify.
The were , especially , which were at their most diverse stage. Avians such as the ancestors of modern-day birds also diversified. They inhabited every continent, and were even found in cold polar latitudes. 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 , but now it is understood avian adaptive radiation is not consistent with pterosaur decline). By the end of the period only three highly specialized families remained; Pteranodontidae, Nyctosauridae, and Azhdarchidae.
The Liaoning lagerstätte (Yixian Formation) in China is an important site, full 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 includes modern birds and their closest non-avian relatives, such as dromaeosaurs, oviraptorosaurs, therizinosaurs, troodontids along with other avialans. Fossils of these dinosaurs from the Liaoning lagerstätte are notable for the presence of hair-like .
diversified during the Cretaceous, and the oldest known , and some , akin to Butterfly and , appeared. , and appeared.
Baculites, an ammonite genus with a straight shell, flourished in the seas along with reef-building rudist clams. Inoceramidae were also particularly notable among Cretaceous bivalves, and they have been used to identify major biotic turnovers such as at the Turonian-Coniacian boundary. Predatory gastropods with drilling habits were widespread. Globotruncanid foraminifera and echinoderms such as sea urchins and Asteroidea thrived. Ostracods were abundant in Cretaceous marine settings; ostracod species characterised by high male sexual investment had the highest rates of extinction and turnover. Thylacocephala, a class of crustaceans, went extinct in the Late Cretaceous. The first radiation of the (generally silicon dioxide shelled, rather than calcareous) in the oceans occurred during the Cretaceous; freshwater diatoms did not appear until the Miocene. Calcareous nannoplankton were important components of the marine microbiota and important as biostratigraphic markers and recorders of environmental change.
The Cretaceous was also an important interval in the evolution of bioerosion, the production of borings and scrapings in rocks, hardgrounds and shells.
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