Cretaceous
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The Cretaceous ( The Cretaceous was a period with a relatively warm birds appeared. 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 historyThe 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 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]
GeologySubdivisionsThe 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:
Boundaries The impact of a meteorite or comet is today widely accepted as the main reason for the 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 Global 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 any chemostratigraphic events, such as isotope excursions (large sudden changes in ratios of isotopes) that could be used to define or correlate a boundary. Calpionellids, an enigmatic group of planktonic protists with urn-shaped calcitic tests briefly abundant during the latest Jurassic to earliest Cretaceous, have been suggested as the most promising candidates for fixing the Jurassic–Cretaceous boundary.[7] In particular, the first appearance Calpionella alpina, coinciding with the base of the eponymous Alpina subzone, has been proposed as the definition of the base of the Cretaceous.[8] The working definition for the boundary has often been placed as the first appearance of the ammonite 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 plesiosaurs and the pterosaurs. The other Cretaceous groups that did not survive into the Cenozoic Era — the ichthyosaurs, last remaining temnospondyls (Koolasuchus), and nonmammalian 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]
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 ItalyThe 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. EuropeIn 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 AmericaDuring 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] PaleogeographyDuring the Cretaceous, the late- .transgression, one-third of Earth's present land area was submerged.[24]
The Cretaceous is justly famous for its 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 lava beds called the Deccan Traps were erupted in the very late Cretaceous and early Paleocene.
ClimatePalynological evidence indicates the Cretaceous climate had three broad phases: a Berriasian–Barremian warm-dry phase, a Aptian–Santonian warm-wet phase, and a Campanian–Maastrichtian cool-dry phase.[27] The cooling trend of the last epoch of the Jurassic continued into the Berriasian, the first age of the Cretaceous. 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.[28] 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.[29][30] isotherms After the end of the first age, however, temperatures increased again, and these conditions were almost constant until the end of the period. 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.[20] 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 today's.[37][38] Floraginkgophytes, 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.[39]
The earliest widely accepted evidence of flowering plants are monosulcate (single-grooved) monocots are known from the Aptian.[39] 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.[42] The oldest known fossils of grasses are from the Albian,[43] with the family having diversified into modern groups by the end of the Cretaceous.[44] 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).[45]
During the Cretaceous, Polypodiales ferns, which make up 80% of living fern species, would also begin to diversify.[46] Terrestrial faunaOn land, dryolestoids dominating South America .
The apex predators were 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[49]). By the end of the period only three highly specialized families remained; Pteranodontidae, Nyctosauridae, and Azhdarchidae.[50]
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.[51]
Rhynchocephalians , a rhyncocephalian from the late Early Cretaceous of ItalyRhynchocephalians (which today only includes the Tuatara) disappeared from North America and Europe after the Early Cretaceous,[52] and were absent from North Africa[53] and northern South America[54] 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.[55] 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.[53] Choristodera , a choristodere from the Early Cretaceous of ChinaChoristoderes, 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.[56] Marine faunaIn the seas, Cenomanian-Turonian anoxic event), plesiosaurs throughout the entire period, and mosasaurs appearing in the Late Cretaceous. Sea turtles in the form of Cheloniidae and Panchelonioidea lived during the period and survived the extinction event. Panchelonioidea is today represented by a single species; the leatherback sea turtle .
See also
ReferencesCitations
Bibliography
External links Wikimedia Commons has media related to Cretaceous.  Look up cretaceous in Wiktionary, the free dictionary.
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