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| Late/Upper Cretaceous | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| 100.5 ± 0.1 – 66.0Ma | |||||||||
.png) A map of Earth as it appeared 85 million years ago during the Late Cretaceous Epoch, Santonian Age | |||||||||
| Chronology | |||||||||
| |||||||||
| Etymology | |||||||||
| Chronostratigraphic name | Upper Cretaceous | ||||||||
| Geochronological name | Late Cretaceous | ||||||||
| Name formality | Formal | ||||||||
| Usage information | |||||||||
| Celestial body | Earth | ||||||||
| Regional usage | Global (ICS) | ||||||||
| Time scale(s) used | ICS Time Scale | ||||||||
| Definition | |||||||||
| Chronological unit | Epoch | ||||||||
| Stratigraphic unit | Series | ||||||||
| Time span formality | Formal | ||||||||
| Lower boundary definition | FAD of thePlanktonic ForaminiferRotalipora globotruncanoides | ||||||||
| Lower boundary GSSP | Mont Risoux,Hautes-Alpes, France 44°23′33″N5°30′43″E / 44.3925°N 5.5119°E /44.3925; 5.5119 | ||||||||
| Lower GSSP ratified | 2002[2] | ||||||||
| Upper boundary definition | Iridium enriched layer associated with a major meteorite impact and subsequentK-Pg extinction event | ||||||||
| Upper boundary GSSP | El Kef Section,El Kef, Tunisia 36°09′13″N8°38′55″E / 36.1537°N 8.6486°E /36.1537; 8.6486 | ||||||||
| Upper GSSP ratified | 1991 | ||||||||
TheLate Cretaceous (100.5–66Ma) is the more recent of twoepochs into which theCretaceousPeriod is divided in thegeologic time scale.Rock strata from this epoch form theUpper CretaceousSeries. The Cretaceous is named aftercreta, the Latin word for the whitelimestone known aschalk. The chalk of northern France and thewhite cliffs of south-eastern England date from the Cretaceous Period.[3]
During the Late Cretaceous, the climate was warmer than present, although throughout the period a cooling trend is evident.[4] Thetropics became restricted to equatorial regions and northernlatitudes experienced markedly more seasonal climatic conditions.[4]
Due toplate tectonics, the Americas were gradually moving westward, causing the Atlantic Ocean to expand. TheWestern Interior Seaway divided North America into eastern and western halves;Appalachia andLaramidia.[4] India maintained a northward course towards Asia.[4] In the Southern Hemisphere, Australia andAntarctica seem to have remained connected and began to drift away from Africa and South America.[4] Europe was an island chain.[4] Populating some of these islands were endemicdwarf dinosaur species.[4]
In the Late Cretaceous, thehadrosaurs,ankylosaurs, andceratopsians experienced success inAsiamerica (Western North America and eastern Asia).Tyrannosaurs dominated the large predator niche in North America.[4] They were also present in Asia, although were usually smaller and more primitive than the North American varieties.[4]Pachycephalosaurs were also present in both North America and Asia.[4]Dromaeosaurids shared the same geographical distribution, and are well documented in both Mongolia and Western North America.[4] Additionallytherizinosaurs (known previously as segnosaurs) appear to have been in North America and Asia.Gondwana held a very different dinosaurian fauna, with most predators beingabelisaurids andcarcharodontosaurids; andtitanosaurs being among the dominant herbivores.[4]Spinosaurids were also present during this time.[5]
Birds became increasingly common, diversifying in a variety ofenantiornithe andornithurine forms. EarlyNeornithes such asVegavis[6] co-existed with forms as bizarre asYungavolucris andAvisaurus.[7] Though mostly small, marineHesperornithes became relatively large and flightless, adapted to life in the open sea.[8]
Though primarily represented byazhdarchids, other forms likepteranodontids,tapejarids (Caiuajara andBakonydraco),nyctosaurids and uncertain forms (Piksi,Navajodactylus) are also present. Historically, it has been assumed that pterosaurs were in decline due to competition with birds, but it appears that neither group overlapped significantly ecologically, nor is it particularly evident that a true systematic decline was ever in place, especially with the discovery of smaller pterosaur species.[9]
Several oldmammal groups began to disappear, with the lasteutriconodonts occurring in theCampanian ofNorth America.[10] In the northern hemisphere,cimolodont,multituberculates,metatherians andeutherians were the dominant mammals, with the former two groups being the most common mammals in North America. In the southern hemisphere there was instead a more complex fauna ofdryolestoids,gondwanatheres and other multituberculates and basaleutherians;monotremes were presumably present, as was the last of theharamiyidans,Avashishta.
Mammals, though generally small, ranged into a variety of ecological niches, from carnivores (Deltatheroida), to mollusc-eater (Stagodontidae), toherbivores (multituberculates,Schowalteria,Zhelestidae andMesungulatidae) to highly atypical cursorial forms (Zalambdalestidae,Brandoniidae).[citation needed]
Trueplacentals evolved only at the very end of the epoch; the same can be said for truemarsupials. Instead, nearly all known eutherian and metatherian fossils belong to other groups.[11]
In the seas,mosasaurs suddenly appeared and underwent a spectacular evolutionary radiation. Modern sharks also appeared and penguin-likepolycotylidplesiosaurs (3 meters long) and huge long-neckedelasmosaurs (13 meters long) also diversified. These predators fed on the numerousteleost fishes, which in turn evolved into new advanced and modern forms (Neoteleostei).Ichthyosaurs andpliosaurs, on the other hand, became extinct during theCenomanian-Turonian anoxic event.[citation needed]
Near the end of the Cretaceous Period,flowering plants diversified. In temperate regions, familiar plants likemagnolias,sassafras androses could be found in abundance.[4]
The Cretaceous–Paleogene extinction event was a large-scalemass extinction of animal and plant species in a geologically short period of time, approximately66 million years ago (Ma). It is widely known as theK–T extinction event and is associated with a geological signature, usually a thin band dated to that time and found in various parts of the world, known as theCretaceous–Paleogene boundary (K–T boundary).K is the traditional abbreviation for the Cretaceous Period derived from the German nameKreidezeit, andT is the abbreviation for theTertiary Period (a historical term for the period of time now covered by thePaleogene andNeogene periods). The event marks the end of theMesozoic Era and the beginning of theCenozoic Era.[12] "Tertiary" being no longer recognized as a formal time or rock unit by theInternational Commission on Stratigraphy, the K-T event is now called the Cretaceous—Paleogene (or K-Pg) extinction event by many researchers.
Non-avian dinosaurfossils are found only below the Cretaceous–Paleogene boundary and became extinct immediately before or during the event.[13] A very small number ofdinosaur fossils have been found above the Cretaceous–Paleogene boundary, but they have been explained asreworked fossils, that is, fossils that have been eroded from their original locations and then preserved in latersedimentary layers.[14][15][16]Mosasaurs,plesiosaurs,pterosaurs and manyspecies of plants andinvertebrates also became extinct.Mammalian and birdclades passed through the boundary with few extinctions, andevolutionary radiation from thoseMaastrichtian clades occurred well past the boundary. Rates of extinction and radiation varied across different clades of organisms.[17]
Many scientists hypothesize that the Cretaceous–Paleogene extinctions were caused by catastrophic events such as the massiveasteroid impact that caused theChicxulub crater, in combination with increasedvolcanic activity, such as that recorded in theDeccan Traps, both of which have been firmly dated to the time of the extinction event. In theory, these events reduced sunlight and hinderedphotosynthesis, leading to a massive disruption in Earth'secology.
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