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Cretaceous傍ertiary extinction event


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Quasar
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« on: June 29, 2009, 01:09:56 pm »

Cretaceous傍ertiary extinction event



Artist's rendering of bolide impact.
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Quasar
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« Reply #1 on: June 29, 2009, 01:10:38 pm »



Badlands near Drumheller, Alberta where erosion has exposed the K–T boundary
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Quasar
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« Reply #2 on: June 29, 2009, 01:11:00 pm »

The Cretaceous–Tertiary extinction event, which occurred approximately 65.5 million years ago (Ma), was a large-scale mass extinction of animal and plant species in a geologically short period of time. Widely known as the K–T extinction event, it is associated with a geological signature known as the K–T boundary, usually a thin band of sedimentation found in various parts of the world. K is the traditional abbreviation for the Cretaceous Period derived from the German name Kreidezeit, and T is the abbreviation for the Tertiary Period (a historical term for the period of time now covered by the Paleogene and Neogene periods). The event marks the end of the Mesozoic Era and the beginning of the Cenozoic Era.[1] With "Tertiary" being discouraged as a formal time or rock unit by the International Commission on Stratigraphy, the K–T event is now called the Cretaceous–Paleogene (or K–Pg) extinction event by many researchers.[2]
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Quasar
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« Reply #3 on: June 29, 2009, 01:11:18 pm »

Non-avian dinosaur fossils are only found below the K–T boundary, indicating that dinosaurs became extinct immediately before, or during the event.[3] A very small number of dinosaur fossils have been found above the K–T boundary, but they have been explained as reworked, that is, fossils that have been eroded from their original locations then preserved in later sedimentary layers.[4][5][6] Mosasaurs, plesiosaurs, pterosaurs and many species of plants and invertebrates also became extinct. Mammalian and bird clades passed through the boundary with few extinctions, and evolutionary radiation from those Maastrichtian clades occurred well past the boundary. Rates of extinction and radiation varied across different clades of organisms.[7]

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Quasar
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« Reply #4 on: June 29, 2009, 01:11:28 pm »

Scientists theorize that the K–T extinctions were caused by one or more catastrophic events, such as massive asteroid impacts (like the Chicxulub impact), or increased volcanic activity. Several impact craters and massive volcanic activity, such as that in the Deccan traps, have been dated to the approximate time of the extinction event. These geological events may have reduced sunlight and hindered photosynthesis, leading to a massive disruption in Earth's ecology. Other researchers believe the extinction was more gradual, resulting from slower changes in sea level or climate.[7]

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Quasar
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« Reply #5 on: June 29, 2009, 01:12:46 pm »



Millions of years ago


Marine extinction intensity through time. The blue graph shows the apparent percentage (not the absolute number) of marine animal genera becoming extinct during any given time interval. It does not represent all marine species, just those that are readily fossilized. The labels of the "Big Five" extinction events are clickable hyperlinks; see Extinction event for more details. (source and image info)
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« Reply #6 on: June 29, 2009, 01:13:25 pm »

Extinction patterns

Even though the boundary event was severe, there was significant variability in the rate of extinction between and within different clades. Because atmospheric particles blocked sunlight, reducing the amount of solar energy reaching the Earth's surface, species that depended on photosynthesis declined or became extinct. Photosynthesizing organisms, including phytoplankton and land plants, formed the foundation of the food chain in the late Cretaceous as they do today. Evidence suggests that herbivorous animals died out when the plants they depended on for food became scarce; consequently, top predators such as Tyrannosaurus rex also perished.[8]

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« Reply #7 on: June 29, 2009, 01:13:37 pm »

Coccolithophorids and molluscs, including ammonites, rudists, freshwater snails and mussels, and those organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example, it is thought that ammonites were the principal food of mosasaurs, a group of giant marine reptiles that became extinct at the boundary.[9]

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 carnivorous mammals. Mammals and birds that survived the extinction fed on insects, worms, and snails, which fed on dead plant and animal matter. Scientists hypothesize that these organisms survived the collapse of plant-based food chains because they fed on detritus.[10][7][11]

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« Reply #8 on: June 29, 2009, 01:13:47 pm »

In stream communities, few groups of animals became extinct; because stream communities rely less directly on food from living plants and more on detritus that washes in from land, buffering them from extinction.[12] Similar, but more complex patterns have been found in the oceans. Extinction was more severe among animals living in the water column, than among animals living on or in the sea floor. 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.[7]

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« Reply #9 on: June 29, 2009, 01:14:13 pm »

The largest air-breathing survivors of the event, crocodilians and champsosaurs, were semi-aquatic and had access to detritus. Modern crocodilians can live as scavengers and can survive for months without food, 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.[10]

After the K–T event, biodiversity required substantial time to recover, despite the existence of abundant vacant ecological niches.[7]

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« Reply #10 on: June 29, 2009, 01:14:36 pm »

Microbiota

The K–T boundary represents one of the most dramatic turnovers in the fossil record for various calcareous nanoplankton that formed the calcium deposits that gave the Cretaceous its name. The turnover in this group is clearly marked at the species level.[13][14] Statistical analysis of marine losses at this time suggests that the decrease in diversity was caused more by a sharp increase in extinctions than by a decrease in speciation.[15] The K–T boundary record of dinoflagellates is not as well-understood, mainly because only microbial cysts provide a fossil record, and not all dinoflagellate species have cyst-forming stages, thereby likely causing diversity to be underestimated.[7] Recent studies indicate that there were no major shifts in dinoflagellates through the boundary layer.[16]

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« Reply #11 on: June 29, 2009, 01:14:48 pm »



Radiolaria have left a geological record since at least the Ordovician times, and their mineral fossil skeletons can be tracked across the K-T boundary. There is no evidence of mass extinction of these organisms, and, there is support for high productivity of these species in Southern high latitudes as a result of cooling temperatures in the early Paleocene.[7] Approximately 46% of diatom species survived the transition from the Cretaceous to the Upper Paleocene. This suggests a significant turnover in species, but not a catastrophic extinction of diatoms, across the K–T boundary.[7][17]

The occurrence of Planktonic foraminifera across the K-T boundary has been studied since the 1930s.[18][19] Research spurred by the possibility of an impact event at the K-T boundary resulted in numerous publications detailing planktonic foraminiferal extinction at the boundary.[7] However, there is debate ongoing between groups that believe the evidence indicates substantial extinction of these species at the K-T boundary,[20] and those who believe the evidence supports multiple extinctions and expansions through the boundary.[21][22]

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« Reply #12 on: June 29, 2009, 01:15:02 pm »

As the biomass in the ocean is thought to have decreased during the K-T event, numerous species of benthic foraminifera went extinct, presumably since they depend on organic debris for nutrients. However, as the marine microbiota recovered, it is thought that increased speciation of benthic foraminifera resulted from the increase in food sources.[7] Phytoplankton recovery in the early Paleocene provided the food source to support large benthic foraminiferal assemblages, which are mainly detritus-feeding. Ultimate recovery of the benthic populations occurred over several stages lasting several hundred thousand years into the early Paleocene.[23][24]

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« Reply #13 on: June 29, 2009, 01:15:28 pm »

Marine invertebrates

There is variability in the fossil record as to the extinction rate of marine invertebrates across the K-T boundary. The apparent rate is influenced by the lack of fossil records rather than actual extinction.[7]

Ostracodes, a class of small crustaceans that were prevalent in the upper Maastrichtian, left fossil deposits in a variety of locations. A review of these fossils shows that ostracode diversity was lower in the Paleocene than any other time in the Tertiary. However, current research cannot ascertain whether the extinctions occurred prior to or during the boundary interval itself.[25][26]

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« Reply #14 on: June 29, 2009, 01:15:39 pm »

Approximately 60% of late-Cretaceous Scleractinia coral genera failed to cross the K-T boundary into the Paleocene. Further analysis of the coral extinctions shows that approximately 98% of colonial species, ones that inhabit warm, shallow tropical waters, went extinct. The solitary corals, which generally do not form reefs and inhabit colder and deeper (below the photic zone) areas of the ocean were less impacted by the K-T boundary. Colonial coral species rely upon symbiosis with photosynthetic algae, which collapsed due to the events surrounding the K-T boundary.[27][28] However, the use of data from coral fossils to support K-T extinction and subsequent Paleocene recovery must be weighed against the changes that occurred in coral ecosystems through the K-T boundary.[7]

The numbers of cephalopod, echinoderm, and bivalve genera exhibited significant diminution after the K-T boundary.[7] Most species of brachiopods, a small phylum of marine invertebrates, survived the K-T event and diversified during the early Paleocene.

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