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The Cretaceous-Tertiary event was the catastrophic mass extinction of extant animal species in a comparatively short period of time. The event occurred approximately 65.5 million years ago. It is widely known as the K-T extinction event, and its geological signature is called the K-T boundary (K is the traditional abbreviation for the Cretaceous Period, to avoid confusion with the Carboniferous Period, abbreviated as C, and the Cambrian period, denoted C).

With a few possible exceptions, there are no non-avian dinosaur fossils that are found later than the K-T boundary, and it appears that all went extinct during or shortly after the event. Many other groups of animals and plants, including mosasaurs, plesiosaurs, pterosaurs, and invertebrates, also became extinct at the K-T boundary. The event marks the end of the Mesozoic Era, and the beginning of the Cenozoic Era.

Extinction patterns

Despite the severity of the K-T extinction event, there was significant variability of the effect on different classes of organisms.

Organisms which depended on photosynthesis became extinct or suffered heavy losses – from photosynthesing plankton (e.g. coccolithophorids) to land plants and organisms whose food chain depended on photosynthesising organisms, e.g. tyrannosaurs (which ate herbivorous dinosaurs). No land animal larger than a cat survived. Animals which built calcium carbonate shells became extinct or suffered heavy losses (coccolithophorids; many groups of molluscs, including ammonites, rudists, freshwater snails and mussels), as did organisms whose food chain depended on these calcium carbonate shell builders. For example, it is thought that ammonites were the principal food of mosasaurs.

Most omnivores, insectivores and carrion-eaters appear to have survived quite well. It is worth noting that at the end of the Cretaceous there seem to have been no purely herbivorous or carnivorous mammals. Many mammals, and the birds which survived the extinction, fed on insects, larvae, worms, snails, etc., which in turn fed on dead plant matter. They survived the collapse of plant-based food chains because they lived in "detritus-based" food chains.

In stream communities few groups of animals became extinct. Stream communities tend to be less reliant on food from living plants and are more dependent on detritus that washes in from land. The stream communities may also have been buffered from extinction by their reliance on detritus-based food chains. Similar, but more complex patterns have been found in the oceans. For example, animals living in the water column are almost entirely dependent on primary production from living phytoplankton. Many animals living on or in the ocean floor feed on detritus, or at least can switch to detritus feeding. Extinction was more severe among those animals living in the water column than among animals living on or in the sea floor.

The largest air-breathing survivors, crocodilians and champsosaurs, were semi-aquatic. Modern crocodilians can live as scavengers and can survive for months without food. Modern crocodilian young are small, grow slowly, and feed largely on invertebrates for their first few years, so they rely on a detritus-based food chain.

Marine animals

Invertebrate groups which became extinct include: Ammonoids (a group of shelled cephalopods), Rudists (reef-building clams) and Inoceramids (giant relatives of modern scallops).

Large vertebrates also becaume extinct at the end of the Cretaceous, including mosasaurs and plesiosaurs, giant aquatic reptiles which were the top marine predators.

Planktonic organisms suffered heavy losses, notably the coccolithophorids (chalk-forming nanoplankton algae which largely gave the Cretaceous period its name). 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.

Terrestrial animals

Groups which became totally extinct include: All non-avian dinosaurs are believed by most paleontologists to have become extinct at the end of the Cretaceous. Most paleontologists regard birds as the only surviving dinosaurs (see Origin of birds). Pterosaurs, large flying reptiles, also became extinct at the K-T event.

Groups which suffered heavy losses include birds, marsupials, freshwater mussels and snails, and insects. All non-Neornithes birds became extinct, including flourishing groups like Enantiornithes and Hesperornithes. The Northern hemisphere family of marsupials became extinct, but those in Australia and South America survived. Analyses of insect damage to leaves suggests that herbivorous insects suffered losses at the K-T boundary.

But some other groups were relatively unaffected: amphibians, turtles, Lepidosaurs (including snakes and lizards),Champsosaurs (semi-aquatic archosauromorphs which died out in the early Oligocene, Crocodilians, Monotremes (egg-laying mammals), Multituberculates, and Placentals, the ancestors of most modern mammals.

Plant species

There is now overwhelming evidence for global disruption of vegetation at the Cretaceous-Paleogene boundary. However, there are important regional differences in the signature of vegetation turnover. The data suggest both massive devastation and mass extinction of plants at many Cretaceous-Paleogene boundary sections in North America but mainly mass-kill of vegetation at Southern Hemisphere high latitudes resulting in dramatic but short-term changes in the relative abundance of plant groups.

In North America, as many as 57% of plant species may have become extinct. The Paleocene recovery of plants began with a "fern spike" like that which signals the recovery from natural disasters (e.g. the 1980 Mount St. Helens eruption).

Fossil evidence

North American dinosaur extinctions

At present the best sequence of fossil-bearing rocks known is in Montana, USA (the Hell Creek, Lance Formation and Scollard Formation), running from about 83.5 MYA (million years ago) to 64.9 MYA and covering the Campanian and Maastrichtian ages of the Cretaceous and the beginning of the Paleocene period. They show changes in dinosaur populations over the last 18M years of the Cretaceous. Although the Hell Creek, Lance and Scollard formations provide a wealth of information, they cover a relatively small area and it cannot be assumed that these formations represent a worldwide picture of dinosaur life at the end of the Cretaceous.

In the middle-late Campanian these formations show a greater diversity of dinosaurs than any other single group of rocks. There was no obvious reduction in dinosaur diversity, not even in the latest part of the Maastrichtian (Fastovsky and Sheehan 1995 and later papers). And the late Maastrichtian rocks contain the largest members of almost every major clade: Tyrannosaurus, Ankylosaurus, Pachycephalosaurus, Triceratops and Torosaurus. This suggests food was plentiful not long before the extinction.

In the sediments below the K-T boundary the dominant plant remains are angiosperm pollen grains, but the actual boundary layer contains no pollen and is dominated by fern spores. Normal pollen levels resume immediately above the boundary layer. This is reminiscent of areas blighted by volcanic eruptions, where the recovery is led by ferns which are later replaced by larger angiosperm plants.

Marine extinctions

The mass extinction of marine plankton appears to have been abrupt and right at the K-T boundary. Ammonite genera became extinct at or near the K-T boundary along with a smaller and slower extinction of ammonites associated with a marine regression shortly before that. The gradual extinction of most inoceramid bivalves began well before the K-T boundary, and a small, gradual reduction in ammonite diversity occurred throughout the very late Cretaceous. Further analysis shows that several processes were ongoing in the late Cretaceous seas and partially overlapped in time, which finished with the abrupt mass extinction.

Length of time for extinction

This is a controversial issue, because some theories about the extinction's causes require a rapid extinction over a relatively short period (from a few years to a few thousand years) while others require longer periods. It is difficult to resolve because the fossil record is so incomplete that most extinct species probably died out a long time after the most recent fossil that has been found (the Signor-Lipps effect).

Scientists have also found very few continuous beds of fossil-bearing rock which cover a time range from several million years before the K-T extinction to a few million years after it. Some evidence suggests different patterns for the terrestrial and marine extinctions, and the marine extinction was complex.

Cause of K-T extinction event

Alvarez hypothesis

In 1980, a team of researchers led by Nobel-prize-winning physicist Luis Alvarez, his son geologist Walter Alvarez and chemists Frank Asaro and Helen Michels discovered that sedimentary layers found all over the world at the Cretaceous-Tertiary boundary contain a concentration of iridium hundreds of times greater than normal. Iridium is extremely rare in the earth's crust because it is very dense, and therefore most of it sank into the earth's core while the earth was still molten. The Alvarez team suggested that an asteroid struck the earth at the time of the K-T boundary. There were other earlier speculations on the possibility of an impact event, but no evidence had been uncovered at that time.

The evidence for the Alvarez impact theory is supported by chondritic meteorites and asteroids which contain a much higher iridium concentration than the earth's crust. The isotopic ratio of iridium in asteroids is similar to that of the K-T boundary layer but significantly different from the ratio in the earth's crust. Chromium isotopic anomalies found in Cretaceous-Tertiary boundary sediments are similar to that of an asteroid or a comet composed of carbonaceous chondrites. Shocked quartz granules, glass spherules and tektites, indicative of an impact event, are common in the K-T boundary, especially in deposits from around the Caribbean. All of these constituents are embedded in a layer of clay, which the Alvarez team interpreted as the debris spread all over the world by the impact.

The Alvarez team then estimated the total amount of iridium in the K-T layer and the size of the asteroid, assuming that it contained the normal percentage of iridium found in chondrites. The answer was about 10 kilometers (6 miles) in diameter, about the size of Manhattan. Such a large impact would have had approximately the force of 100 trillion tons of TNT, i.e. about 2 million times as great as the most powerful thermonuclear bomb ever tested.

The most obvious consequence of such an impact would be a vast dust cloud which would block sunlight and prevent photosynthesis for a few years. This would account for the extinction of plants and phytoplankton and of all organisms dependent on them (including predatory dinosaurs as well as herbivores). But small creatures whose food chains were based on detritus would have a reasonable chance of survival. It is estimated that sulfuric acid aerosols were injected into the stratosphere, leading to a 10-20% reduction of solar transmission normal for that period. It would have taken at least 10 years for those aerosols to dissipate.

Global firestorms may have resulted as incendiary fragments from the blast fell back to Earth. Analyses of fluid inclusions in ancient amber suggest that the oxygen content of the atmosphere was very high (30-35%) during the late Cretaceous. This high O₂ level would have supported intense combustion. The level of atmospheric O₂ plummeted in the early Tertiary Period. If widespread fires occurred, they would have increased the CO₂ content of the atmosphere and caused a temporary greenhouse effect once the dust cloud settled, and this would have exterminated the most vulnerable survivors of the "long winter".

The impact may also have produced acid rain, depending on what type of rock the asteroid struck. However, recent research suggests this effect was relatively minor. Chemical buffers would have limited the changes, and the survival of animals vulnerable to acid rain effects (such as frogs) indicate this was not a major contributor to extinction.

Impact theories can only explain very rapid extinctions, since the dust clouds and possible sulphuric aerosols would wash out of the atmosphere in a fairly short time - possibly under 10 years.

Although further studies of the K-T layer consistently show the excess of iridium, the idea that the dinosaurs were exterminated by an asteroid remained a matter of controversy among geologists and paleontologists for more than a decade.

Chicxulub Crater

One issue with the "Alvarez hypothesis" (as it came to be known) was that no documented crater matched the event. This was not a lethal blow to the theory; although the crater resulting from the impact would have been larger than 250 kilometres in diameter, Earth's geological processes tend to hide or destroy craters over time.

Subsequent research, however, found what many thought was "the smoking gun" - the Chicxulub Crater buried under Chicxulub on the coast of Yucatan, Mexico. Identified in 1990 based on the work of Glen Penfield done in 1978, this crater is oval, with an average diameter of about 180km, about the size calculated by the Alvarez team.

The shape and location of the crater indicate further causes of devastation in addition to the dust cloud. The asteroid landed right on the coast and would have caused gigantic tsunamis, for which evidence has been found all round the coast of the Caribbean and eastern United States - marine sand in locations which were then inland, and vegetation debris and terrestrial rocks in marine sediments dated to the time of the impact. The asteroid landed in a bed of gypsum (calcium sulphate), which would have produced a vast sulphur dioxide aerosol. This would have further reduced the sunlight reaching the earth's surface and then precipitated as acid rain, killing vegetation, plankton and organisms which build shells from calcium carbonate (notably some plankton species and many species of mollusk). The crater's shape suggests that the asteroid landed at an angle of 20° to 30° from horizontal and travelling north-west. This would have directed most of the blast and solid debris into the central part of what is now the United States. Most paleontologists now agree that an asteroid did hit the Earth about 65 million years ago, but many dispute whether the impact was the sole cause of the extinctions.

Gerta Keller suggests that the Chicxulub impact occurred approximately 300,000 years before the K-T boundary. This dating is based on evidence collected in Northeast Mexico, detailing multiple stratigraphic layers containing impact spherules, the earliest of which occurs some 10 metres below the K-T boundary. This chronostratigraphic thickness is thought to represent 300,000 years. This finding supports the theory that one or many impacts were contributary, but not causal, to the K-T boundary mass extinction. However, many scientists reject Keller's analysis, some arguing the 10 metre layer on top of the impact spherules should be attributed to tsunami activity resulting from impact.

Deccan Traps

Several scientists think the extensive volcanic activity in India known as the Deccan Traps may have been responsible for, or contributed to, the extinction.

Before 2000, arguments that the Deccan Traps flood basalts caused the extinction were usually linked to the view that the extinction was gradual, as the flood basalt events were thought to have started around 68MYA and lasted for over 2M years. However, in 2000, Hofman et al. provided evidence that two-thirds of the Deccan Traps were created in 1M years about 65.5.MYA. So these eruptions would have caused a fairly rapid extinction, over a period of thousands of years, but over a longer time period than one caused entirely by an impact event.

The Deccan Traps would have caused extinction through several mechanism, including the release of dust and sulphuric aerosols into the air which blocked sunlight and thereby reducing photosynthesis in plants. In addition, carbon dioxide emissions which would have increased the greenhouse effect when the dust and aerosols cleared from the atmosphere.

In the years when the Deccan Traps theory was linked to a slower extinction, Luis Alvarez (who died in 1988) replied that paleontologists were being misled by sparse data. His assertion did not go over well at first, but later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely or at least partly due to a massive Earth impact. However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as a drop in sea level and massive volcanic eruptions in India (Deccan Traps sequence) and these may have contributed to the extinctions.

A very large crater has been recently reported in the sea floor off the west coast of India. The Shiva crater, 450-600 kilometres in diameter, has also been dated at about 65 million years at the K-T boundary. The impact may have been the triggering event for the Deccan Traps. However, this feature has not yet been accepted by the geologic community as an impact crater and may just be a sinkhole depression caused by salt withdrawal.

Multiple impact event

Several other craters also appear to have been formed at the K-T boundary. This suggests the possibility of near simultaneous multiple impacts, perhaps from a fragmented asteroidal object, similar to the Shoemaker-Levy 9 cometary impact with Jupiter. Among these are the Boltysh crater (24 km diameter, 65.17 ± 0.64 Ma old) in Ukraine; the Silverpit crater (20 km diameter, 60-65 Ma old) in the North Sea; the Eagle Butte crater (10 km diameter, < 65 Ma old) in Alberta, Canada; and the Vista Alegre crater (9.5 km diameter, < 65 Ma old) in Paraná State, Brazil.

Maastrichtian sea-level regression

There is clear evidence that sea levels fell in the final stage of the Cretaceous by more than at any other time in the Mesozoic era. In some Maastrichtian rock sequences from various parts of the world the latest rocks are terrestrial; earlier ones represent shorelines and the earliest represent seabeds. These layers do not show the tilting and distortion associated with mountain building, hence by far the likeliest explanation is a regression (drop in sea level). There is no direct evidence for the cause of the regression, but most probably the mid-ocean ridges became less active and therefore sank under their own weight.

A severe regression would have greatly reduced the continental shelf area, which is the most species-rich part of the sea, and therefore could have been enough to cause a marine mass extinction. However research concludes that this change would have been insufficient to cause the observed level of ammonite extinction.

The regression would also have caused climate changes, partly by disrupting winds and ocean currents and partly by reducing the earth's albedo and therefore increasing global temperatures. These would have caused some extinctions on land, especially among herbivores because of changes in the vegetation available. But the North American Maastrichtian fossil record for dinosaurs shows continued high diversity with gains and losses rather than a prolonged mass extinction and a continuing increase in dinosaur sizes, which suggests the total food available was not reduced even if its composition changed.

Supernova hypothesis

Another proposed cause for the K-T extinction event was cosmic radiation from a relatively nearby supernova explosion. The iridium anomaly at the boundary could support this hypothesis. The fallout from a supernova explosion should contain the plutonium isotope Pu-244, the longest-lived plutonium isotope (half-life 81 Myr). Detectable traces of Pu-244 would then be detected from rocks deposited at the time. However, analysis of the boundary layer sediments revealed the absence of Pu-244, thus essentially disproving this hypothesis.

Composite theories

Two of the best-supported theories, based on the Chixculub impact and the Deccan Traps, are not mutually exclusive in the present stage of our knowledge:

• We only know of one sequence of rocks, the Hell Creek and Lance formations around Montana (USA), which gives a detailed and continuous record of the final stages of the Cretaceous. The evidence of these rocks appears to favour a very quick extinction, most probably caused by the Chixculub impact.
• We do not know how fast the extinction was in other parts of the world. There is good reason to hope that discoveries in China will add to our knowledge of the K-T extinction. But we have virtually no information about what happened in the southern hemisphere.
• It is not certain that a catastrophe in the northern hemisphere would have been able to cause a mass extinction in the southern hemisphere - in today's earth the two hemispheres share a single ocean current system but have largely separate wind systems, which would have made it difficult for debris from Chixculub to cause a "long winter" in the south. Perhaps the southern extinction was mainly caused by the Deccan Traps at the same time but rather more slowly.
• The impact on plants appears to have been different in the northern and southern hemispheres - many species of plants were wiped out in the north, while in the south there was a large reduction in the number of plants but few species became extinct.
• There is evidence on many different planetery bodies that an impact of a certain size can cause massive antipodean fractures; the Deccan Traps and the Chicxulub crater fit this model in gross terms, but the primary pulse of the Deccan Traps, at 66.9 ± 0.2 million years ago preceded the Chicxulub impact by about 1.7 million years.

And it is quite possible that the marine extinctions could have been caused by some combination of impact(s), the Deccan Traps and a severe sea-level regression.

Possible post-KT extinction event dinosaurs

It was suggested by Sloan et al. in 1986 that some dinosaurs survived into the Paleocene and therefore the extinction of dinosaurs was gradual (they said nothing about other aspects of the K-T extinction). Their arguments were based on the finding of dinosaur remains in the Hell Creek Formation up to 1.3 metres above (40,000 years later than) the K-T boundary. Similar reports have come from other parts of the world, including China.

Recently, there is possible evidence of a Dead Clade Walking: in 2001, evidence was presented by Fassett et al. that pollen samples recovered near a fossilized hadrosaur femur recovered in the Ojo Alamo Sandstone at the San Juan River indicate that the animal lived in Tertiary times, approximately 64.5 million years ago or about 1 million years after the K-T event.

Many scientists dismiss the "Paleocene dinosaurs" as re-worked, i.e. washed out of their original locations and then re-buried in much later sediments. Remains of archosaurs and icthyosaurs have been found in sediments from as late as the Miocene.

See also

• Signor-Lipps effect
• Shocked quartz
• Tektite
• Chicxulub Crater
• Deccan Traps
• Permian-Triassic extinction event - a much larger extinction event
• Tunguska event
• Paleogene

Footnotes

1. Favstovsky, D.E., and Sheehan, P.M. (2005). "The extinction of the dinosaurs in North America". GSA Today. 15 (3): 4–10. Retrieved 2007-05-18.{{cite journal}}: CS1 maint: multiple names: authors list (link)
2. Kauffman, E (2004). "Mosasaur Predation on Upper Cretaceous Nautiloids and Ammonites from the United States Pacific Coast". Palaios. 19 (1). Society for Sedimentary Geology: 96–100. doi:10.1669/0883-1351(2004)019<0096:MPOUCN>2.0.CO;2. Retrieved 2007-06-17. {{cite journal}}: Cite has empty unknown parameter: |coauthors= (help)
3. Sheehan and Fastovsky, Geology, v. 20, p. 556-560.
4. Bambach, R.K.; Knoll, A.H.; Wang, S.C. (December 2004), "Origination, extinction, and mass depletions of marine diversity", Paleobiology, 30 (4): 522–542
5. Pope, KO, D'Hondt, SL & Marshall, CR (1998). "Meteorite impact and the mass extinction of species at the Cretaceous/Tertiary boundary". PNAS. 95 (19): 11028–11029. PMID 9736679. Retrieved 2007-06-15.{{cite journal}}: CS1 maint: multiple names: authors list (link)
6. ^ Marshall, C. R. & Ward, PD (1996). "Sudden and Gradual Molluscan Extinctions in the Latest Cretaceous of Western European Tethys". Science. 274 (5291): 1360–1363. doi:10.1126/science.274.5291.1360.{{cite journal}}: CS1 maint: multiple names: authors list (link)
7. ^ Alvarez, LW, Alvarez, W, Asaro, F, and Michel, HV (1980). "Extraterrestrial cause for the Cretaceous-Tertiary extinction". Science. 208 (4448): 1095–1108. doi:10.1126/science.208.4448.1095.{{cite journal}}: CS1 maint: multiple names: authors list (link)
8. De Laubenfels, MW (1956). "Dinosaur Extinctions: One More Hypothesis". Journal of Paleontology. 30 (1): 207–218. Retrieved 2007-05-22.
9. Ocampo, A, Vajda, V & Buffetaut, E (2006). Unravelling the Cretaceous-Paleogene (KT) Turnover, Evidence from Flora, Fauna and Geology in Biological Processes Associated with Impact Events (Cockell, C, Gilmour, I & Koeberl, C, editors). SpringerLink. pp. 197–219. ISBN 978-3-540-25735-6. Retrieved 2007-06-17.{{cite book}}: CS1 maint: multiple names: authors list (link)
10. Kring, DA (2003). "Environmental consequences of impact cratering events as a function of ambient conditions on Earth". Astrobiology. 3 (1): 133–152. PMID 12809133.
11. ^ Keller, G (2005). "Impacts, volcanism and mass extinction: random coincidence or cause and effect?". Australian Journal of Earth Sciences. 52: 725–757. doi:10.1080/08120090500170393.
12. Hofman, C, Féraud, G & Courtillot, V (2000). "40Ar/39Ar dating of mineral separates and whole rocks from the Western Ghats lava pile: further constraints on duration and age of the Deccan traps". Earth and Planetary Science Letters. 180: 13–27. doi:10.1016/S0012-821X(00)00159-X.{{cite journal}}: CS1 maint: multiple names: authors list (link)
13. Chatterjee, S, Guven, N, Yoshinobu, A, & Donofrio, R (2006). "Shiva structure: a possible KT boundary impact crater on the western shelf of India" (PDF). Special Publications of the Museum of Texas Tech University (50). Retrieved 2007-06-15.{{cite journal}}: CS1 maint: multiple names: authors list (link)
14. Sloan, R. E., Rigby, K,. Van Valen, L. M., Gabriel, Diane (1986). "Gradual dinosaur extinction and simultaneous ungulate radiation in the Hell Creek formation". Science. 232: 629–633. doi:10.1126/science.232.4750.629. Retrieved 2007-05-18. {{cite journal}}: Check |doi= value (help); Unknown parameter |isue= ignored (help)CS1 maint: multiple names: authors list (link)
15. Fassett, JE, Lucas, SG, Zielinski, RA, and Budahn, JR (2001). "Compelling new evidence for Paleocene dinosaurs in the Ojo Alamo Sandstone, San Juan Basin, New Mexico and Colorado, USA" (PDF). Catastrophic events and mass extinctions, Lunar and Planetary Contribution. 1053: 45–46. Retrieved 2007-05-18.{{cite journal}}: CS1 maint: multiple names: authors list (link)

References

• Vajda, V., Raine, I. & Hollis., C. (2001). Indication of Global deforestation at the Cretaceous-Tertiary Boundary by New Zealand Fernspike." Science 294: 1700-1702.
• Vajda, V. & McLoughlin S. (2004). Fungal Proliferation at the Cretaceous-Tertiary Boundary. Science, 303: 1489.
• Fortey, Richard. Life. New York: Vintage Books, 1998. 238-260.

External links

• Chicxulub
• Dinosaur dust-up
• The Deccan Traps Volcanism-Greenhouse Dinosaur Extinction Theory
• Deccan traps
• Understanding the K-T Boundary - NASA-related website
• Shiva crater: Chatterjee et al. 2002 Volcanism, India-Seychelles Rifting, Dinosaur Extinction, and Petroleum Entrapment at the KT Boundary (GSA abstract)
• List of 172+ impact craters in Earth Impact Database with Crater name, Diameter, Age, Country, Latitude, Longitude, etc.
• Air bubbles, amber, and dinosaurs
• Earth Impact Database
• "The KT Boundary" - BBC Radio 4 Broadcast, In Our Time, 23rd June 2005 - hosted by Melvyn Bragg (duration: approximately 45 minutes)
• "Chicxulub impact predates the K-T boundary mass extinction"
• "The Deccan Traps Volcanism-Greenhouse Dinosaur Extinction Theory"
• Tom Holtz' lecture notes (Holtz is a leading dinosaur expert)
• Richard Cowen's analysis of the K-T extinction
• UC Berkeley's survey of theories covers a wider range of theories, including some which are now discredited.

News

• BBC News: Dinosaur impact theory challenged
• National Geographic: Gerda Keller's claim that the Chixculub impact was 300,000 years before the K-T extinction
• Spaceref: More Than a Meteor Likely Killed Dinosaurs 65 Million Years Ago

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• Extinction events
• KT boundary
• Cretaceous
• Paleogene