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

With some controversial exceptions, all non-avian dinosaurs became extinct during or immediately after the event; therefore, non-avian dinosaur fossils are not found later than the K–T boundary, save for a few fossils described as Paleocene dinosaurs whose dating are subject to intense discussion. Many other groups of animals and plants, including mosasaurs, plesiosaurs, pterosaurs, and some 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.

The cause of the event has centered on an large impact event, increased volcanic activity or other causes (in combination or separately). Several impact craters and massive volcanic activity in the Deccan traps have all been dated to the approximate time of the extinction event. These geological events caused extensive weather disruptions which reduced sunlight and led to reduced photosynthesis. Therefore, the amount of plant material available to herbivorous animals decreased, leading to a massive disruption in Earth's ecology.

Extinction patterns

Despite the severity of the K–T extinction event, there was significant variability in the rate of extinction between different classes of organisms. Organisms which depended on photosynthesis became extinct or suffered heavy losses due to reduced sunlight. Photosynthesizing organisms, from plankton (e.g. coccolithophorids) to land plants, formed the primary part of the food chain in the late Cretaceous. Herbivorous animals, which depended on plants and plankton as their food, died out as their food sources became scarce; consequently, top predators such as Tyrannosaurus rex also began dying.

Animals which built calcium carbonate shells (for example, coccolithophorids along with many groups of molluscs, including ammonites, rudists, freshwater snails and mussels), as well as organisms whose food chain depended on these calcium carbonate 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.

Omnivores, insectivores and carrion-eaters survived the extinction event, 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. Many mammals, and the birds which survived the extinction, fed on insects, larvae, worms, and snails, which in turn fed on dead plant and animal matter. They survived the collapse of plant-based food chains because they were a part of the "detritus-based" food chains.

In stream communities, few groups of animals became extinct. Stream communities tend to rely less on food from living plants and more on detritus that washes in from land. This 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 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 many animals living on or in the ocean floor feed on detritus or can switch to detritus feeding.

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 and dead organisms or fragments of organisms for their first few years. This has been linked to crocodilian survival at the end of the Cretaceous.

Microbiota

The K–T boundary represents one of the most dramatic turnovers in the fossil record for various calcareous nanoplankton (which formed the calcium deposits that gave the Cretaceous its name). The change in plankton is well-established at the species level. 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.

The K–T boundary record of dinoflagellates is not as well-understood, mainly because only two terrestrial sites have provided a fossil record of dinoflagellates. Furthermore, only cysts provide a fossil record, and not all dinoflagellate species have cyst-forming stages, thereby causing an underestimate of the total number of species.

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.

Benthic organisms, such as foraminifera, that depend on organic debris for nutrients, exhibited some extinctions in the early Paleocene as a result of lower biomass. However, as marine microbiota recovered, increased speciation of foraminifera resulted from the increase in nutrient sources.

Marine invertebrates

There is variability in the fossil record as to the extinction rate of marine invertebrates, but most fossils indicate significant rates. For example, approximately 60% of Cretaceous coral genera became extinct. Other invertebrate groups, including ammonoids (a group of shelled cephalopods), rudists (reef-building clams), and inoceramids (giant relatives of modern scallops), also became extinct. Many other invertebrate groups, including cephalopods, echinoderms, gastropods and other bivalves, and brachiopods, exhibited significant diminution in genera after the K-T boundary.

Marine vertebrates

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

There are substantial fossil records of jawed fishes across the K–T boundary, which provides good evidence of extinction patterns of these classes of marine vertebrates. Within cartilaginous fish, approximately 80% of the (sharks, rays, and skates) families survived. For teleost fishes (bony fishes) families, less than 10% became extinct.

Amphibians

There is no evidence of K–T boundary mass extinctions of amphibians, and there is strong evidence that most amphibians survived the event relatively unscathed. Several in-depth studies of salamander genera in fossil beds in Montana show that six of seven genera were unchanged after the event. Frog species appear to have survived into the Paleocene with little extinction; however, the fossil record for many of the families of frogs is uneven. An extensive survey of three genera of frogs in Montana show that all three were unaffected by the K–T event and survived apparently unchanged. The data shows little or no evidence for extinction of amphibian families that bracket the K–T event.

Terrestrial reptiles

The three living reptile groups, Crocodilia, Chelonia (turtles) and Lepidosauria (snakes, lizards, and worm lizards), along with champsosaurs (semi-aquatic archosauromorphs which died out in the early Oligocene) survived the K–T boundary, with only the lepidosaurs being primarily terrestrial. Archosaurs, except for birds and several crocodile families, were in significant decline before the K–T boundary. Groups which became totally extinct include all non-avian dinosaurs, which 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). However, all non-Neornithes birds became extinct, including flourishing groups like Enantiornithes and Hesperornithes.

Mammals

The Northern hemisphere family of marsupials became extinct, but those in Australia and South America survived. Other mammalian families, including monotremes (egg-laying mammals), multituberculates, and placentals, the ancestors of most modern mammals, survived the K–T event. Mammalian species began diversifying approximately 30 million years prior to the K-T boundary; however, further diversification actually stalled across the boundary, which indicates that mammals filled ecological niches during the Cretaceous that were least impacted by the extinction event. A few orders of mammals did diversify right at the K-T boundary, including Chiroptera (bats) and Cetartiodactyla (whales and dolphins and Even-toed ungulates), as a result of the reduced competition in those niches.

Terrestrial invertebrates

Insect damage to fossilized leaves from flowering plants, found in fourteen sites in North America as a proxy for insect diversity across the K–T boundary, were analyzed to determine the rate of extinction. Researchers found that Cretaceous sites, prior to the extinction event, had rich plant and insect-feeding diversity. However, during the early Paleocene, the flora was relatively diverse, with little predation from insects, even 1.7 million years after the extinction event.

Terrestrial plants

There is overwhelming evidence of global disruption of vegetation at the Cretaceous–Paleogene boundary. However, there are important regional differences in the signature of vegetation turnover. In North America, the data suggest massive devastation and mass extinction of plants at many Cretaceous–Paleogene boundary sections, although there were substantial megafloral changes before the boundary. In high southern hemisphere latitudes, such as New Zealand and Antarctica, mainly mass-kill of vegetation caused no significant turnover in species, but dramatic and short-term changes in the relative abundance of plant groups.

In North America, as many as 57% of plant species became 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).

Due to the wholesale destruction of vegetation at the K–T boundary, there was a significant proliferation of saprophytic organisms such as fungi which do not require photosynthesis and need organic substrates provided by the dead vegetation. The dominance of fungal species would have lasted only a few years until ferns and other plant species recovered once the atmosphere cleared and favored photosynthetic organisms.

Extinction evidence

North American fossils

The extinction event is best represented by the abundance of fossil records of dinosaur taxa prior to the K-T boundary, and the nearly complete absence of fossils immediately thereafter. Because of the rich biodiversity in North America during the late Cretaceous, rich, fossil-bearing strata can be found in several locations.

At present the best sequence of fossil-bearing rocks are found in Hell Creek, Lance and Scollard formations located in Montana, USA. The fossil beds are found in strata from about 83.5 MYA (million years ago) to 64.9 MYA, covering the Campanian and Maastrichtian ages of the Cretaceous through the beginning of the Paleocene period. They show changes in dinosaur populations over the last 18 million 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.

The middle–late Campanian formations show a greater diversity of dinosaurs than any other single group of rocks. The late Maastrichtian rocks contain the largest members of many major clades: Tyrannosaurus, Ankylosaurus, Pachycephalosaurus, Triceratops and Torosaurus, which suggests prey were plentiful immediately prior to the extinction.

In addition to rich dinosaur fossils, there are also plant fossils that illustrate the reduction in plant species across the K-T boundary. In the sediments below the K–T boundary the dominant plant remains are angiosperm pollen grains, but the actual boundary layer contains little pollen and is dominated by fern spores. Normal pollen levels gradually resume 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 fossils

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; however, there was a smaller and slower extinction of ammonite genera prior to the boundary that was associated with a late Cretaceous marine regression. 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

The length of time taken for the extinction to occur 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. The issue 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.

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.

Using estimates of the total amount of iridium in the K–T layer, and assuming that the asteroid contained the normal percentage of iridium found in chondrites, the Alvarez team went on to calculate the size of the asteroid. 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 animals 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 ten 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 ten 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 180 kilometres, 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 contributory, but not causal, to the K–T boundary mass extinction. However, many scientists reject Keller's analysis, some arguing that the 10 metre layer on top of the impact spherules should be attributed to tsunami activity resulting from impact.

Deccan Traps

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 68 MYA and lasted for over 2 million years. However, there is evidence that two-thirds of the Deccan Traps were created in 1 million years about 65.5 MYA, so these eruptions would have caused a fairly rapid extinction, possibly a period of thousands of years, a longer time period than one caused entirely by an impact event.

The Deccan Traps would have caused extinction through several mechanisms, 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.

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 kilometre diameter, 65.17 ± 0.64 MYA) in Ukraine; the Silverpit crater (20 kilometre diameter, 60–65 MYA) in the North Sea; the Eagle Butte crater (10 km diameter, <65 MYA) in Alberta, Canada; and the Vista Alegre crater (9.5 kilometre diameter, <65 MYA) in Paraná State, Brazil. Any other craters that might have formed in the Tethys waterway , would have been obscured by tectonic events like the relentless northward drift of Africa and India.

A very large crater has been recently reported in the sea floor off the west coast of India. The Shiva crater, 450–600 kilometre in diameter, has also been dated at about 65 MYA 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.

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.

Marine regression also resulted in the loss of epeiric seas, such as the Western Interior Seaway of North America. The loss of these seas greatly altered habitats, removing coastal plains that ten million years before had been host to diverse communities such as are found in rocks of the Dinosaur Park Formation. Another consequence was an expansion of freshwater environments as waterways now had longer distances to travel before reaching oceans. While this change was favorable to freshwater vertebrates, those that had marine ties, such as sharks, suffered.

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, the longest-lived plutonium isotope (with a half-life of 81 million years). Detectable traces of Pu would then be detected from rocks deposited at the time. However, analysis of the boundary layer sediments revealed the absence of Pu, thus essentially disproving this hypothesis.

Multiple causes

In a review article, J. David Archibald and David E. Fastovsky discussed a scenario combining three major postulated causes: volcanism, marine regression, and extraterrestrial impact. In this scenario, terrestrial and marine communities are stressed by the changes in and loss of habitats; dinosaurs, as the largest vertebrates, show stresses first, in declining diversity. At the same time, particulates from volcanism cool and dry areas of the globe. Then, an impact event occurred, causing collapses in photosynthesis-based food chains (already stressed on land) as under other the impact-alone hypothesis. The major difference between this hypothesis and single-cause hypotheses is that proponents view the suggested single causes as either not sufficient in strength to cause the extinctions or not likely to produce the taxonomic pattern of the extinction.

Possible post-KT extinction event dinosaurs

Main articles: Paleocene dinosaurs

Several researchers have stated that some dinosaurs survived into the Paleocene and therefore the extinction of dinosaurs was gradual. 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 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, however, describe these apparent "Paleocene dinosaurs" as re-worked, i.e. washed out of their original locations and then re-buried in much later sediments.

See also

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

Footnotes

1. Favstovsky, D.E., and Sheehan, P.M. (2005). "The extinction of the dinosaurs in North America". GSA Today. 15 (3): 4–10. doi:10.1130/1052-5173(2005)015%3C4:TEOTDI%3E2.0.CO;2. Retrieved 2007-05-18.{{cite journal}}: CS1 maint: multiple names: authors list (link)
2. Wilf, P & Johnson KR (2004). "Land plant extinction at the end of the Cretaceous: a quantitative analysis of the North Dakota megafloral record". Paleobiology. 30 (3): 347–368. doi:10.1666/0094-8373(2004)030%3C0347:LPEATE%3E2.0.CO;2.
3. Keller, G & MacLeod, N (1996). Cretaceous–Tertiary Mass Extinctions: Biotic and Environmental Changes. WW Norton & Company. ISBN 978-0393966572.{{cite book}}: CS1 maint: multiple names: authors list (link)
4. 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%3C0096:MPOUCN%3E2.0.CO;2. Retrieved 2007-06-17. {{cite journal}}: Cite has empty unknown parameter: |coauthors= (help)
5. ^ Sheehan, Peter (1986). "Detritus feeding as a buffer to extinction at the end of the Cretaceous". Geology. 14 (10): 868–870. Retrieved 2007-07-04. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
6. Sheehan, PM & Fastovsky, DE (1992). "Major extinctions of land-dwelling vertebrates at the Cretaceous–Tertiary boundary, eastern Montana". Geology. 20 (6): 556–560. Retrieved 2007-06-22.{{cite journal}}: CS1 maint: multiple names: authors list (link)
7. ^ MacLeod, N, Rawson, PF, Forey, PL, Banner, FT, Boudagher-Fadel, MK, Bown, PR, Burnett, JA, Chambers, P, Culver, S, Evans, SE, Jeffery, C, Kaminski, MA, Lord, AR, Milner, AC, Milner, AR, Morris, N, Owen, E, Rosen, BR, Smith, AB, Taylor, PD, Urquhart, E & Young, JR (1997). "The Cretaceous–Tertiary biotic transition". Journal of the Geological Society. 154 (2): 265–292. doi:10.1144/gsjgs.154.2.0265.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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14. Ward, PD, Kennedy, WJ, MacLeod, KG, and Mount, JF (1991). "Ammonite and inoceramid bivalve extinction patterns in Cretaceous/Tertiary boundary sections of the Biscay region (southwestern France, northern Spain)". Geology. 19 (12): 1181–1184.{{cite journal}}: CS1 maint: multiple names: authors list (link)
15. Chatterjee, Sankar (1989). "New plesiosaurs from the Upper Cretaceous of Antarctica". Geological Society, London, Special Publications. 47: 197–215. doi:10.1144/GSL.SP.1989.047.01.15. Retrieved 2007-07-04. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
16. Patterson, C (1993). Osteichthyes: Teleostei. In: The Fossil Record 2 (Benton, MJ, editor). Springer. ISBN 978-0412393808.
17. ^ Archibald, JD & Bryant, LJ (1990). "Differential Cretaceous–Tertiary extinction of nonmarine vertebrates; evidence from northeastern Montana. In: Global Catastrophes in Earth History: an Interdisciplinary Conference on Impacts, Volcanism, and Mass Mortality (Sharpton, VL and Ward, PD, editors)". Geological Society of America, Special Paper. 247: 549–562.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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19. ^ Dodson, P. (1996). The Horned Dinosaurs: A Natural History. Princeton: Princeton University Press. pp. 279–281. ISBN 0-691-05900-4.
20. Bininda-Emonds, ORP, Cardillo, M, Jones, KE, MacPhee, RDE, Beck, RMD, Grenyer, R, Price, SA, Vos, RA, Gittleman, JL Y Purvis, A (2007). "The delayed rise of present-day mammals". Nature. 446: 507–512. doi:10.1038/nature05634.{{cite journal}}: CS1 maint: multiple names: authors list (link)
21. Springer, MS, Murphy, WJ, Eizirik, E, & O'Brien, SJ (2003). "Placental mammal diversification and the Cretaceous–Tertiary boundary" (PDF). PNAS. 100 (3): 1056–1061. doi:10.1073/pnas.0334222100. Retrieved 2007-07-20.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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References

• Fortey, R (1999). Life: A Natural History of the First Four Billion Years of Life on Earth. Vintage. pp. 238–260. ISBN 978-0375702617.
• Fortey, R (2005). Earth: An Intimate History. Vintage. ISBN 978-0375706202.

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.
• 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

Template:Link FA

• Extinction events
• KT boundary
• Cretaceous
• Paleogene
• Dinosaurs
• Volcanoes
• Mammals
• Climate forcing agents
• Impact events
• Planetary science
• History of climate