Cretaceous-Tertiary extinction event

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Badlands near Drumheller, Alberta where erosion has exposed the KT boundary.
Badlands near Drumheller, Alberta where erosion has exposed the KT boundary.
KT boundary exposure (marked by the white line) in Trinidad Lake State Park
KT boundary exposure (marked by the white line) in Trinidad Lake State Park

The Cretaceous-Tertiary extinction was the rapid dying off of an extremely large number of species in a comparatively short period of time about 65.5 million years ago. It is also known as the K-T extinction event, and its geological signature is the K-T boundary ("K" is the traditional abbreviation for the Cretaceous Period, to avoid confusion with the Carboniferous Period, abbreviated as "C").

Contents

[edit] Casualties and survivors of the K-T extinction

[edit] Marine

Groups which became totally extinct include:

  • Ammonoids, which are currently thought to have fed on (zoo)plankton[citation needed].
  • Rudists, a group of clams which were the major reef-builders of the Cretaceous and also fed on plankton.
  • Inoceramids, giant relatives of modern scallops - they also fed on plankton.
  • Mosasaurs, giant aquatic reptiles which were the top marine predators.
  • Plesiosaurs, another group of large reptilian marine predators.

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

[edit] Terrestrial

Groups which became totally extinct include:

Groups which suffered heavy losses include:

  • Birds. Some groups became extinct, including Enantiornithes and Hesperornithiformes.
  • Marsupials. The Northern hemisphere family of marsupials became extinct, but those in Australia and South America survived.
  • Freshwater mussels and snails also suffered heavy losses in North America.

But some other groups were relatively unaffected:

[edit] Vegetation record

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).

[edit] Possible patterns and trends

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Despite its overall severity, the K-T extinction was rather patchy. This raises the question of why some groups died out while others did not.

There do seem to be some general trends:

  • Organisms which depended on photosynthesis became extinct or suffered heavy losses - from photosynthesing plankton (e.g. coccolithophorids) to land plants. And so did organisms whose food chain depended on photosynthesising organisms, e.g. tyrannosaurs (which ate vegetarian dinosaurs, which ate plants).
  • Organisms which built calcium carbonate shells became extinct or suffered heavy losses (coccolithophorids; many groups of molluscs, including ammonites, rudists, freshwater snails and mussels). And so 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.
  • 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 vegetarian 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. So 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. (See Sheehan and Fastovsky, Geology, v. 20, p. 556-560.)
  • 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.
  • No land animal larger than a cat survived.
  • The largest air-breathing survivors, crocodilians and champsosaurs, were semi-aquatic. Modern crocodilians can live as scavengers and can survive for as long as a year without a meal. And modern crocodilians' young are small, grow slowly and feed largely on invertebrates for their first few years - so they rely on a detritus-based food chain.

[edit] How long did the K-T extinction take?

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. And 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 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.
  • The little evidence we have suggests different patterns for the terrestrial and marine extinctions, and the marine extinction was complex.

[edit] Possible early Paleocene dinosaurs

Sloan et al have published papers (1986 onwards) which suggested 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.

Most scientists now dismiss the "Paleocene dinosaurs" as re-worked, i.e. washed out 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 ([1]). In the case of the "Paleocene dinosaurs":

  • All of the remains are fragments, which could have been re-worked. There are no complete dinosaurs and no collections of bone which could have come from one individual.
  • The vast majority of the dinosaur remains in Paleocene rocks are teeth, which are the most susceptible of all fossils to re-working because they are small and durable enough to survive without visible effects stresses which would destroy other bones.

[edit] Evidence about North American dinosaurs

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:

  • Some groups declined and others grew more diverse.
  • In the middle-late Campanian these formations show a greater diversity of dinosaurs than any other single group of rocks.
  • There is 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.

Although the Hell Creek, Lance and Scollard formations provide a wealth of information, they cover a relatively small area and it is dangerous to assume that they tell us what happened world-wide.

[edit] Evidence about marine extinctions

Pope, D'Hondt and Marshall (1998) summarised the evidence then available about marine extinctions:

  • The mass extinction of marine plankton appeared to be abrupt and right at the K-T boundary.
  • Marshall and Ward (1996) found a major extinction of ammonites at or near the K-T boundary, a smaller and slower extinction of ammonites associated with a marine regression shortly before that, gradual extinction of most inoceramid bivalves well before the K-T boundary and a small, gradual reduction in ammonite diversity throughout the very late Cretaceous.

Marshall and Ward's analysis shows that several processes were going on in the late Cretaceous seas and partially overlapped in time, and finished with an abrupt mass extinction.

[edit] Theories

This section concentrates on theories which scientists currently or recently have seriously considered. UC Berkeley's survey of theories covers a wider range of theories, including some which are now discredited.

[edit] What makes a good theory?

A good theory of the K-T extinction should:

  • explain all of the losses, not just focus on a few groups such as dinosaurs.
  • explain the selectivity of the extinction, i.e. why particular groups of organisms died out and why others survived.
  • provide killing mechanisms which are strong enough to cause a mass extinction but not a total extinction.
  • be based on events or processes that can be shown to have happened, not just inferred from the extinction.

[edit] Alvarez hypothesis

Artistic depiction of asteroidal impact
Artistic depiction of asteroidal impact

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. The impact theory can also be traced back to M. W. DeLaubenfels' "Dinosaur Extinctions: One More Hypothesis," a paper published in the Journal of Paleontology, Vol 30, No 1, p 207-218 January 1956. There is some question as to why Alvarez's 1980 paper does not give any credit to DeLaubenfels.

The Alvarez impact theory idea is supported by the composition of the K-T boundary layer:

  • chondritic meteorites and asteroids contain a much higher iridium concentration than the earth's crust because they have about the same concentration of iridium as the whole earth.
  • the isotopic composition of iridium in asteroids is similar to that of the K-T boundary layer but differs from that of iridium in the earth's crust.
  • chromium isotopic anomalies found in Cretaceous-Tertiary boundary sediments also strongly support the impact theory and suggest that the impact object must have been an asteroid or a comet composed of material similar to carbonaceous chondrites.
  • shocked quartz granules, glass spherules and tektites are common, 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.
  • 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.

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 [2]. 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 (Kring, 2000), 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.

[edit] Chicxulub Crater

Main article: Chicxulub Crater
Radar topography reveals the 180 kilometre (112 mile) wide ring of the crater
Radar topography reveals the 180 kilometre (112 mile) wide ring of the crater

One problem 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 150 to 200 kilometres in diameter,[citation needed] Earth's geological processes tend to hide or destroy craters over time.

But subsequent research found what many thought was "the smoking gun" - the Chicxulub Crater buried under Chicxulub on the coast of Yucatan. This crater is oval, with an average diameter of about 180km, about the size calculated by the Alvarez team. Its shape and location 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 USA - 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. The Chicxulub crater remains in the centre of a very large controversy.

[edit] Deccan Traps

Main article: 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. But Hofman, Féraud and Courtillot (2000) 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 still much slower than one caused entirely by an impact.

The killing mechanisms would have been:

  • dust which blocked sunlight and stopped photosynthesis.
  • sulphur gases which first formed aerosols which also blocked sunlight and then precipitated as acid rain.
  • carbon dioxide emissions which would have increased the greenhouse effect when the dust and aerosols cleared.

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 2. This, the Shiva crater, 450-600 kilometres in diameter, has also been dated at about 65 million years at the K-T boundary. The researchers suggest that 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. [3].

[edit] 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.

Note: Ma ("mega-annum") means million years.

[edit] 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. But Marshall and Ward's analysis (1996) suggests this was not enough to exterminate the ammonites.

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.
  • a continuing increase in dinosaur sizes, which suggests the total food available was not reduced even if its composition changed.

[edit] 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), that is not found in earth rocks. However, analysis of the boundary layer sediments revealed the absence of Pu-244, thus essentially disproving this hypothesis.

[edit] 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.

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.

[edit] Why did small predatory dinosaurs die out?

The widely-supported theories explain why large predatory dinosaurs became extinct - a catastrophe killed the vast majority of land plants, so large vegetarians and then large predators starved to death.

But if there were small predatory dinosaurs at the end of the Cretaceous, one might have expected them to survive because:

  • Their most likely prey (small invertebrates and mammals) survived.
  • Small animals are generally less vulnerable to extinction than large animals with similar life-styles, because they require a smaller quantity of food and other resources to support a viable breeding population.

There were small predatory dinosaurs in earlier times, e.g. chicken-sized Compsognathus (late Jurassic), Microraptor (early Cretaceous), Parvicursor (Campanian, i.e. first half of late Cretaceous). In fact it seems that the earliest known dinosaurs were all small predators.

The real puzzle is the apparent absence or extreme rarity of small predatory dinosaurs (other than birds) in the terrestrial Maastrichtian fossil beds. Troodon used to be cited as the main example of a late-Maastrichtian small predatory dinosaur, but the fact that its teeth were leaf-shaped has led many scientists to believe that it was probably a vegetarian. All the apparent small predatory dinosaur fossils found so far in the Hell Creek formation are teeth, which is a little suspicious - teeth are small but very durable fossils, so they are more prone than other fossils to being washed out of earlier beds and then re-buried.

At present we seem to face a dilemma:

  • If there were small predatory dinosaurs at the end of the Cretaceous, why did they die out at the same time as large dinosaurs?
  • If there were no small predatory dinosaurs at the end of the Cretaceous, why not? What was so unusual about the latest Cretaceous ecosystems?

[edit] See also

[edit] References

(1998) "Meteorite impact and the mass extinction of species at the Cretaceous/Tertiary boundary" 'PNAS' Vol. 95, Issue 19, 11028-11029

  • 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 (abstract online).
  • 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.

[edit] External links

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Cretaceous-Tertiary extinction event
Craters found at the K-T boundary
Boltysh crater Chicxulub Crater
Eagle Butte crater Shiva crater
Silverpit crater Vista Alegre crater

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