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Cretaceous-Tertiary extinction event

2007 Schools Wikipedia Selection. Related subjects: Ancient History,
Classical History and Mythology; Geology and geophysics

   Badlands near Drumheller, Alberta where erosion has exposed the KT
   boundary.
   Enlarge
   Badlands near Drumheller, Alberta where erosion has exposed the KT
   boundary.
   KT boundary exposure in Trinidad Lake State Park
   Enlarge
   KT boundary exposure in Trinidad Lake State Park

   The Cretaceous-Tertiary extinction occurred about 65.5 million years
   ago. It is also known as the K-T extinction event and its geological
   signature as the K-T boundary ("K" is the traditional abbreviation for
   the Cretaceous Period, to avoid confusion with the Carboniferous
   Period, abbreviated as "C"). Since the label "Tertiary" is no longer
   recognized by most geologists (for example, the International
   Commission on Stratigraphy) as a geologic 'Period', the K-T demise
   might also be called the Cretaceous- Paleogene (or K-Pg) extinction
   event.

Significance of the K-T extinction

   The KT extinction event, labeled here as "End K", is shown in
   comparison to the impact of other events on the extinction intensity
   for marine fossilerferous genera.
   Enlarge
   The KT extinction event, labeled here as "End K", is shown in
   comparison to the impact of other events on the extinction intensity
   for marine fossilerferous genera.

   It was one of the "Big Five" extinctions. Although not the largest, it
   may be one of the most significant, because:
     * It caused a major change in both marine and land ecosystems. Before
       the K-T extinction about 50% of known marine species were sessile,
       and after it only about 33% were sessile. On the land it most
       famously exterminated the dinosaurs and so made it possible for
       mammals to become the dominant land vertebrates - and hence paved
       the way for human evolution.
     * It marks the end of the Cretaceous Period and the beginning of the
       Tertiary Period. In fact it changed ecosystems so much that it is
       regarded as the boundary between the Mesozoic and Cenozoic eras.

Casualties and survivors of the K-T extinction

Marine

   Groups which became totally extinct include:
     * Ammonoids, which are currently thought to have fed on plankton.
     * 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 lizards 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).

Terrestrial

   Groups which became totally extinct include:
     * Non- avian dinosaurs. Note: most paleontologists regard birds as
       the surviving dinosaurs.
     * Pterosaurs

   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.
     * 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).

   But some other groups were relatively unaffected:
     * Insects. These show no sign of reduced diversity, unlike at the
       Permo-Triassic extinction.
     * Amphibians
     * Turtles
     * Lepidosaurs. These include tuataras, lizards, snakes and
       amphisbaenians.
     * Champsosaurs (semi-aquatic archosauromorphs) - they later died out
       in the early Oligocene.
     * Crocodilians
     * Modern birds ( Aves)
     * Monotremes, egg-laying mammals.
     * Multituberculates, although they later became extinct in the early
       Oligocene.
     * Placentals, the ancestors of most modern mammals.

Is there a pattern?

   Despite its overall severity, the K-T extinction was rather patchy.
   This raises the question, "Why did some groups die out and others
   survive?"

   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 carnivorius
       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 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.

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). And it is difficult
   to resolve because of:
     * The Signor-Lipps effect, i.e. 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 shortage of 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.

   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 Maastrictian 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 happended world-wide.

Theories

   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.

Alvarez hypothesis

   Artistic depiction of asteroidal impact
   Enlarge
   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 the 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
       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
       10km (6 miles) in diameter, about the size of Manhattan. Such a
       large impact would have had approximately the force of
       100,000,000,000,000 tones of TNT, i.e. about 2,000,000 times as
       great as the most powerful H-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 vegetarians). 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 . This high O[2] level would have
   supported intense combustion. The level of atmospheric O[2] plummeted
   in the early Tertiary Period. If widespread fires occurred, they would
   have increased the CO[2] 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 (see
   Kring, D.A. GSA Today v. 10, no.8).

   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

   Radar topography reveals the 180 kilometre (112 mile) wide ring of the
   crater
   Enlarge
   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, 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 Carribbean 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 concentrated most of the blast and solid debris in the central
       part of the USA.

   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.

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. A partial reason for the rejection of the impact theory
   may have been a certain general distrust that a group of physicists was
   intruding into the paleontologists' domain of expertise.

   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. .

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.
     * Boltysh crater (24 km diam., 65.17 ± 0.64 Ma old) in Ukraine
     * Silverpit crater (20 km diam., 60-65 Ma old) in the North Sea
     * Eagle Butte crater (10 km diam., < 65 Ma old) in Alberta, Canada
     * Vista Alegre crater (9.5 km diam., < 65 Ma old) in Paraná State,
       Brazil

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

Maastrichtian 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
   likely 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.

   It 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 vegetarians 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.

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.

Composite theories

   The two 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 to-day'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.

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