The CRETACEOUS–PALEOGENE (K–PG) EXTINCTION EVENT, also known as
the CRETACEOUS–TERTIARY (K–T) EXTINCTION, was a mass extinction
of some three-quarters of the plant and animal species on
occurred over a geologically short period of time approximately 66
million years ago. With the exception of some ectothermic species
like the leatherback sea turtle and crocodiles , no tetrapods weighing
more than 25 kilograms (55 lb) survived. It marked the end of the
Cretaceous period and with it, the entire
Mesozoic Era, opening the
Cenozoic Era that continues today.
In the geologic record , the K–Pg event is marked by a thin layer
of sediment called the
K–Pg boundary , which can be found throughout
the world in marine and terrestrial rocks. The boundary clay shows
high levels of the metal iridium , which is rare in the Earth\'s crust
but abundant in asteroids .
As originally proposed in 1980 by a team of scientists led by Luis
Walter Alvarez , it is now generally thought that the
K–Pg extinction was caused by a massive comet or asteroid impact ,
estimated to be 10 km (6.2 mi) to 15 km (9.3 mi) wide, 66 million
years ago and its catastrophic effects on the global environment,
including a lingering impact winter that made it impossible for plants
and plankton to carry out photosynthesis . The impact hypothesis,
also known as the
Alvarez hypothesis , was bolstered by the discovery
of the 180-kilometre-wide (112 mi)
Chicxulub crater in the Gulf of
Mexico in the early 1990s, which provided conclusive evidence that
K–Pg boundary clay represented debris from an asteroid impact.
The fact that the extinctions occurred at the same time as the impact
provides strong situational evidence that the K–Pg extinction was
caused by the asteroid. It was possibly accelerated by the creation
Deccan Traps . However, some scientists maintain the extinction
was caused or exacerbated by other factors, such as volcanic
eruptions, climate change, or sea level change, separately or
A wide range of species perished in the K–Pg extinction. The
best-known victims are the non-avian dinosaurs . However, the
extinction also destroyed a plethora of other terrestrial organisms,
including certain mammals , pterosaurs , birds , lizards, insects,
and plants. In the oceans, the K–Pg extinction killed off
plesiosaurs and the giant marine lizards (Mosasauridae ) and
devastated fish, sharks , mollusks (especially ammonites , which
became extinct) and many species of plankton. It is estimated that 75%
or more of all species on
Earth vanished. Yet the devastation caused
by the extinction also provided evolutionary opportunities. In the
wake of the extinction, many groups underwent remarkable adaptive
radiations —a sudden and prolific divergence into new forms and
species within the disrupted and emptied ecological niches resulting
from the event. Mammals in particular diversified in the
producing new forms such as horses, whales, bats, and primates. Birds,
fish and perhaps lizards also radiated.
* 1 Microbiota
* 2.3 Terrestrial invertebrates
* 2.4 Terrestrial plants
* 2.5 Amphibians
* 2.6 Non-archosaur reptiles
* 2.7 Archosaurs
* 2.7.1 Crocodyliforms
* 2.7.4 Birds
* 2.7.5 Non-avian dinosaurs
* 2.8 Mammals
* 3 Evidence
* 3.1 North American fossils
* 3.2 Marine fossils
* 3.3 Megatsunamis
* 4 Duration
* 5 Chicxulub impact
* 5.1 Evidence for impact
* 5.2 Effects of impact
* 6 Alternative hypotheses
* 6.2 Multiple impact event
Maastrichtian sea-level regression
* 6.4 Multiple causes
* 7 Recovery and radiation
* 8 See also
* 9 References and notes
* 10 Further reading
* 11 External links
K–Pg boundary represents one of the most dramatic turnovers in
the fossil record for various calcareous nanoplankton that formed the
calcium deposits that gave the
Cretaceous its name. The turnover in
this group is clearly marked at the species level. 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–Pg boundary record of
dinoflagellates is not as well understood, mainly because only
microbial cysts provide a fossil record, and not all dinoflagellate
species have cyst-forming stages, thereby likely causing diversity to
be underestimated. Recent studies indicate that there were no major
shifts in dinoflagellates through the boundary layer.
Marine extinction intensity during the Phanerozoic
% Millions of years ago (H) K–Pg Tr–J P–Tr Cap Late D
O–S The blue graph shows the apparent percentage
(not the absolute number) of marine animal genera becoming extinct
during any given time interval. It does not represent all marine
species, just those that are readily fossilized. The labels of the
traditional "Big Five" extinction events and the more recently
End-Capitanian extinction event are clickable hyperlinks;
Extinction event for more details. (source and image info )
The K–Pg extinction event was severe, global, rapid, and selective.
In terms of severity, the event eliminated a vast number of species.
Based on marine fossils, it is estimated that 75% or more of all
species were made extinct by the K–Pg extinction event.
The event appears to have affected all continents at the same time.
Non-avian dinosaurs, for example, are known from the
North America, Europe, Asia, Africa,
South America and Antarctica,
but are unknown from the
Cenozoic anywhere in the world. Similarly,
fossil pollen shows devastation of the plant communities in areas as
far apart as New Mexico, Alaska, China, and New Zealand.
Even though the boundary event was severe, there was significant
variability in the rate of extinction between and within different
Species that depended on photosynthesis declined or became
extinct as atmospheric particles blocked sunlight and reduced the
solar energy reaching the Earth's surface. This plant extinction
caused a major reshuffling of the dominant plant groups.
insectivores and carrion -eaters survived the extinction event,
perhaps because of the increased availability of their food sources.
No purely herbivorous or carnivorous mammals seem to have survived.
Rather, the surviving mammals and birds fed on insects , worms , and
snails, which in turn fed on dead plant and animal matter. Scientists
hypothesize that these organisms survived the collapse of plant-based
food chains because they fed on detritus (non-living organic
In stream communities , few animal groups became extinct because such
communities rely less directly on food from living plants and more on
detritus that washes in from the land, buffering 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 animals living on or in the ocean floor feed on
detritus or can switch to detritus feeding.
mollusks (including ammonites , rudists , freshwater snails and
mussels ), and those organisms whose food chain included these shell
builders, became extinct or suffered heavy losses. For example, it is
thought that ammonites were the principal food of mosasaurs , a group
of giant marine reptiles that became extinct at the boundary. The
largest air-breathing survivors of the event, crocodyliforms and
champsosaurs , were semi-aquatic and had access to detritus. Modern
crocodilians can live as scavengers and can survive for months without
food, and their young are small, grow slowly, and feed largely on
invertebrates and dead organisms or fragments of organisms for their
first few years. These characteristics have been linked to crocodilian
survival at the end of the Cretaceous.
After the K–Pg extinction event, biodiversity required substantial
time to recover, despite the existence of abundant vacant ecological
Radiolaria have left a geological record since at least the
Ordovician times, and their mineral fossil skeletons can be tracked
across the K–Pg boundary. There is no evidence of mass extinction of
these organisms, and there is support for high productivity of these
species in southern high latitudes as a result of cooling temperatures
in the early Paleocene. 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–Pg boundary.
The occurrence of planktonic foraminifera across the K–Pg boundary
has been studied since the 1930s. Research spurred by the possibility
of an impact event at the
K–Pg boundary resulted in numerous
publications detailing planktonic foraminiferal extinction at the
boundary. However, there is debate ongoing between groups that think
the evidence indicates substantial extinction of these species at the
K–Pg boundary, and those who think the evidence supports multiple
extinctions and expansions through the boundary.
Numerous species of benthic foraminifera became extinct during the
K–Pg extinction event, presumably because they depend on organic
debris for nutrients, since the biomass in the ocean is thought to
have decreased. However, as the marine microbiota recovered, it is
thought that increased speciation of benthic foraminifera resulted
from the increase in food sources.
Phytoplankton recovery in the
Paleocene provided the food source to support large benthic
foraminiferal assemblages, which are mainly detritus-feeding. Ultimate
recovery of the benthic populations occurred over several stages
lasting several hundred thousand years into the early Paleocene.
Discoscaphites iris ammonite from the Owl Creek Formation (Upper
Cretaceous), Owl Creek, Ripley, Mississippi.
There is significant variation in the fossil record as to the
extinction rate of marine invertebrates across the K–Pg boundary.
The apparent rate is influenced by the lack of fossil records rather
than actual extinction.
Ostracods , a class of small crustaceans that were prevalent in the
upper Maastrichtian, left fossil deposits in a variety of locations. A
review of these fossils shows that ostracod diversity was lower in the
Paleocene than any other time in the Cenozoic. However, current
research cannot ascertain whether the extinctions occurred prior to or
during the boundary interval itself.
Approximately 60% of late-
Scleractinia coral genera failed
to cross the
K–Pg boundary into the Paleocene. Further analysis of
the coral extinctions shows that approximately 98% of colonial
species, ones that inhabit warm, shallow tropical waters, became
extinct. The solitary corals, which generally do not form reefs and
inhabit colder and deeper (below the photic zone ) areas of the ocean
were less impacted by the K–Pg boundary. Colonial coral species rely
upon symbiosis with photosynthetic algae , which collapsed due to the
events surrounding the K–Pg boundary. However, the use of data
from coral fossils to support K–Pg extinction and subsequent
Paleocene recovery must be weighed against the changes that occurred
in coral ecosystems through the K–Pg boundary.
The numbers of cephalopod , echinoderm , and bivalve genera exhibited
significant diminution after the K–Pg boundary. Most species of
brachiopods , a small phylum of marine invertebrates, survived the
K–Pg extinction event and diversified during the early Paleocene.
Rudist bivalves from the Late
Cretaceous of the Omani Mountains,
United Arab Emirates. Scale bar is 10 mm
Except for nautiloids (represented by the modern order
and coleoids (which had already diverged into modern octopodes ,
squids , and cuttlefish ) all other species of the molluscan class
Cephalopoda became extinct at the K–Pg boundary. These included the
ecologically significant belemnoids , as well as the ammonoids , a
group of highly diverse, numerous, and widely distributed shelled
cephalopods. Researchers have pointed out that the reproductive
strategy of the surviving nautiloids, which rely upon few and larger
eggs, played a role in outsurviving their ammonoid counterparts
through the extinction event. The ammonoids utilized a planktonic
strategy of reproduction (numerous eggs and planktonic larvae), which
would have been devastated by the K–Pg extinction event. Additional
research has shown that subsequent to this elimination of ammonoids
from the global biota, nautiloids began an evolutionary radiation into
shell shapes and complexities theretofore known only from ammonoids.
Approximately 35% of echinoderm genera became extinct at the K–Pg
boundary, although taxa that thrived in low-latitude, shallow-water
environments during the late
Cretaceous had the highest extinction
rate. Mid-latitude, deep-water echinoderms were much less affected at
the K–Pg boundary. The pattern of extinction points to habitat loss,
specifically the drowning of carbonate platforms , the shallow-water
reefs in existence at that time, by the extinction event.
Other invertebrate groups, including rudists (reef-building clams)
and inoceramids (giant relatives of modern scallops ), also became
extinct at the K–Pg boundary.
This article NEEDS ATTENTION FROM AN EXPERT IN PALAEONTOLOGY OR
FISH. The specific problem is: THE PARAGRAPH ON SHARKS AND SURVIVAL
THROUGH THE K-T EVENT SIMPLY DOES NOT MAKE SENSE. IT CONTRADICTS THE
ARTICLE ON SHARKS, AND SELF-CONTRADICTS WITH RESPECT TO BOTH SHARKS
AND BATOIDS. IT ALSO REQUIRES SOME LANGUAGE CLEANUP. WikiProject
Palaeontology or WikiProject
Fish may be able to help recruit an
expert. (March 2017)
There are substantial fossil records of jawed fishes across the
K–Pg boundary, which provides good evidence of extinction patterns
of these classes of marine vertebrates. While the deep sea realm was
able to remain seemingly unaffected, there was an equal loss between
the open marine apex predators and the durophagous demersal feeders on
the continental shelf.
Within cartilaginous fish , approximately 7 out of the 41 families of
neoselachians (modern sharks , skates and rays) disappeared after this
event and batoids (skates and rays) lost nearly all the identifiable
species, while more than 90% of teleost fish (bony fish) families
Maastrichtian age, 28 shark families and 13 batoid families
thrived, of which 25 and 9 survived the K-T boundary event,
respectively. Forty-seven of all neoselachian genera cross the K/T
boundary, 85% being sharks. Batoids display with 15% a comparably low
There is evidence of a mass kill of bony fishes at a fossil site
immediately above the
K–Pg boundary layer on
Seymour Island near
Antarctica, apparently precipitated by the K–Pg extinction event.
However, the marine and freshwater environments of fishes mitigated
environmental effects of the extinction event.
Insect damage to the fossilized leaves of flowering plants from
fourteen sites in North America were used as a proxy for insect
diversity across the
K–Pg boundary and 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, flora were relatively diverse
with little predation from insects, even 1.7 million years after the
There is overwhelming evidence of global disruption of plant
communities at the K–Pg boundary. Extinctions are seen both in
studies of fossil pollen, and fossil leaves. In North America, the
data suggests massive devastation and mass extinction of plants at the
K–Pg boundary sections, although there were substantial megafloral
changes before the boundary. In North America, approximately 57% of
plant species became extinct. In high southern hemisphere latitudes,
such as New Zealand and Antarctica, the mass die-off of flora caused
no significant turnover in species, but dramatic and short-term
changes in the relative abundance of plant groups. In some regions,
Paleocene recovery of plants began with recolonizations by fern
species, represented as a fern spike in the geologic record; this same
pattern of fern recolonization was observed after the 1980 Mount St.
Helens eruption .
Due to the wholesale destruction of plants at the K–Pg boundary,
there was a proliferation of saprotrophic organisms, such as fungi ,
that do not require photosynthesis and use nutrients from decaying
vegetation. The dominance of fungal species lasted only a few years
while the atmosphere cleared and there was plenty of organic matter to
feed on. Once the atmosphere cleared, photosynthetic organisms, like
ferns and other plants, returned.
Polyploidy appears to have enhanced
the ability of flowering plants to survive the extinction, probably
because the additional copies of the genome such plants possessed
allowed them to more readily adapt to the rapidly changing
environmental conditions that followed the impact.
There is limited evidence for extinction of amphibians at the K–Pg
boundary. A study of fossil vertebrates across the
K–Pg boundary in
Montana concluded that no species of amphibian became extinct. Yet
there are several species of
Maastrichtian amphibian, not included as
part of this study, which are unknown from the Paleocene. These
include the frog Theatonius lancensis and the albanerpetontid
Albanerpeton galaktion; therefore some amphibians do seem to have
become extinct at the boundary. The relatively low levels of
extinction seen among amphibians probably reflect the low extinction
rates seen in freshwater animals.
Kronosaurus Hunt by Dmitry Bogdanov, 2008. Large marine
reptiles, including plesiosaurians like these, became extinct at the
end of the Cretaceous.
The two living non-archosaurian reptile taxa, testudines (turtles)
and lepidosaurians (lizards and tuataras ), along with choristoderes
(semi-aquatic archosauromorphs that would die out in the early Miocene
), survived across the K–Pg boundary. Over 80% of
species passed through the K–Pg boundary. Additionally, all six
turtle families in existence at the end of the
Paleogene and are represented by living species. Living
lepidosaurs include the tuataras (the only living rhynchocephalians )
and the squamates . The rhynchocephalians were a widespread and
relatively successful group of lepidosaurians during the early
Mesozoic, but began to decline by the mid-Cretaceous, though they were
very successful in the Late
South America . They are
represented today by a single genus located exclusively in New Zealand
The order Squamata, which is represented today by lizards, including
snakes and amphisbaenians (worm lizards), radiated into various
ecological niches during the
Jurassic and was successful throughout
the Cretaceous. They survived through the
K–Pg boundary and are
currently the most successful and diverse group of living reptiles
with more than 6,000 extant species. Many families of terrestrial
squamates became extinct at the boundary, such as monstersaurians and
polyglyphanodonts , and fossil evidence indicates they suffered very
heavy losses in the KT event, only recovering 10 million years after
it. Giant non-archosaurian aquatic reptiles such as mosasaurs and
plesiosaurs , which were the top marine predators of their time,
became extinct by the end of the Cretaceous. The ichthyosaurs had
already disappeared before the mass extinction occurred.
The archosaur clade includes two surviving groups, crocodilians and
birds , along with the various extinct groups of non-avian dinosaurs
and pterosaurs .
Ten families of crocodilians or their close relatives are represented
Maastrichtian fossil records, of which five died out prior to
the K–Pg boundary. Five families have both
Paleocene fossil representatives. All of the surviving families of
crocodyliforms inhabited freshwater and terrestrial
environments—except for the
Dyrosauridae , which lived in freshwater
and marine locations. Approximately 50% of crocodyliform
representatives survived across the K–Pg boundary, the only apparent
trend being that no large crocodiles survived. Crocodyliform
survivability across the boundary may have resulted from their aquatic
niche and ability to burrow, which reduced susceptibility to negative
environmental effects at the boundary. Jouve and colleagues suggested
in 2008 that juvenile marine crocodyliforms lived in freshwater
environments like modern marine crocodile juveniles, which would have
helped them survive where other marine reptiles became extinct;
freshwater environments were not as strongly affected by the K–Pg
extinction event as marine environments.
Choristodera , a generally crocodile-like group of uncertain
phylogeny (possibly archosaurian) also survived the event, only to
become extinct in the Miocene. Studies on
Champsosaurus ' palatal
teeth suggest that there were dietary changes among the various
species across the KT event.
One family of pterosaurs ,
Azhdarchidae , was definitely present in
Maastrichtian , and it likely became extinct at the K–Pg
boundary. These large pterosaurs were the last representatives of a
declining group that contained 10 families during the mid-Cretaceous.
Several other pterosaur lineages may have been present during the
Maastrichtian, such as the ornithocheirids , pteranodontids and/or
nyctosaurids , as well as a possible tapejarid , though they are
represented by fragmentary remains that are difficult to assign to any
given group. While this was occurring, modern birds were undergoing
diversification; traditionally it was thought that they replaced
archaic birds and pterosaur groups, possibly due to direct
competition, or they simply filled empty niches, but there is no
correlation between pterosaur and avian diversities that are
conclusive to a competition hypothesis, and small pterosaurs were
present in the Late Cretaceous.
Most paleontologists regard birds as the only surviving dinosaurs
Origin of birds ). It is thought that all non-avian theropods
became extinct, including then-flourishing groups like
enantiornithines and hesperornithiforms . Several analyses of bird
fossils show divergence of species prior to the K–Pg boundary, and
that duck, chicken and ratite bird relatives coexisted with non-avian
dinosaurs. Large collections of bird fossils representing a range of
different species provides definitive evidence for the persistence of
archaic birds to within 300,000 years of the K–Pg boundary. The
absence of these birds in the
Paleogene is evidence that a mass
extinction of archaic birds took place there. A small fraction of the
Cretaceous bird species survived the impact, giving rise to today's
birds. The only bird group known for certain to have survived the
K–Pg boundary is the
Aves . Avians may have been able to survive
the extinction as a result of their abilities to dive, swim, or seek
shelter in water and marshlands. Many species of avians can build
burrows, or nest in tree holes or termite nests, all of which provided
shelter from the environmental effects at the K–Pg boundary.
Long-term survival past the boundary was assured as a result of
filling ecological niches left empty by extinction of non-avian
Tyrannosaurus was among the dinosaurs living on
Earth before the
Excluding a few controversial claims, scientists agree that all
non-avian dinosaurs became extinct at the K–Pg boundary. The
dinosaur fossil record has been interpreted to show both a decline in
diversity and no decline in diversity during the last few million
years of the Cretaceous, and it may be that the quality of the
dinosaur fossil record is simply not good enough to permit researchers
to distinguish between the options. There is no evidence that late
Maastrichtian non-avian dinosaurs could burrow, swim or dive, which
suggests they were unable to shelter themselves from the worst parts
of any environmental stress that occurred at the K–Pg boundary. It
is possible that small dinosaurs (other than birds) did survive, but
they would have been deprived of food, as herbivorous dinosaurs would
have found plant material scarce and carnivores would have quickly
found prey in short supply.
The growing consensus about the endothermy of dinosaurs (see dinosaur
physiology ) helps to understand their full extinction in contrast
with their close relatives, the crocodilians . Ectothermic
("cold-blooded") crocodiles have very limited needs for food (they can
survive several months without eating) while endothermic
("warm-blooded") animals of similar size need much more food to
sustain their faster metabolism. Thus, under the circumstances of food
chain disruption previously mentioned, non-avian dinosaurs died,
while some crocodiles survived. In this context, the survival of other
endothermic animals, such as some birds and mammals, could be due,
among other reasons, to their smaller needs for food, related to their
small size at the extinction epoch.
Whether the extinction occurred gradually or suddenly has been
debated, as both views have support from the fossil record. A study of
29 fossil sites in Catalan
Pyrenees of Europe in 2010 supports the
view that dinosaurs there had great diversity until the asteroid
impact, with over 100 living species. However, more recent research
indicates that this figure is obscured by taphonomical biases and the
sparsity of the continental fossil record. The results of this study,
which were based on estimated real global biodiversity, showed that
between 628 and 1078 non-avian dinosaur species were alive at the end
Cretaceous and underwent sudden extinction after the
Paleogene extinction event. Alternatively,
interpretation based on the fossil-bearing rocks along the Red Deer
Alberta supports the gradual extinction of non-avian
dinosaurs; during the last 10 million years of the
there, the number of dinosaur species seems to have decreased from
about 45 to about 12. Other scientists have pointed out the same.
Several researchers support the existence of
Paleocene dinosaurs .
Evidence of this existence is based on the discovery of dinosaur
remains in the
Hell Creek Formation up to 1.3 m (4.3 ft) above and 40
thousand years later than the K–Pg boundary. Pollen samples
recovered near a fossilized hadrosaur femur recovered in the Ojo Alamo
Sandstone at the San Juan River indicate that the animal lived during
the Cenozoic, approximately 7015203546520000000♠64.5 Ma (about 1
million years after the K–Pg extinction event). If their existence
K–Pg boundary can be confirmed, these hadrosaurids would be
considered a dead clade walking . Scientific consensus is that these
fossils were eroded from their original locations and then re-buried
in much later sediments (also known as reworked fossils). Hell
Cretaceous mammalian lineages, including monotremes
(egg-laying mammals), multituberculates , metatherians , eutherians ,
dryolestoideans , and gondwanatheres survived the K–Pg extinction
event, although they suffered losses. In particular, metatherians
largely disappeared from North America, and the Asian deltatheroidans
became extinct. In the
Hell Creek beds of North America, at least
half of the ten known multituberculate species and all eleven
metatherians species are not found above the boundary.
Multituberculates in Europe and North America survived relatively
unscathed and quickly bounced back in the Palaeocene, but Asian forms
were decimated, never again to represent a significant component on
Mammalian species began diversifying approximately 30 million years
prior to the K–Pg boundary. Diversification of mammals stalled
across the boundary. Current research indicates that mammals did not
explosively diversify across the K–Pg boundary, despite the
environment niches made available by the extinction of dinosaurs.
Several mammalian orders have been interpreted as diversifying
immediately after the K–Pg boundary, including
Chiroptera (bats) and
Cetartiodactyla (a diverse group that today includes whales and
dolphins and even-toed ungulates ), although recent research
concludes that only marsupial orders diversified after the K–Pg
K–Pg boundary mammalian species were generally small, comparable in
size to rats ; this small size would have helped them find shelter in
protected environments. In addition, it is postulated that some early
monotremes, marsupials, and placentals were semiaquatic or burrowing,
as there are multiple mammalian lineages with such habits today. Any
burrowing or semiaquatic mammal would have had additional protection
K–Pg boundary environmental stresses.
NORTH AMERICAN FOSSILS
In North American terrestrial sequences, the extinction event is best
represented by the marked discrepancy between the rich and relatively
Maastrichtian palynomorph record and the post-boundary
At present the most informative sequence of dinosaur-bearing rocks in
the world from the
K–Pg boundary is found in western North America,
particularly the late
Hell Creek Formation of
Montana . This formation, when compared with the older (approximately
75 Ma) Judith River /
Dinosaur Park Formations (from
Alberta respectively) provides information on the changes in dinosaur
populations over the last 10 million years of the Cretaceous. These
fossil beds are geographically limited, covering only part of one
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 several major clades:
Torosaurus , which suggests food was plentiful immediately prior to
In addition to rich dinosaur fossils, there are also plant fossils
that illustrate the reduction in plant species across the K–Pg
boundary. In the sediments below the
K–Pg boundary the dominant
plant remains are angiosperm pollen grains, but the actual boundary
layer contains little pollen and is dominated by fern spores. More
usual pollen levels gradually resume above the boundary layer. This is
reminiscent of areas blighted by modern volcanic eruptions, where the
recovery is led by ferns, which are later replaced by larger
The mass extinction of marine plankton appears to have been abrupt
and right at the K–Pg boundary.
Ammonite genera became extinct at
or near the K–Pg 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–Pg
boundary, and a small, gradual reduction in ammonite diversity
occurred throughout the very late Cretaceous. Further analysis shows
that several processes were in progress in the late
and partially overlapped in time, then ended with the abrupt mass
extinction. The diversity of marine life decreased when the climate
near the K-T boundary increased in temperature. The temperature
increased about three to four degrees very rapidly between 65.4 and
65.2 million years ago, which is around the time of the extinction
event. Not only did the climate temperature increase, but the water
temperature decreased causing a drastic decrease in marine diversity.
The scientific consensus is that the asteroid impact at the K–Pg
boundary left tsunami deposits and sediments around the area of the
Caribbean Sea and Gulf of Mexico. These deposits have been identified
in the La Popa basin in northeastern
Mexico , platform carbonates in
northeastern Brazil, and Atlantic deep-sea sediments. The
megatsunami has been estimated to be over 100 metres (330 ft) tall, as
the asteroid fell in an area of relatively shallow sea; in deep sea it
would have been 4.6 kilometres (2.9 mi) tall.
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 of the
Signor–Lipps effect ; that is, the fossil record is so incomplete
that most extinct species probably died out long after the most recent
fossil that has been found. Scientists have also found very few
continuous beds of fossil-bearing rock which cover a time range from
several million years before the K–Pg extinction to a few million
years after it. The sedimentation rate and thickness of K-Pg clay
from three sites suggest short duration of event, perhaps less than
ten thousand years.
Cretaceous–Paleogene boundary ,
Alvarez hypothesis ,
EVIDENCE FOR IMPACT
Location of Chicxulub crater,
In 1980, a team of researchers consisting of
Nobel Prize –winning
physicist Luis Alvarez , his son geologist
Walter Alvarez , and
Frank Asaro and
Helen Michel discovered that sedimentary
layers found all over the world at the Cretaceous–
contain a concentration of iridium many times greater than normal (30,
160 and 20 times in three sections originally studied).
extremely rare in Earth\'s crust because it is a siderophile element ,
and therefore most of it traveled with the iron as it sank into
Earth\'s core during planetary differentiation . As iridium remains
abundant in most asteroids and comets , the Alvarez team suggested
that an asteroid struck the
Earth at the time of the K–Pg boundary.
There were earlier speculations on the possibility of an impact event
, but this was the first hard evidence of an impact. The K–Pg
boundary exposure in
Trinidad Lake State Park , in the
Raton Basin of
Colorado , shows an abrupt change from dark- to light-colored rock.
White line added to mark the transition.
This hypothesis was viewed as radical when first proposed, but
additional evidence soon emerged. The boundary clay was found to be
full of minute spherules of rock, crystallized from droplets of molten
rock formed by the impact.
Shocked quartz and other minerals were
also identified in the K–Pg boundary. Shocked minerals have their
internal structure deformed, and are created by intense pressures such
as those associated with nuclear blasts or meteorite impacts. The
identification of giant tsunami beds along the Gulf Coast and the
Caribbean also provided evidence for impact, and suggested that the
impact may have occurred nearby—as did the discovery that the K–Pg
boundary became thicker in the southern United States, with
meter-thick beds of debris occurring in northern New Mexico.
Radar topography reveals the 180 km –wide (112 mi) ring of the
Chicxulub Crater .
Further research identified the giant
Chicxulub crater , buried under
Chicxulub on the coast of Yucatán , as the source of the K–Pg
boundary clay. Identified in 1990 based on work by geophysicist Glen
Penfield in 1978, the crater is oval, with an average diameter of
roughly 180 kilometres (110 mi), about the size calculated by the
Alvarez team. The discovery of the crater—a necessary prediction of
the impact hypothesis—provided conclusive evidence for a K–Pg
impact, and strengthened the hypothesis that the extinction was caused
by an impact.
In 2007, a hypothesis was put forth that argued the impactor that
killed the dinosaurs belonged to the
Baptistina family of asteroids.
Concerns have been raised regarding the reputed link, in part because
very few solid observational constraints exist of the asteroid or
family. Indeed, it was recently discovered that 298 Baptistina does
not share the same chemical signature as the source of the K–Pg
impact. Although this finding may make the link between the
Baptistina family and K–Pg impactor more difficult to substantiate,
it does not preclude the possibility. A 2011 WISE study of reflected
light from the asteroids of the family estimated the break-up at 80
Ma, giving it insufficient time to shift orbits and impact the Earth
by 66 Ma.
In a 2013 paper,
Paul Renne of the Berkeley Geochronology Center
reported that the date of the asteroid event is
7001660430000000000♠66.043±0.011 million years ago, based on
argon–argon dating . He further posits that the mass extinction
occurred within 32,000 years of this date.
EFFECTS OF IMPACT
In March 2010, an international panel of scientists endorsed the
asteroid hypothesis, specifically the Chicxulub impact, as being the
cause of the extinction. A team of 41 scientists reviewed 20 years of
scientific literature and in so doing also ruled out other theories
such as massive volcanism . They had determined that a
10-to-15-kilometre (6.2 to 9.3 mi) space rock hurtled into
Chicxulub on Mexico's
Yucatán Peninsula . The collision would have
released the same energy as 100 teratonnes of TNT (420 ZJ ), over a
billion times the energy of the atomic bombings of Hiroshima and
The consequences of the Chicxulub impact were of global extent. Some
of these phenomena were brief occurrences that immediately followed
the impact, but there were also long-term geochemical and climatic
disruptions that were catastrophic to the ecology.
The reentry of ejecta into Earth's atmosphere would include a brief
(hours long) but intense pulse of infrared radiation , killing exposed
organisms. A paper in 2013 by a prominent nuclear winter modeler
suggested that the global debris layer deposited by the impact
contains enough soot to hint that the entire terrestrial biosphere
burned, with an implication of this being that this would have caused
a global soot-cloud blocking out the sun, creating the nuclear winter
effect. However the suggestion that global firestorms occurred is
debated, with opponents arguing that while ferocious fires probably
did result locally, ferocious fires do not necessarily equal
firestorms , and any such ferocious fires were instead limited to, the
immediate American continent. This disagreement between researchers is
termed the "Cretaceous-Palaeogene firestorm debate ."
Aside from the hypothesized fire/nuclear winter effects, the impact
would have inhibited photosynthesis by creating a dust cloud that
blocked sunlight for up to a year. Further, the asteroid struck a
region of sulfur-rich carbonate rock, much of which was vaporized,
thereby injecting sulfuric acid aerosols into the stratosphere , which
might have reduced sunlight reaching the Earth's surface by more than
50%, and would have caused rain and ocean water to become acidic.
The acidification of the oceans would kill many organisms that build
shells from calcium carbonate . At Brazos section, the paleo-sea
surface temperature dropped as much as 7℃ for decades after the
impact. It would take at least ten years for such aerosols to
dissipate, and would account for the extinction of plants and
phytoplankton , and of organisms dependent on them (including
predatory animals as well as herbivores ). Some creatures whose food
chains were based on detritus would have a reasonable chance of
If widespread fires occurred, they would have increased the CO
2 content of the atmosphere and caused a temporary greenhouse effect
once the dust clouds and aerosol settled, and this would have
exterminated the most vulnerable organisms that survived the period
immediately after the impact.
Most paleontologists now agree that an asteroid did hit the
approximately the end of the Cretaceous, but there is an ongoing
dispute whether the impact was the sole cause of the extinctions.
In a 2016 study, a team from Potsdam Institute for Climate Impact
Research announced that the main cause for mass extinction was a
severe drop in global temperatures caused by concentrations of large
amount of droplets of sulfuric acid in the atmosphere. Freezing
temperatures lasted for at least three years. The river bed at
the Moody Creek Mine, 7 Mile Creek / Waimatuku, Dunollie, New Zealand
contains evidence of a devastating event on terrestrial plant
communities at the Cretaceous-
Tertiary boundary, confirming the
severity and global nature of the event.
The fact that the extinctions occurred at the same time as the
Chicxulub asteroid impact strongly supports the impact hypothesis of
extinction. However, some scientists continue to dispute the role of
the Chicxulub impact in driving the extinction, and to suggest that
other events may have contributed to the end-
extinction. In particular, volcanic eruptions, climate change, sea
level change, and other impact events have been suggested to play a
role in driving the K–Pg 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 68 Mya and lasted more than 2 million years. The most recent
evidence shows that the traps erupted over a period of 800,000 years
spanning the K–Pg boundary, and therefore may be responsible for the
extinction and the delayed biotic recovery thereafter.
Deccan Traps could have caused extinction through several
mechanisms, including the release of dust and sulfuric aerosols into
the air, which might have blocked sunlight and thereby reduced
photosynthesis in plants. In addition, Deccan Trap volcanism might
have resulted in carbon dioxide emissions that increased the
greenhouse effect when the dust and aerosols cleared from the
In the years when the
Deccan Traps hypothesis was linked to a slower
extinction, Luis Alvarez (who died in 1988) replied that
paleontologists were being misled by sparse data . While his assertion
was not initially well-received, 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 that produced the Indian
Deccan Traps, and these may have contributed to the extinctions. The
duration of the K-T extinction event was less than 10 ky, and the time
span is too short to be explained by Deccan volcanism. Geophysical
models and high-precision radiometric dating suggest that the
Chicxulub impact could have triggered some of the largest Deccan
eruptions, and potentially could have triggered eruptions at active
volcanoes anywhere on Earth.
MULTIPLE IMPACT EVENT
One other crater also appears to have been formed at about the time
of the K–Pg boundary. Other crater-like topographic features have
also been proposed as impact craters formed in connection with
Paleogene extinction. This suggests to some the possibility
of near-simultaneous multiple impacts, perhaps from a fragmented
asteroidal object, similar to the Shoemaker–Levy 9 impact with
Jupiter . In addition to the 180 km (110 mi)
Chicxulub Crater , there
is the 24 km (15 mi)
Boltysh crater in Ukraine
(7015205660879200000♠65.17±0.64 Ma), the 20 km (12 mi) Silverpit
crater in the
North Sea (7015187767720000000♠59.5±14.5 Ma) possibly
formed by bolide impact, and the controversial and much larger 600 km
Shiva crater . Any other craters that might have formed in
Tethys Ocean would have been obscured by tectonic events like the
northward drift of Africa and India.
MAASTRICHTIAN SEA-LEVEL REGRESSION
There is clear evidence that sea levels fell in the final stage of
Cretaceous by more than at any other time in the
Mesozoic era. In
Maastrichtian stage rock layers from various parts of the world,
the later layers are terrestrial; earlier layers represent shorelines
and the earliest layers represent seabeds. These layers do not show
the tilting and distortion associated with mountain building ,
therefore, the likeliest explanation is a "regression", that is, a
drop in sea level. There is no direct evidence for the cause of the
regression, but the explanation currently accepted as most likely is
that 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
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, since continental runoff now had
longer distances to travel before reaching oceans. While this change
was favorable to freshwater vertebrates , those that prefer marine
environments, such as sharks , suffered.
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 were stressed by the
changes in and loss of habitats. Dinosaurs, as the largest
vertebrates, were the first affected by environmental changes, and
their diversity declined. At the same time, particulate materials from
volcanism cooled and dried areas of the globe. Then, an impact event
occurred, causing collapses in photosynthesis-based food chains, both
in the already-stressed terrestrial food chains and in the marine food
chains. The major difference between this hypothesis and the
single-cause hypotheses is that its 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. Recent work at Seymour Island, Antarctica showed two
separate extinction events near the Cretaceous-
with one correlating to Deccan Trap volcanism and the correlated with
the Chicxulub impact. Sierra Peterson has recently published a
combined extinction patterns as a result of a new clumped isotope
temperature record from a hiatus-free, expanded KPg boundary section
from Seymour Island, Antarctica. They documented a 7.8±3.3 °C
warming synchronous with the onset of
Deccan Traps volcanism and a
second, smaller warming at the time of meteorite impact. They suggest
"Local warming may have been amplified due to simultaneous
disappearance of continental or sea ice. Intra-shell variability
indicates a possible reduction in seasonality after Deccan eruptions
began, continuing through the meteorite event.
Species extinction at
Seymour Island occurred in two pulses that coincide with the two
observed warming events, directly linking the end-Cretaceous
extinction at this site to both volcanic and meteorite events via
RECOVERY AND RADIATION
The K–Pg extinction had a profound effect on the evolution of life
Earth . The elimination of dominant
Cretaceous groups allowed other
organisms to take their place, spurring a remarkable series of
adaptive radiations in the Paleogene. The most striking example is
the replacement of dinosaurs by mammals. After the K–Pg extinction,
mammals evolved rapidly to fill the niches left vacant by the
dinosaurs. Within mammalian genera, new species were approximately
9.1% larger after the K–Pg boundary.
Other groups also underwent major radiations. Based on molecular
sequencing and fossil dating, Neoaves appeared to radiate after the
K–Pg boundary. They even produced giant, flightless forms, such as
Dromornithidae , and the predatory
Phorusrhacidae . The extinction of
Cretaceous lizards and snakes may
have led to the radiation of modern groups such as iguanas, monitor
lizards, and boas. On land, giant boid and enormous madtsoiid snakes
appeared, and in the seas, giant sea snakes radiated. Teleost fish
diversified explosively, filling the niches left vacant by the
extinction. Groups appearing in the
Paleocene and Eocene include
billfish, tunas, eels, and flatfish. Major changes are also seen in
Paleogene insect communities. Many groups of ants were present in the
Cretaceous, but in the Eocene ants became dominant and diverse, with
larger colonies. Butterflies diversified as well, perhaps to take the
place of leaf-eating insects wiped out by the extinction. The advanced
Termitidae , also appear to have risen in
* Climate across
* List of unconfirmed impact craters on
Earth – for unconfirmed
craters similar to or larger than Chicxulub
* Nature timeline
Silurian extinction events
Triassic extinction event
* Timeline of Cretaceous-
Paleogene extinction event research
Jurassic extinction event
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