The Cretaceous–Paleogene (K–Pg) extinction
event, also known as the Cretaceous–Tertiary
(K–T) extinction, was a sudden mass extinction
of some three-quarters of the plant and animal
species on Earth, approximately 66 million
years ago.
With the exception of some ectothermic species
such as 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 Alvarez and his son Walter Alvarez,
it is now generally thought that the K–Pg
extinction was caused by the impact of a massive
comet or asteroid 10 to 15 km (6 to 9 mi)
wide, 66 million years ago, which devastated
the global environment, mainly through a lingering
impact winter which halted photosynthesis
in plants and plankton.
The impact hypothesis, also known as the Alvarez
hypothesis, was bolstered by the discovery
of the 180-kilometer-wide (112 mi) Chicxulub
crater in the Gulf of Mexico's Yucatán Peninsula
in the early 1990s, which provided conclusive
evidence that the K–Pg boundary clay represented
debris from an asteroid impact.
The fact that the extinctions occurred simultaneously
provides strong evidence that they were caused
by the asteroid.
A 2016 drilling project into the Chicxulub
peak ring, confirmed that the peak ring comprised
granite ejected within minutes from deep in
the earth, but contained hardly any gypsum,
the usual sulfate-containing sea floor rock
in the region: the gypsum would have vaporized
and dispersed as an aerosol into the atmosphere,
causing longer-term effects on the climate
and food chain.
Other causal or contributing factors to the
extinction may have been the Deccan Traps
and other volcanic eruptions, climate change,
and sea level change.
A wide range of species perished in the K–Pg
extinction, the best-known being the non-avian
dinosaurs.
It 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 extinction also provided evolutionary
opportunities: in its wake, many groups underwent
remarkable adaptive radiation—sudden and
prolific divergence into new forms and species
within the disrupted and emptied ecological
niches.
Mammals in particular diversified in the Paleogene,
evolving new forms such as horses, whales,
bats, and primates.
Birds, fish, and perhaps lizards also radiated.
== Extinction patterns ==
The K–Pg extinction event was severe, global,
rapid, and selective, eliminating a vast number
of species.
Based on marine fossils, it is estimated that
75% or more of all species were made extinct.The
event appears to have affected all continents
at the same time.
Non-avian dinosaurs, for example, are known
from the Maastrichtian of 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.Despite
the event's severity, there was significant
variability in the rate of extinction between
and within different clades.
Species that depended on photosynthesis declined
or became extinct as atmospheric particles
blocked sunlight and reduced the solar energy
reaching the ground.
This plant extinction caused a major reshuffling
of the dominant plant groups.
Omnivores, 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 detritus (dead plant and animal matter).In
stream communities, few animal groups became
extinct, because such communities rely less
directly on food from living plants, and more
on detritus washed in from the land, protecting
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 on the ocean
floor always or sometimes feed on detritus.
Coccolithophorids and 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 survive for months without food, and their
young are small, grow slowly, and feed largely
on invertebrates and dead 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
niches.
=== Microbiota ===
The K–Pg boundary represents one of the
most dramatic turnovers in the fossil record
for various calcareous nanoplankton that formed
the calcium deposits for which the Cretaceous
is named.
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 so well understood, mainly because
only microbial cysts provide a fossil record,
and not all dinoflagellate species have cyst-forming
stages, which likely causes diversity to be
underestimated.
Recent studies indicate that there were no
major shifts in dinoflagellates through the
boundary layer.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, a significant turnover in
species but not a catastrophic extinction.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 ongoing debate 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 event,
presumably because they depend on organic
debris for nutrients, while biomass in the
ocean is thought to have decreased.
As the marine microbiota recovered, however,
it is thought that increased speciation of
benthic foraminifera resulted from the increase
in food sources.
Phytoplankton recovery in the early 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.
=== Marine invertebrates ===
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 a lack
of fossil records, rather than extinctions.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.
Current research cannot ascertain, however,
whether the extinctions occurred prior to,
or during, the boundary interval.Approximately
60% of late-Cretaceous 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.
Except for nautiloids (represented by the
modern order Nautilida) 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.
=== Fish ===
There are substantial fossil records of jawed
fishes across the K–Pg boundary, which provide
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 survived.In the Maastrichtian age,
28 shark families and 13 batoid families thrived,
of which 25 and 9, respectively, survived
the K–T boundary event.
Forty-seven of all neoselachian genera cross
the K–T boundary, with 85% being sharks.
Batoids display with 15% a comparably low
survival rate.There is evidence of a mass
extinction 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.
=== Terrestrial invertebrates ===
Insect damage to the fossilized leaves of
flowering plants from fourteen sites in North
America was 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.
During the early Paleocene, however, flora
were relatively diverse with little predation
from insects, even 1.7 million years after
the extinction event.
=== Terrestrial plants ===
There is overwhelming evidence of global disruption
of 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, the 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
plenty of organic matter to feed on was present.
Once the atmosphere cleared, photosynthetic
organisms, initially ferns and other ground-level
plants, returned.
Just two species of fern appear to have dominated
the landscape for centuries after the event.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.
=== Amphibians ===
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.
=== Non-archosaurs ===
==== Choristodere ====
The choristoderes (semi-aquatic archosauromorphs)
survived across the K–Pg boundary but would
die out in the early Miocene.
Studies on Champsosaurus' palatal teeth suggest
that there were dietary changes among the
various species across the KT event.
==== Turtles ====
More than 80% of Cretaceous turtle species
passed through the K–Pg boundary.
Additionally, all six turtle families in existence
at the end of the Cretaceous survived into
the Paleogene and are represented by living
species.
==== Lepidosauria ====
The living non-archosaurian reptile taxa,
lepidosaurians (lizards and tuataras), survived
across the K–Pg boundary.
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, although they were
very successful in the Late Cretaceous of
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 K–T 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 disappeared from fossil
records before the mass extinction occurred.
=== Archosaurs ===
The archosaur clade includes two surviving
groups, crocodilians and birds, along with
the various extinct groups of non-avian dinosaurs
and pterosaurs.
==== Crocodyliforms ====
Ten families of crocodilians or their close
relatives are represented in the Maastrichtian
fossil records, of which five died out prior
to the K–Pg boundary.
Five families have both Maastrichtian and
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 as do modern marine crocodile
juveniles, which would have helped them survive
where other marine reptiles became extinct;
freshwater environments were not so strongly
affected by the K–Pg extinction event as
marine environments were.
==== Pterosaurs ====
One family of pterosaurs, Azhdarchidae, was
definitely present in the Maastrichtian, and
it likely became extinct at the K–Pg boundary.
These large pterosaurs were the last representatives
of a declining group that contained ten families
during the mid-Cretaceous.
Several other pterosaur lineages may have
been present during the Maastrichtian, such
as the ornithocheirids, pteranodontids, 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.
In fact, at least some niches previously held
by birds were reclaimed by pterosaurs prior
to the K–Pg event.
==== Birds ====
Most paleontologists regard birds as the only
surviving dinosaurs (see Origin of birds).
It is thought that all non-avian theropods
became extinct, including then-flourishing
groups such as 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 dinosaurs.
==== Non-avian dinosaurs ====
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 more than 100 living species.
More recent research indicates that this figure
is obscured by taphonomical biases, however,
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 1,078 non-avian dinosaur
species were alive at the end of the Cretaceous
and underwent sudden extinction after the
Cretaceous–Paleogene extinction event.
Alternatively, interpretation based on the
fossil-bearing rocks along the Red Deer River
in Alberta, Canada, supports the gradual extinction
of non-avian dinosaurs; during the last 10
million years of the Cretaceous layers there,
the number of dinosaur species seems to have
decreased from about 45 to approximately 12.
Other scientists have made the same assessment
following their research.Several researchers
support the existence of Paleocene non-avian
dinosaurs.
Evidence of this existence is based on the
discovery of dinosaur remains in the Hell
Creek Formation up to 1.3 m (4 ft 3.2 in)
above and 40,000 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 in Colorado,
indicate that the animal lived during the
Cenozoic, approximately 64.5 Ma (about 1 million
years after the K–Pg extinction event).
If their existence past the K–Pg boundary
can be confirmed, these hadrosaurids would
be considered a dead clade walking.
Scientific consensus, however, is that these
fossils were eroded from their original locations
and then re-buried in much later sediments
(also known as reworked fossils).
=== Mammals ===
All major 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 (aside from the lineage leading
to Gurbanodelta).
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 faunas.
A recent study indicates that metatherians
suffered the heaviest losses at the K–T
event, followed by multituberculates, while
eutherians recovered the quickest.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 boundary.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 from K–Pg
boundary environmental stresses.
== Evidence ==
=== 
North American fossils ===
In North American terrestrial sequences, the
extinction event is best represented by the
marked discrepancy between the rich and relatively
abundant late-Maastrichtian palynomorph record
and the post-boundary fern spike.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 Maastrichtian-age Hell Creek Formation
of Montana.
Comparison with the older Judith River Formation
(Montana) and Dinosaur Park Formation (Alberta),
which both date from approximately 75 Ma,
provides information on the changes in dinosaur
populations over the last 10 million years
of the Cretaceous.
However, these fossil beds are geographically
limited, covering only part of one continent.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: Tyrannosaurus,
Ankylosaurus, Pachycephalosaurus, Triceratops,
and Torosaurus, which suggests food was plentiful
immediately prior to the extinction.
In addition to rich dinosaur fossils, there
are also plant fossils that illustrate the
reduction in plant species across the K–Pg
boundary.
In the sediments below the K–Pg boundary
the dominant plant remains are angiosperm
pollen grains, but the 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 angiosperm plants.
=== Marine fossils ===
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 Cretaceous
seas and partially overlapped in time, then
ended with the abrupt mass extinction.
The diversity of marine life decreased when
the climate near the K–Pg 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 very near 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.
=== Megatsunamis ===
The scientific consensus is that the asteroid
impact at the K–Pg boundary left megatsunami
deposits and sediments around the area of
the Caribbean Sea and Gulf of Mexico, from
the colossal waves created by the impact.
These deposits have been identified in the
La Popa basin in northeastern Mexico, platform
carbonates in northeastern Brazil, in Atlantic
deep-sea sediments, and in the form of the
thickest-known layer of graded sand deposits,
around 100 m (330 ft), in the Chicxulub crater
itself, directly above the shocked granite
ejecta.
The megatsunami has been estimated at more
than 100 m (330 ft) tall, as the asteroid
fell into relatively shallow seas; in deep
seas it would have been 4.6 km (2.9 mi) tall.
== Duration ==
The rapidity of the extinction is a controversial
issue, because some theories about the extinction's
causes imply a rapid extinction over a relatively
short period (from a few years to a few thousand
years) while others imply 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 that 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 rapid extinction,
perhaps less than ten thousand years.
At one site in the Denver Basin of Colorado,
the 'fern spike' lasted about one thousand
years (no more than 71 thousand years); the
earliest Cenozoic mammals appeared about 185,000
years (no more than 570,000 years) after the
K–Pg boundary layer was deposited.
== Chicxulub impact ==
=== Evidence for impact ===
In 1980, a team of researchers consisting
of Nobel Prize-winning physicist Luis Alvarez,
his son, geologist Walter Alvarez, and chemists
Frank Asaro and Helen Michel discovered that
sedimentary layers found all over the world
at the Cretaceous–Paleogene boundary contain
a concentration of iridium many times greater
than normal (30, 160, and 20 times in three
sections originally studied).
Iridium is extremely rare in Earth's crust
because it is a siderophile element which
mostly sank along with iron 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.
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.
The identification of giant tsunami beds along
the Gulf Coast and the Caribbean provided
more evidence, 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.
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 km
(110 mi), about the size calculated by the
Alvarez team.
The discovery of the crater—a prediction
of the impact hypothesis—provided conclusive
evidence for a K–Pg impact, and strengthened
the hypothesis that it caused the extinction.
In a 2013 paper, Paul Renne of the Berkeley
Geochronology Center dated the impact at 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.In
2007, it was proposed that the impactor belonged
to the Baptistina family of asteroids.
This link has been doubted, though not disproved,
in part because of a lack of observations
of the asteroid and its family.
It was recently discovered that 298 Baptistina
does not share the chemical signature of the
K–Pg impactor.
Further, a 2011 Wide-field Infrared Survey
Explorer (WISE) study of reflected light from
the asteroids of the family estimated their
break-up at 80 Ma, giving them insufficient
time to shift orbits and impact Earth by 66
Ma.
=== Effects of impact ===
In March 2010, an international panel of 41
scientists reviewed 20 years of scientific
literature and endorsed the asteroid hypothesis,
specifically the Chicxulub impact, as the
cause of the extinction, ruling out other
theories such as massive volcanism.
They had determined that a 10-to-15-kilometer
(6 to 9 mi) asteroid hurtled into Earth at
Chicxulub on Mexico's Yucatán Peninsula.
The collision would have released the same
energy as 100 teratonnes of TNT (420 zettajoules)—more
than a billion times the energy of the atomic
bombings of Hiroshima and Nagasaki.The Chicxulub
impact caused a global catastrophe.
Some of the phenomena were brief occurrences
immediately following the impact, but there
were also long-term geochemical and climatic
disruptions that devastated the ecology.
The re-entry of ejecta into Earth's atmosphere
would include a brief (hours-long) but intense
pulse of infrared radiation, cooking exposed
organisms.
A paper in 2013 by a prominent modeler of
nuclear winter suggested that, based on the
amount of soot in the global debris layer,
the entire terrestrial biosphere might have
burned, implying a global soot-cloud blocking
out the sun and creating an impact winter
effect.
This is debated, however, with opponents arguing
that local ferocious fires, probably limited
to North America, fall short of global firestorms.
This is the "Cretaceous–Palaeogene firestorm
debate".
Aside from the hypothesized fire and/or impact
winter effects, the impact would have created
a dust cloud that blocked sunlight for up
to a year, inhibiting photosynthesis.
The asteroid hit an area of carbonate rock
containing a large amount of combustible hydrocarbons
and sulphur, 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 acid rain.
The resulting acidification of the oceans
would kill many organisms that grow shells
of calcium carbonate.
At Brazos section, the sea surface temperature
dropped as much as 7 °C (13 °F) 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 subsequently herbivores and their predators.
Creatures whose food chains were based on
detritus would have a reasonable chance of
survival, however.
Freezing temperatures probably lasted for
at least three years.If widespread fires occurred,
they would have increased the CO2 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.Although most
paleontologists now agree that an asteroid
did hit the Earth at approximately the end
of the Cretaceous, there is an ongoing dispute
whether the impact was the sole cause of the
extinctions.
=== 2016 Chicxulub crater drilling project
===
In 2016, a scientific drilling project obtained
deep rock-core samples from the peak ring
around the Chicxulub impact crater.
The discoveries confirmed that the rock comprising
the peak ring had been shocked by immense
pressure and melted in just minutes from its
usual state into its present form.
Unlike sea-floor deposits, the peak ring was
made of granite originating much deeper in
the earth, which had been ejected to the surface
by the impact.
Gypsum is a sulfate-containing rock usually
present in the shallow seabed of the region;
it had been almost entirely removed, vaporized
into the atmosphere.
Further, the event was immediately followed
by a megatsunami sufficient to lay down the
largest known layer of sand separated by grain
size directly above the peak ring.
These findings strongly support the impact's
role in the extinction event.
The impactor was large enough to create a
190-kilometer-wide (120 mi) peak ring, to
melt, shock, and eject deep granite, to create
colossal water movements, and to eject an
immense quantity of vaporized rock and sulfates
into the atmosphere, where they would have
persisted for a long time.
This worldwide dispersal of dust and sulfates
would have affected climate catastrophically,
led to large temperature drops, and devastated
the food chain.
== Alternative hypotheses ==
Although the concurrence of the end-Cretaceous
extinctions with the Chicxulub asteroid impact
strongly supports the impact hypothesis, some
scientists continue to support other contributing
causes: volcanic eruptions, climate change,
sea level change, and other impact events.
The end-Cretaceous event is the only mass
extinction known to be associated with an
impact, and other large impacts, such as the
Manicouagan Reservoir impact, do not coincide
with any noticeable extinction events.
=== Deccan Traps ===
Before 2000, arguments that the Deccan Traps
flood basalts caused the extinction were usually
linked to the view that the extinction was
gradual, as the flood basalt events were thought
to have started around 68 Mya and lasted more
than 2 million years.
The most recent evidence shows that the traps
erupted over a period of only 800,000 years
spanning the K–Pg boundary, and therefore
may be responsible for the extinction and
the delayed biotic recovery thereafter.The
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 atmosphere.In
the years when the Deccan Traps hypothesis
was linked to a slower extinction, Luis Alvarez
(d.
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.
Even Walter Alvarez acknowledged that other
major changes may have contributed to the
extinctions.Combining these theories, some
geophysical models suggest that the impact
contributed to the Deccan Traps.
These models, combined with high-precision
radiometric dating, suggest that the Chicxulub
impact could have triggered some of the largest
Deccan eruptions, as well as eruptions at
active volcanoes anywhere on Earth.
=== Multiple impact event ===
Other crater-like topographic features have
also been proposed as impact craters formed
in connection with Cretaceous-Paleogene extinction.
This suggests 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 (65.17±0.64 Ma), the 20
km (12 mi) Silverpit crater in the North Sea
(59.5±14.5 Ma) possibly formed by bolide
impact, and the controversial and much larger
600 km (370 mi) Shiva crater.
Any other craters that might have formed in
the Tethys Ocean would have been obscured
by the northward tectonic drift of Africa
and India.
=== 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 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,
a drop in sea level.
There is no direct evidence for the cause
of the regression, but the currently accepted
explanation is that the mid-ocean ridges became
less active and sank under their own weight.A
severe regression would have greatly reduced
the continental shelf area, the most species-rich
part of the sea, and therefore could have
been enough to cause a marine mass extinction;
however, this change would not have sufficed
to cause the extinction of 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 increasing global temperatures.Marine
regression also resulted in the loss of epeiric
seas, such as the Western Interior Seaway
of North America.
The loss of these seas greatly altered habitats,
removing coastal plains that ten million years
before had been host to diverse communities
such as are found in rocks of the Dinosaur
Park Formation.
Another consequence was an expansion of freshwater
environments, 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.
=== Multiple causes ===
Proponents of multiple causation view the
suggested single causes as either too small
to produce the vast scale of the extinction,
or not likely to produce its observed taxonomic
pattern.
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.
Recent work led by Sierra Peterson at Seymour
Island, Antarctica, showed two separate extinction
events near the Cretaceous-Paleogene boundary,
with one correlating to Deccan Trap volcanism
and one correlated with the Chicxulub impact.
The team analyzed combined extinction patterns
using a new clumped isotope temperature record
from a hiatus-free, expanded K–Pg boundary
section.
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 climate change.
== Recovery and radiation ==
The K–Pg extinction had a profound effect
on the evolution of life on 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.
Also significant, within the 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 the herbivorous Gastornis and 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 mound-building termites, Termitidae,
also appear to have risen in importance.
== See also ==
== Notes
