Everything about Cretaceous Tertiary Extinction Event totally explained
The
Cretaceous–Tertiary extinction event was a large-scale
mass extinction of animal and plant species in a geologically short period of time, approximately (Ma). It is widely known as the
K–T extinction event and is associated with a geological signature, usually a thin band dated to that time and found in various parts of the world, known as the
K–T boundary.
K is the traditional abbreviation for the
Cretaceous Period derived from the
German name
Kreidezeit, and
T is the abbreviation for the
Tertiary Period (an historical term for the period of time now covered by the
Paleogene and
Neogene periods). The event marks the end of the
Mesozoic Era and the beginning of the
Cenozoic Era. "Tertiary" is no longer recognized as a formal time or rock unit by the
International Commission on Stratigraphy, the K-T event is now called the
Cretaceous-Paleogene (or K-Pg) extinction event by many researchers.
Non-
avian dinosaur fossils are only found below the K–T boundary and became extinct immediately before or during the event. A very small number of dinosaur fossils have been found above the K–T boundary, but they've been explained as
reworked, that is, fossils that have been eroded from their original locations then preserved in later
sedimentary layers.
Mosasaurs,
plesiosaurs,
pterosaurs and many
species of
plants and
invertebrates also became extinct.
Mammalian and bird
clades passed through the boundary with few extinctions, and
evolutionary radiation from those
Maastrichtian clades occurred well past the boundary. Rates of extinction and radiation varied across different clades of organisms.
Scientists
theorize that the K–T extinctions were caused by one or more catastrophic events such as massive
asteroid impacts or increased
volcanic activity. Several
impact craters and massive volcanic activity in the
Deccan traps have been dated to the approximate time of the extinction event. These geological events may have reduced
sunlight and hindered
photosynthesis, leading to a massive disruption in Earth's
ecology. Other researchers believe the extinction was more gradual, resulting from slower changes in
sea level or
climate.
Coccolithophorids and
molluscs, 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's thought that ammonites were the principal food of
mosasaurs, a group of giant marine
reptiles that became extinct at the boundary.
Omnivores,
insectivores and
carrion-eaters survived the extinction event, perhaps because of the increased availability of their food sources. At the end of the Cretaceous there seem to have been no purely herbivorous or
carnivorous mammals. Mammals and birds which survived the extinction fed on
insects,
worms, and snails, which 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.
In
stream communities, few groups of animals became extinct; because stream communities rely less directly on food from living plants and more on detritus that washes in from 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. Statistical analysis of
marine losses at this time suggests that the decrease in diversity was caused more by a sharp increase in extinctions than by a decrease in
speciation. The K–T boundary record of
dinoflagellates isn't 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.
Radiolaria have left a geological record since at least the
Ordovician times, and their mineral fossil skeletons can be tracked across the K-T boundary. There is no evidence of mass extinction of these organisms, and, there's support for high productivity of these species in
Southern high latitudes as a result of cooling temperatures in the early Paleocene.
The occurrence of
Planktonic
foraminifera across the K-T boundary has been studied since the 1930s. Research spurred by the possibility of an impact event at the K-T boundary resulted in numerous publications detailing planktonic foraminiferal extinction at the boundary. and those who believe the evidence supports multiple extinctions and expansions through the boundary.
As the
biomass in the ocean is thought to have decreased during the K-T event, numerous species of
benthic foraminifera went extinct, presumably since they depend on organic debris for nutrients. However, as the marine microbiota recovered, it's thought that increased speciation of benthic foraminifera resulted from the increase in food sources.
Marine invertebrates
There is variability in the fossil record as to the extinction rate of
marine invertebrates across the K-T boundary. The apparent rate is influenced by the lack of fossil records rather than actual extinction.
Approximately 60% of late-Cretaceous
Scleractinia coral genera failed to cross the K-T boundary into the Paleocene. Further analysis of the coral extinctions show that approximately 98% of colonial species, ones that inhabit warm, shallow
tropical waters, went extinct. The solitary corals, which generally don't form reefs and inhabit colder and deeper (below the
photic zone) areas of the ocean were less impacted by the K-T boundary. Colonial coral species rely upon
symbiosis with photosynthetic
algae, which collapsed due to the events surrounding the K-T boundary. However, the use of data from coral fossils to support K-T extinction and subsequent Paleocene recovery must be weighed against the changes that occurred in coral ecosystems through the K-T boundary.
Approximately 35% of
echinoderm genera went extinct at the K-T boundary, although
taxa that thrived in low-latitude, shallow-water environments during late Cretaceous had the highest extinction rate. Mid-latitude, deep-water echinoderms were much less affected at the K-T 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 completely extinct at the K-T boundary.
Fish
There are substantial fossil records of
jawed fishes across the K–T boundary, which provides good evidence of extinction patterns of these classes of marine vertebrates. Within
cartilaginous fish, approximately 80% of the
sharks,
rays, and
skates families survived the extinction event, There is evidence of a mass kill of bony fishes at a fossil site immediately above the K-T boundary layer on
Seymour Island near Antarctica. It is speculated that fish were undergoing environmental stresses and the K-T boundary event may have precipitated the mass extinction. However, the marine and freshwater environments of fishes mitigated environmental effects of the extinction event.
Terrestrial plants
There is overwhelming evidence of global disruption of plant communities at the K-T boundary. However, there were important regional differences in plant succession. In North America, the data suggest massive devastation and mass extinction of plants at the K-T boundary sections, although there were substantial megafloral changes before the boundary.
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.
Due to the wholesale destruction of plants at the K–T boundary, there was a significant proliferation of
saprotrophic organisms such as
fungi which don't require
photosynthesis and utilize 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.
Amphibians
There is no evidence of K–T boundary mass extinctions of
amphibians, and there's strong evidence that most amphibians survived the event relatively unscathed.
Frog species appear to have survived into the
Paleocene with few species becoming extinct. However, the fossil record for frog families and genera is uneven.
Living lepidosaurs include
Rhynchocephalia and
Squamata. The Rhynchocephalia, or
tuatara, were a widespread and relatively successful group of lepidosaurs in the early Mesozoic, but began to decline by the mid-Cretaceous. They are represented today by a single genus located exclusively in
New Zealand.
The order Squamata, which is represented today by lizards, snakes, and amphisbaenia, radiated into various ecological niches during the
Jurassic and were successful throughout the Cretaceous. They survived through the K-T boundary and are currently the most successful and diverse group of living reptiles with more than 6,000 extant species. No known family of terrestrial squamates went extinct at the boundary, and fossil evidence indicates they didn't suffer any significant decline in numbers. Their small size, adaptable metabolism, and ability to move to more favorable habitats were key factors in their survivability during the late Cretaceous and early Paleocene.
Archosaurs
The
archosaur clade includes two living orders,
crocodilians (of which
Alligatoridae,
Crocodylidae and
Gavialidae are the only surviving families) and
birds, along with the extinct
dinosaurs and
pterosaurs.
Crocodilians
Ten crocodile families are represented in the Maastrichtian fossil records, of which five died out prior to the K-T boundary. Five families have both Maastrichtian and Paleocene fossil representatives. All of the surviving families of crocodilians inhabited freshwater and terrestrial environments, except for the
Dyrosauridae which lived in freshwater and marine locations. Approximately 50% of crocodilian representatives survived across the K-T boundary, the only apparent trend being that no large crocodiles, such as the giant North American crocodile
Deinosuchus, survived.
Pterosaurs
Only one family of pterosaurs,
Azhdarchidae, was definitely present in the Maastrichtian, and it went extinct at the K-T boundary. These large pterosaurs were the last representatives of a declining group that contained 10 families during the mid-Cretaceous. Smaller pterosaurs went extinct prior to the Maastrichtian during a period that saw a decline in smaller animal species while larger species became more prevalent. While this was occurring,
modern birds were undergoing diversification and replacing archaic birds and pterosaur groups, possibly due to direct competition, or they simply filled empty niches. Several analyses of bird fossils show divergence of species prior to the K-T boundary, and that duck, chicken and ratite bird relatives coexisted with non-avian dinosaurs. Neornithine birds survived the K-T boundary as a result of their abilities to dive, swim, or seek shelter in water and marshlands. Many species of birds can build burrows, or nest in tree holes or termite nests, all of which provided shelter from the environmental effects at the K-T boundary. Long-term survival past the boundary was assured as a result of filling ecological niches left empty by extinction of dinosaurs. Since there's no evidence that late Maastrichtian nonavian dinosaurs could burrow, swim or dive, they were unable to shelter themselves from the worst parts of any environmental stress that occurred at the K-T boundary. It is possible that small dinosaurs (other than birds) did survive, but they'd have been deprived of food as both herbivorous dinosaurs would have found plant material scarce, and carnivores would have shortly found prey in short supply. Mammalian species began diversifying approximately 30 million years prior to the K-T boundary; however, further diversification actually stalled across the boundary, which indicates that mammals filled ecological niches during the Cretaceous that were least impacted by the extinction event. A few orders of mammals did diversify right at the K-T boundary, including
Chiroptera (bats) and
Cetartiodactyla (
whales and dolphins and
Even-toed ungulates), as a result of the reduced competition in those niches.
K-T boundary mammalian species were relatively small (most less than 1 kg) which allowed them to find shelter in a number of different environments. In addition, many early monotremes and marsupials were semiaquatic or burrowing which also provided protection from K-T 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. Normal pollen levels gradually resume above the boundary layer. This is reminiscent of areas blighted by volcanic eruptions, where the recovery is led by ferns which are later replaced by larger angiosperm plants.
Marine fossils
The mass extinction of marine plankton appears to have been abrupt and right at the K–T boundary.
Ammonite genera became extinct at or near the K–T boundary; however, there was a smaller and slower extinction of ammonite genera prior to the boundary that was associated with a late Cretaceous marine regression. The gradual extinction of most inoceramid bivalves began well before the K–T boundary, and a small, gradual reduction in ammonite diversity occurred throughout the very late Cretaceous. Further analysis shows that several processes were in progress in the late Cretaceous seas and partially overlapped in time, then ended with the abrupt mass extinction. Scientists have also found very few continuous beds of fossil-bearing rock which cover a time range from several million years before the K–T extinction to a few million years after it. There were other earlier speculations on the possibility of an
impact event, but no evidence had been uncovered at that time. There is evidence of a breakup of the parent-body asteroid of
298 Baptistina, which is conjectured to have occurred about 160 mya. It is hypothesized that several
fragments from this breakup eventually impacted to form Chicxulub Crater on Earth, and
Tycho crater on the Moon.
The consequence of an impact would be a dust cloud which would block
sunlight and inhibit photosynthesis for a few years, which would account for the extinction of
plants and
phytoplankton, and of
organisms dependent on them (including
predatory animals as well as
herbivores). Small creatures whose food chains were based on detritus had a reasonable chance of survival. It is estimated that
sulfuric acid aerosols were injected into the
stratosphere, leading to a 10–20% reduction in sunlight reaching the earth's surface. It would have taken at least ten years for those aerosols to dissipate. The consequences of reentry of ejecta into Earth's atmosphere included a brief (hours long) but intense pulse of
infrared radiation of an intensity similar to an
oven set to
broil, killing exposed organisms.
The impact may also have produced
acid rain, depending on what type of rock the asteroid struck. However, recent research suggests this effect was relatively minor.
Chemical buffers would have limited the changes, and the survival of animals vulnerable to
acid rain effects (such as
frogs) indicate this wasn't a major contributor to extinction. Impact theories can only explain very rapid extinctions, since the dust clouds and possible sulfuric aerosols would wash out of the atmosphere in a fairly short time—possibly under ten years. Subsequent research, however, identified the Chicxulub Crater buried under
Chicxulub on the coast of
Yucatan,
Mexico as the impact crater which matched the Alvarez hypothesis dating. Identified in 1990 based on the work of Glen Penfield done in 1978, this crater is oval, with an average diameter of about, about the size calculated by the Alvarez team.
The shape and location of the crater indicate further causes of devastation in addition to the dust cloud. The asteroid landed in the ocean and would have caused
tsunamis, for which evidence has been found in several locations in the Caribbean and eastern United States—marine sand in locations which were then inland, and vegetation debris and terrestrial rocks in marine sediments dated to the time of the impact. The asteroid landed in a bed of
gypsum (calcium sulfate), which would have produced a vast sulfur dioxide
aerosol. This would have further reduced the sunlight reaching the earth's surface and then precipitated as acid rain, killing vegetation, plankton and organisms which build shells from calcium carbonate (
coccolithophores and
molluscs). Most paleontologists now agree that an asteroid did hit the Earth about 65 mya, but there's an ongoing dispute whether the impact was the sole cause of the extinctions.
Deccan Traps
Before 2000, arguments that the
Deccan Traps flood basalts caused the extinction were usually linked to the view that the extinction was gradual, as the flood basalt events were thought to have started around 68 mya and lasted for over 2 million years. The most recent evidence shows that the traps were in fact erupted over 800,000 years spanning the K-T 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 which would have increased the
greenhouse effect when the dust and aerosols cleared from the atmosphere.
In the years when the Deccan Traps theory was linked to a slower extinction, Luis Alvarez (who died in 1988) replied that
paleontologists were being misled by
sparse data. While his assertion wasn't 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.
Multiple impact event
Several other craters also appear to have been formed about the time of the K–T boundary. This suggests the possibility of near simultaneous multiple impacts, perhaps from a fragmented asteroidal object, similar to the
Shoemaker-Levy 9 cometary impact with
Jupiter. Among these are the
Boltysh crater, a diameter impact crater in
Ukraine, and the
Silverpit crater, a diameter suspected impact crater in the
North Sea Any other craters that might have formed in the
Tethys Ocean would have been obscured by tectonic events like the relentless northward 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 ones are terrestrial; earlier ones represent shorelines and the earliest represent seabeds. These layers don't 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 which is currently accepted as the 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 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.
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