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The Cretaceous–Tertiary extinction event, which occurred approximately 65.5 million years ago (Ma), was a large-scale mass extinction of animal and plant species in a geologically short period of time. Widely known as the K–T extinction event, it is associated with a geological signature known as the K–T boundary, usually a thin band of sedimentation found in various parts of the world. K is the traditional abbreviation for the Cretaceous Period derived from the German name Kreidezeit, and T is the abbreviation for the Tertiary Period (a 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. With "Tertiary" being discouraged 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 have 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.
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 wiped out by the K–Pg extinction. In terrestrial ecosystems all animals weighing more than 2.2 pounds (1 kg) disappeared.
The event appears to have hit all continents at the same time. 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 show 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 clades. 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. 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 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 material).
In stream communities few animal groups 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. 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 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 niches.
The 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.
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 believe the evidence indicates substantial extinction of these species at the K–Pg boundary, and those who believe 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 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.
There is variability 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-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.
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. Within cartilaginous fish, approximately 7 out of the 41 families of Neoselachian, modern sharks, suffered during this event and Batoids, skates and rays, lost nearly all the identifiable species, while more than 90% of teleost fish (bony fish) families survived.
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. The loss and origination of sharks and batoids at family level are more pronounced. Sharks and Batoids first appeared in the Danian Age 66 to 61.6 million years ago, starting with two shark families (Carcharhinidae, Isuridae) and a single batoid family (Torpedinidae), resulting in a total origination percentage of only 8% of marine animals. Only 25 shark families and nine batoid families survived the K-T boundary event. In the late cretaceous period, the Maastrichtian age, 28 shark families and 13 batoid families thrived, before the event occurred. Forty-seven of all marine genera cross the K/T boundary, 85% being sharks. Batoids display with 15% a comparably low survival rate.
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 extinction event.
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.
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 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. 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 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 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 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 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.
The Choristodera, a generally crocodile-like group of uncertain phylogeny (possibly archosaurian) also survived the event, only to go extinct in the Miocene.
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 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 (see Origin of birds). It was thought that all non-avian theropods became extinct, including then-flourishing groups like enantiornithines and hesperornithiforms. More recent research, however, suggests that one or more non-avian ornithuromorph species may have survived the extinction event. 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. None of them are known to survive into the Paleogene, and their persistence into the latest Maastrichtian therefore provides strong evidence for a mass extinction of archaic birds coinciding with the Chicxulub asteroid impact. A small fraction of the Cretaceous bird species survived the impact, giving rise to today's birds. So far, only a single bird species, which has not been named, has been confidently identified from both above and below the K–Pg boundary (it is present in the Maastrichtian Hell Creek Formation and Danian Fort Union Formation). The only bird group known for certain to have survived the K–Pg boundary is the Aves (though one Paleogene species, Qinornis paleocenica, may represent a surviving non-neornithine bird). 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.
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. Since there is no evidence that late Maastrichtian non-avian 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–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 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 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 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 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 is that these fossils were eroded from their original locations and then re-buried in much later sediments (also known as reworked fossils).
All major Cretaceous mammalian lineages, including monotremes (egg-laying mammals), multituberculates, marsupials and placentals, dryolestoideans, and gondwanatheres survived the K–Pg extinction event, although they suffered losses. In particular, marsupials largely disappeared from North America, and the Asian deltatheroidans, primitive relatives of extant marsupials, became extinct. In the Hell Creek beds of North America, at least half of the ten known multituberculate species and all eleven marsupial 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.
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.
North American fossilsEdit
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. This formation, when compared with the older (approximately 75 Ma) Judith River/Dinosaur Park Formations (from Montana and 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 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 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 angiosperm plants.
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-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 temperate 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.