A study of 29 fossil sites in Catalan Pyrenees of Europe in supports the view that dinosaurs there had great diversity until the asteroid impact, with more than living species. The results of this study, which were based on estimated real global biodiversity, showed that between and 1, non-avian dinosaur species were alive at the end of the Cretaceous and underwent sudden extinction after the Cretaceous—Paleogene extinction event. 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. If their existence past the K—Pg boundary can be confirmed, these hadrosaurids would be considered a dead clade walking. 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. Mammalian species began diversifying approximately 30 million years prior to the K—Pg boundary.
Diversification of mammals stalled across the 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. 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.
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. 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 temperature increased about three to four degrees very rapidly between Not only did the climate temperature increase, but the water temperature decreased, causing a drastic decrease in marine diversity.
The scientific consensus is that the asteroid impact at the K—Pg boundary left megatsunami deposits and sediments around the area of the Caribbean Sea and Gulf of Mexico, from the colossal waves created by the impact. Recently, fossiliferous sedimentary rocks deposited during the K—Pg impact have been found in the Gulf of Mexico area, including tsunami wash deposits carrying remains of a mangrove -type life system, evidence that after the impact water sloshed back and forth repeatedly in the Gulf of Mexico, and dead fish left in shallow water but not disturbed by scavengers: "this layer is the K-T boundary".
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.
In , 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, , 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.
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. He further posits that the mass extinction occurred within 32, years of this date. In , it was proposed that the impactor belonged to the Baptistina family of asteroids.
In March , 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. The collision would have released the same energy as teratonnes of TNT 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.
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The re-entry of ejecta into Earth's atmosphere would include a brief hours-long but intense pulse of infrared radiation , cooking exposed organisms. This is the " Cretaceous—Paleogene firestorm debate ". A paper in 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.
Creatures whose food chains were based on detritus would have a reasonable chance of survival, however. If widespread fires occurred, they would have increased the CO 2 content of the atmosphere and caused a temporary greenhouse effect once the dust clouds and aerosol settled, and, this would have exterminated the most vulnerable organisms that survived the period immediately after the impact. 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.
In , 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 [d] 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. This worldwide dispersal of dust and sulfates would have affected climate catastrophically, led to large temperature drops, and devastated the food chain.
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.
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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. Before , 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 , 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.
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In the years when the Deccan Traps hypothesis was linked to a slower extinction, Luis Alvarez d. 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.
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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. 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.
Any other craters that might have formed in the Tethys Ocean would have been obscured by the northward tectonic drift of Africa and India. 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. 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.
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.
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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. They documented a 7. They suggest local warming may have been amplified due to simultaneous disappearance of continental or sea ice.