Alvarez and others reported spikes in the level of platinum group metals like iridium at the Cretaceous-Tertiary boundary in Italy, Denmark, and New Zealand. They interpreted this sudden introduction of rare-earth metals as evidence for an asteroid impact, to which they attributed the mass extinction at the end of the Cretaceous Period.[1]
Smit and Hertogen independently reported the presence of an iridium spike at the Cretaceous-Tertiary boundary in Spain, which they also attributed to the impact of an extra terrestrial body and credited with the Cretaceous-Tertiary extinctions.[1]
May: Smit and Hertogen published the results of their research on the K-T boundary at Caravaca and proposed that an asteroid impact at the end of the Cretaceous triggered the coeval mass extinction.[2]
June 6th: Alvarez and others published their hypothesis that an impact event cause the extinction of the dinosaurs.[3]
Penfield wrote to Walter Avarez, suggesting the Yucatan structure as the possible crater of the end-Cretaceous impactor, but received no response.[4]
Hsu suggested that a comet may have caused the extinction of the dinosaurs.
Orth and others reported a sudden "spike" in the quantity of fossil fern spores near the Cretaceous-Tertiary boundary, "just above the iridium-bearing clay".[5] They also reported an iridium spike at the K-T Boundary in Colorado and Utah. Since these rocks were deposited by freshwater, their discovery bolstered the impact hypothesis by refuting attempts to explain away the K-T boundary's high iridium concentrations as a result of chemical or sedimentary processes ocurring in the ocean.[6]
Philip Kerourio debunked Erben and others' suggestion that an increase in the incidence of pathological eggs in dinosaurs led to their extinction. He found that only 0.5-2.5% of eggs in the area Erben and the others studied had multiple shell layers and observed no evidence that these pathologies became more common through the Late Cretaceous.[7]
A conference dedicated to the end-Cretaceous extinction event was held at Utah's Snowbird Ski resort.[8] By this point in time, 36 K-T boundary sites with anomalously high iridium levels had been identified.[9] At the conference, Yale geochemist Karl Turekian disputed the impact hypothesis. He expressed interest in debunking the idea by demonstrating that the isotope ratios of osmium in the rocks of the K-T Boundary were typical for rocks of the earth's crust but inconsistent with those in meteorites.[10]
Bruce Bohor applied for a G. K. Gilbert Geological Survey fellowship so that he could search for shocked quartz at the K-T Boundary but was rejected.[11]
Wezel and others reported high iridium levels at Gubbio both far above and below the K-T boundary.[12] They also reported spherules likewise above and below the boundary layer and therefore concluded that the spherules could not have been produced by a meteor impact.[13]
Smit and Klaver reported the presence of feldspar spherules at Caravaca.[13]
Penfield and Camargo reported the existence a crater dating to the K-T boundary in the Yucatan Peninsula during a presentation to the Society of Exploration Geophysicists. They proposed that this crater may have been caused by the same impact event to which Alvarez had recently attributed the mass extinction at the end of the period.[14]
December 13th: An interview of Penfield and Camargo about their work on the Yuacatan crater and its connection to the extinction of the dinosaurs was published in the Houston Chronicle.[15]
Paleontologist Peter Ward reported in a presentation to colleagues at Berkeley that his research supported the idea of a rapid extinction of the ammonites at the Cretaceous-Tertiary boundary.[16]
Clemons, Archibald and others published one of the first rebuttals to the Alvarez hypothesis. They argued that the fossil record of contemporary plants shows a gradual progressive adaptation of the flora to colder temperatures as the Cretaceous ended and the Tertiary began.[17]
Hickey argued based on a study of 1,000 fossil leaves that plant life changed gradually across the Cretaceous-Tertiary boundary in a manner incompatible with a catastrophic extinction event.[18]
Orth and others reported a sudden extreme decrease in the amount of flower pollen and a simultaneous increase in the abundance of fern spores at the K-T boundary. This is consistent with the impact event because ferns have been observed to rapidly recolonize areas rendered desolate by modern natural disasters.[19]
Krassilov suggested that dinosaurs went extinct as their habitats were replaced with forests they were not adapted to.[20]
Philip Signor and Jere Lipps argued that extinctions can appear more gradual in the fossil record than actually transpired because any given level in the stratum will preserve fewer than the interval overall.[21] The Signor-Lipps effect has been regarded as one of the most important discoveries in paleontology. They observed a strong correlation between the area of rock deposited during a given time interval and that time interval's biodiversity. This observation is attributable to the obvious fact that the biodiversity of a time interval can only be inferred from fossils preserved in rocks deposited then. If fewer rocks are known from a given time, then there are also fewer potential sources of fossils.[22] This can mislead scientists into thinking that the biodiversity of a taxon was declining, when in actuality there are simply fewer sources of fossils for the later members of the group. Signor and Lipps noted ammonites as an example; the number of known ammonites declined from the middle to Late Cretaceous. However, this decline in diversity also closely parallels a decline in known rocks of that age from which their fossils could be collected, and may have no connection with their actual historical biodiversity.[23]
Toon and others argued that dust ejected into the atmosphere by an asteroid impact at the end of the Cretaceous would have lowered temperatures on land to near freezing levels for 45 days to six months. This scenario is known as "impact winter". The oceans however would only see a slight temperature drop due to their greater heat capacity.[24]
Hsu and others argued based on carbon isotopic evidence that photosynthesis in ocean plankton nearly completely halted at the Cretaceous-Tertiary boundary. They nicknamed this scenario the "Strangelove Ocean".[25]
Charles Drake published a critique of some of the research presented at the K-T extinction conference held at Snowbird.[26]
March: An article in Sky and Telescope magazine discussed Penfield and Camargo's hypthesis that the Yucatan crater was linked to the end-Cretaceous mass extinction.[15]
Dale Russell argued that since the Campanian age was twice as long as the Maastrichtian, one would expect it to have twice as many dinosaur species, so a disparity between the two is not necessarily evidence that they were in decline.[27]
Schopf attributed the demise of the dinosaurs to gradualistic mundane causes and rejected catastrophist interpretations of their extinction.[28]
Archibald and Clemens argued that the floral and faunal turnover from the Mesozoic to Cenozoic was gradual.[29] They rejected the impact hypothesis, regarding either a super nova or an influx of Arctic seawater into more southerly waters that lowered global temperatures.[30]
October: Luis Alvarez made "a pre-emptive declaration of victory" for the impact hypothesis to the National Academy of Sciences. This brash claim would earn him ire from geologists and paleontologists.[31]
Ward reported that after additional fieldwork he had changed his mind about the pace and timing of ammonite extinction and had concluded that they died out before the end of the Cretaceous.[16]
Hans Thierstein found that 97% of foraminiferan species and 92% of their genera went extinct at the K-T boundary.[32]
Jan Smit reported that the only foraminiferan species to survive the Cretaceous was Guembelitria cretacea, and that all subsequent foraminiferans were its descendants.[32]
Ferguson and Joanen proposed that an increasingly hot and dry climate could have skewed the ratio of male to female dinosaur hatchlings, leading to their extinction.[33]
McLean attributed the extinction of the dinosaurs to volcanism at the end of the Cretaceous.[34]
Anomalously high quantities of platinum group metals were discovered in terrestrial deposits laid down at the time of the Cretaceous-Tertiary boundary in the western United States. The presence of these metals in terrestrial rocks bolstered the asteroid impact hypothesis by overturning alternative explanations for the iridium spike as resulting from earthly chemical processes concentrating them in seawater.[1]
Pollock and others estimated that the asteroid impact that caused the Cretaceous-Paleogene extinction ejected into the atmosphere brought on 3 months of darkness.[35]
Ehrlich and others (including Carl Sagan) published the results of a computer simulation inspired by the Alvarez hypothesis for the end-Cretaceous mass extinction event to study the consequences of nuclear war. The researchers concluded That a nuclear war between the United States and the Soviet Union involving only half of these countries' nuclear stockpiles would be sufficient to cause a similar mass extinction event that would endanger the survival of the human species.[36]
Luck and Turekian demonstrated that the isotope ratios of osmium in the rocks of the K-T Boundary were more typical for a meteorite than those of the earth's crust, confirming rather than debunking the impact hypothesis.[10]
By the end of the year, 50 K-T boundary sites with anomalously high iridium levels had been identified.[9]
Officer and Drake published their first attack on the impact hypothesis.[37] They synthesized previously published data on 15 core samples containing the Cretaceous-Tertiary boundary taken from various places around the world, including undersea. They found three of the samples to have been formed during periods of different polarities of earth's magnetic field. This meant that the rock record of the Cretaceous-Tertiary transition had a different absolute age at different locations and any physical commonality shared between these rocks of different ages could not have resulted from a single instantaneous event.[26] They also argued that the elevated iridium concentrations at the K-T boundary were spread gradually across about 60 cm of the stratigraphic column, rather than increasing sharply in a "spike" right at the boundary itself.[12]
Montanari and others interpreted the feldspar spherules from Caravaca as impact ejecta that had melted and rehardened.[13]
The paper that served as the basis for Luis Alvarez's declaration of victory speech to the National Academy of sciences was published.[31] He expressed shock that paleontologists lacked sufficient "respect" to see dinosaurs as capable of persisting in the face of mundane environmental changes compared to his own view that only a devastating catastrophe like an impact event could have lead to their extinction. He harshly criticized Clemens and Archibald for advancing the super nova hypothesis or the climatic effects of changes in ocean circulation as viable explanations for the extinction of the dinosaurs, saying that they lacked "appreciation for the scientific method" and that these hypotheses were "as dead as the phlogiston theory of chemistry".[30]
Luis Alvarez gave a presentation to the National Academy of Sciences where he proposed that all of earth's mass extinctions were due to impact events.[38]
Peter Ward described his recent conclusions about ammonite extinction to Scientific American. He reported that at Zumaya, Spain 10 m of rock separated the uppermost ammonite fossil from the K-T Boundary, so these cephalopods could not have been victims of an asteroid impact at the end of the period.[39]
Dott jokingly suggested that the appearance of pollen-producing angiosperms afflicted them with "terminal hay fever".[20]
Keith proposed that increasing levels of carbon dioxide in Earth's atmosphere caused oceans to stagnate, which lead to the extinction of the dinosaurs.[34]
Yayanos published an additional paper advocating ionizing radiation the cause of the extinction of the dinosaurs.[40]
Russell argued that the apparent decline in dinosaur diversity during the Late Cretaceous was an illusion attributable to the older Late Cretaceous rocks being more thoroughly explored than the more recent ones closest to the period's boundary with the Cenozoic.[41]
Ekdale and Bromley observed that the deposits at Stevns Klint, Denmark were formed from clay that exhibits a complex stratigraphy and preserves many trace fossils left by burrowing animals.[24]
Despite previously being rejected for a G. K. Gilbert Geological Survey Fellowship to enable him to look for shocked quartz at the K-T Boundary, Bruce Bohor did just that and published his results with a team of other researchers. Bohor and his team found the K-T boundary at a centimeter-thick claystone in Montana. Their examination uncovered evidence for the disappearance from the rock record of many different kinds of fossil pollen, as well as anomalously high iridium levels.[11] Bohor and his colleagues' status as geologists and familiar methodology helped the impact hypothesis gain credibility among fellow researchers who were reluctant to consider proposals from scholars outside the field.[42]
Charles Drake became president of the American Geophysical Union.[37]
Alvarez and others published a rebuttal to Officer and Drake's 1983 paper that attempted to refute the impact hypothesis through magnetostratigraphy. They criticized Officer and Drake for having ignored the research presented at the first Snowbird conference, despite Drake having attended and even previously publishing on some of that very research.[43] The Alvarez team also criticized Officer and Drake for relying on data published by other workers who questioned their own results. For instance, one of the K-T boundary-bearing core samples that supposedly formed at a different time than the others was heavily bioturbated according to the researchers who first studied it. These previous workers acknowledged that the modifications the sampled sediments experienced between deposition and lithification made them unreliable for paleomagnetic dating.[44] Alvarez and his collaborators concluded that Officer and Drake were cherry picking the available data for any evidence that could be marshaled against the impact hypothesis while ignoring the vast quantity that supported it.[45] They also reported the results of their attempt to relocate the high iridium concentrations that Wezel and others reported from sections of the rocks at Gubbio other than the K-T boundary. Despite their re-examinations of the rocks there, they could find no evidence of high iridium levels anywhere other than the boundary itself. They concluded that the Wezel team's anomalous iridium readings were the result of contamination.[12]
Dewey McLean claims to have endured a campaign of persecution from Luis Alvarez resulting in so much stress that he spent this entire year suffering from crippling joint pain.[46]
Bevan French estimated that the end-Cretaceous impact must have occurred within 3500 km of Montana, based on the shocked quartz discovered there.[47]
Summer: A poll of more than 600 paleontologists and other earth scientists found 24% to support the impact hypothesis of the Cretaceous-Paleogene extinction event, 38% agreed that the impact occurred but was not the true cause of the mass extinction, 26% denied that any impact had occurred and 12% completely denied the ocurrence of a mass extinction at all.[48]
Smit and Van der Kars argued that the K-T boundary occurred 2-12 m lower than researchers had previously realized, giving the illusory impression that dinosaurs had died out before the end of the period. They also argued that the "Z" coal beds of the formation used to mark the beginning of the Cenozoic were actually different ages at different exposures and were not useful stratigraphic demarcators.[49]
Hallam attributed the extinction of the dinosaurs to a lack of oxygen at the bottom of the seas as they began expanding and the later withdrawl of these same seas.[34]
Neruchev suggested that the dinosaurs were killed by uranium poisoning.
Renard and Rocchia attributed the extinction of the dinosaurs to an "[i]nterstellar dust cloud".[40]
April 2nd: An editorial criticizing the asteroid impact hypothesis of the end-Cretaceous mass extinction was published by the New York Times. The author compared the hypothesis to astrology because it sought to explain earthly events in terms of celestial bodies.[50]
November 1985: Stephen Jay Gould lampooned the New York Times editorial in a satirical rebuttal comparing the author to the Catholic clergy who forced Galileo to recant of his astronomical findings.[51]
Wolbach and others reported the results of their attempt to locate noble gases at the K-T Boundary in Denmark which could have been left by a meteorite. Serendipitously they found high concentrations of soot at the boundary. If the boundary layer had indeed formed rapidly, then this soot may have been left by wildfires that consumed up to 90% of earth's terrestrial biomass.[52]
Officer and Drake published their second attack on the impact hypothesis.[37] They argued that the high iridium reported from the K-T boundary was introduced gradually by volcanic activity, not suddenly by a meteor impact.[53] They also disputed the attribution of fracture planes in shocked quartz to the forces generated by the supposed end-Cretaceous impact event and instead argued that these fracture planes could have been generated by mundane geologic forces like mount-building and metamorphism. They argued that since geologic structures preserved at Sudbury and Vredefort preserve shocked quartz of terrestrial origin, it cannot be used as evidence for an impact.[54] They observed that volcanologists studying the Kilauea Volcano in Hawaii found the aerosols it emitted to contain iridium levels similar to those of meteorites.[55]
Smit and Kyte criticized Officer and Drake's interpretation of the effects bioturbation would have on sediments laid down at the K-T boundary. Officer and Drake operated under the assumption that bioturbation would only affect a few centimeters of sediments, so the activities of animals living in the sediment would not penetrate deeply enough to spread rapidly deposited iridium that far down. However, Smit and Kyte pointed out that tektites present across a 60 span at the boundary. They argued that since the tektites must have been deposited rapidly and were reworked to that depth, rapidly deposited iridium could have been as well.[56]
Bevan French, an expert on shock metamorphism, rejected Officer and Drake's claim that mountain-building or volcanism could account for the fracture planes in the shocked quartz found at the Cretaceous-Tertiary boundary.[57]
Officer presented Wezel's report of spherules away from the K-T boundary in an address to a meeting of the American Geophysical Union. After the presentation Walter Alvarez pointed out that some of the purported spherules were actually modern insect eggs that the researchers had failed to clean off their specimens. Luis Alvarez claims that at this point the entire audience burst into laughter. Officer himself has since denied that this occurred.[13]
Smit and Romein interpreted a turbidite deposit from Brazos, Texas as the probable legacy of an impact-generated tsunami. They attributed the Texan turbidite to the tsunami because of its close association with the iridium-bearing K-T boundary and its status as the only turbidite deposit in the region.[58]
October 29th: Michael Browne published an article in the New York Times describing the vigorous controversy over the cause of the end-Cretaceous mass extinction during a meeting of the Society of Vertebrate Paleontologists at Rapid City, South Dakota. Most of the assembled paleontologists remained faithful to a gradualist interpretation of the extinction.[59] James Lawrence Powell characterized Robert T. Bakker as "the most vicious" critic of the impact hypothesis at the meeting. Bakker characterized pro-impact researchers as scientifically ignorant and possessing "unbelievable" "arrogance".[60]
Sloan and others argued that dinosaurs went extinct gradually from the Hell Creek Formation through the Bug Creek strata.[21] Further, they argued that dinosaurs already showed signs of being supplanted by diversifying mammals.[61]
Sheehan and Hansen observed that taxa dependent on photosynthesis-based food chains experienced greater losses than those which could relyon detritus. Examples of taxa that suffered major or complete extinctions include ammonites, plankton, and some mollusks.[35]
Officer and Ekdale disputed the interpretation of deposits at Stevns Klint, Denmark as soot rapidly deposited by global wildfires in the wake of an asteroid impact. They argued that the complex stratigraphy and abundant burrow fossils they observed in these deposits suggested that the strata took much longer to form than can be accounted for by the wildfire hypothesis.[24]
McCartney and Nienstedt criticized the idea of an end-Cretaceous asteroid impact in a journal aimed at educators and argued that it did not provide a good explanation for the mass extinction.[50]
Walter Alvarez argued that uniformitarianism should be seen as a byproduct of Occam's Razor rather than an unquestionable dogma.[62]
Kyte and Wasson examined the iridium contents of a long core sample extracted from the Pacific Ocean. This sample contained sediments ranging from 35 to 67 million years in age. The researchers found very low levels of iridium throughout the sample, except for at the K-T Boundary. This bolstered the impact hypothesis by demonstrating the scarcity of iridium in earth's crust over time, which is consistent with the interpretation that it originated with an unusual event.[63]
Charles Drake's tenure as president of the American Geophysical Union ended.[37]
A team of researchers led by Neville Carter that included Officer claimed that feldspar and mica originating in the Toba volcano showed shock fractures.[57]
Naslund and others also reported spherules above and below the K-T boundary at Gubbio. They estimated that the spherule-bearing interval took about 22 million years to be deposited and the the spherules couldn't have been a result of an impact event.[13]
Hoyle and Wickramasinghe jokingly suggested that the dinosaurs became so promiscuous near the end of their reign that they all gave eachother AIDS and died.[64]
Fastovsky reported that the end-Cretaceous iridium spike did not always correspond with the presence of coal deposits researchers once thought to form the boundary between the Late Cretaceous Hell Creek Formation and the Paleocene Tullock Formation. This finding suggested that these deposits don't demarcate the actual boundary between the epochs as researchers had previously believed.[65]
Prinn and Fegley argued that the energy of an asteroid impact at the end of the Cretaceous period would have led atmospheric nitrogen and oxygen to react, forming large quantities of nitric acid that would have fallen back to earth in the form of acid rain.[24]
Bohor and others reported shocked quartz from seven more K-T boundary exposures. They also studied quartz from Mount Toba, where shock fractures were much less common and simpler in structure than quartz from the K-T boundary.[57]
December: Brian Huber disembarked on a ship from Mauritius to Desolation Island off the coast of Antarctica in order to drill core samples from the seafloor. The sample taken off the coast of Desolation Island showed a sharp K-T boundary with abundant foraminiferan fossils below it and few above it. The finding convinced Huber of the impact hypothesis.[66]
Rigby and others reported dinosaur bones from Paleocene sediments in the Hell Creek Formation and argued that this implied that dinosaurs survived the K-T transition.[49]
Fastovsky published further support for the contention that the Z coal beds of the Hell Creek Formation were actually of different age at different sites and couldn't be used as a stratigraphic boundary marker.[49]
Wolbach and others argued that there was evidence for widespread nearly global wildfires following the asteroid impact at the end of the Cretaceous period.[24]
A conference dedicated to the end-Cretaceous extinction event was held at Utah's Snowbird Ski resort.[8]
A team led by Kim Crocket including Officer re-examined the iridium levels at Gubbio.[56] They found high iridium concentrations spanning from 2 m below to 2 m above the K-T boundary. They estimated that this much sediment must have taken as much as 300,000 years to be deposited and argued that bioturbation could not have reworked iridium over such a great distance.[67]
A team of scientists from many different countries and holding different opinions on the impact hypothesis was lead by Rocchia back to Gubbio to once more re-examine the levels of iridium around the Cretaceous-Tertiary boundary there. This team did not find any evidence for high iridium concentrations anywhere but the boundary itself.[68]
Alexopoulos and others compared quartz grains from rocks that had been subjected to various types of geologic forces like bolide impact, volcanism, or tectonic deformation with quartz from the K-T boundary layer. They found that quartz could exhibit shock fractures resulting from any of the studied forces, but the shock fractures exhibited by the impact site and the K-T boundary were both identical to eachother and distinct from those found in the other rocks.[57]
Felitsyn and Vaganov found high levels of iridium in volcanic ejecta from Kamchatka. This provided evidence that terrestrial geologic processes could leave high levels of iridium behind in the rock record without need for an impact to explain them.[55]
Kevin Pope and Charles Duller presented their discovery of a configuration of small ponds "arranged along the arc of an almost perfect circle" in satelite images of the Yucatan peninsula.[69] Geologist Adriana Ocampo suggested that the arc of ponds may represent the surface evidence of a buried impact crater and the researchers began a collaboration to investigate the possibility.[70]
Bourgeois and others attributed the Texan turbidite deposit studied by Smit and Romein to a tsunami 50-100m high.[58]
January 19th: An editorial published in the New York Times quoted Luis Alvarez accusing critics of the impact hypothesis of holding onto their own pet hypotheses despite indisputible evidence to the contrary.[31] He also criticized the paleontological community of lacking scientific rigor justifying Ernest Rutherford's division of science into physics and stamp collecting. Alvarez additionally revealed that he was suffering from terminal esophogeal cancer.[71]
Ward reported that ammonites persisted up to the Cretaceous-Tertiary boundary after all. After finding a partial ammonite fossil "within inches of the boundary" at Zumaya, Ward began prospecting at other places in Europe where the K-T Boundary was exposed. At Hendaye, France he nearly instantly found abundant ammonites near the boundary, leading him to conclude that the scarcity of ammonites at Zumaya was purely local and unrelated to their overall extinction.[73]
Hickey and Kirk Johnson reported that after studying more than 25,000 plant fossils collected across western North America they had concluded that 79% of contemporary plants went extinct at the Cretaceous-Tertiary boundary. Hickey and Johnson embraced the idea of a catastrophic end-Cretaceous mass extinction after having previously denouncing it. Even Archibald was forced to admit that there had been a catastrophic extinction of plant life at the end of the Cretaceous due to this study.[19]
Gerta Keller reported her findings on foraminiferans after having collected their fossils from the Brazos region of Texas and El Kef, Tunisia. She found that 35-40% of foraminiferans had gones extinct 300,00-400,000 years prior to the K-T boundary. She argued that this ruled out the possibility that they were victims of a catastrophic mass extinction event.[74]
Anderson argued that oxygen levels in Earth's atmosphere were high enough during the Mesozoic to fuel huge wildfires ignited by the end-Cretaceous impact event and these wildfires were what killed off the dinosaurs.[33]
Hut and others suggested that the impact at the end of the Cretaceous might actually have been on of a series of impacts that all contributed the Cretaceous-Paleogene extinction event.[40]
Zachos and others found evidence that photosynthesis nearly halted in Earth's oceans and seas near the Cretaceous-Tertiary boundary.[24]
Molnar and O'Reagan listed a wide variety of hypotheses that had been put for to explain the extinction of the dinosaurs.[75]
Paladino and others hypothesized that if dinosaurs had temperature-dependent sex determination then rapid climate change at the end of the Cretaceous could have led to strongly imbalanced sex ratios among the ensuing generations. If the male to female ratio was sufficiently imbalanced, there may not have been enough prospective mates to go around and the population could crash, leading to their extinction.[76]
Gostin and others reported gold and platinum group metals at a 600 million year old near Acraman, Australia. This proved that impact events could introduce elevated iridium levels to the rock record.[77]
Carter and Officer rejected "all[sic]" of the conclusions published the previous year by Alexopoulos and others regarding shocked quartz.[57]
Toutain and Meyer found evidence for high levels of iridium in volcanic emissions on Reunion island. This provided evidence that terrestrial geologic processes could leave high levels of iridium behind in the rock record without need for an impact to explain them.[55]
Koeberl reported the presence of high iridium levels in volcanic dust under antarctic ice. This provided evidence that terrestrial geologic processes could leave high levels of iridium behind in the rock record without need for an impact to explain them.[55]
Kunk and others radiometrically dated the subsurface crater at Manson, Iowa right to the Cretaceous-Tertiary boundary raising the possiblity that it was the result of the extinction-triggering impact at the end of the period.[78]
June: Alan Hildebrand visited Florentin Maurasse, a geologist who had reported the discovery of intriguing Cretaceous-Tertiary rocks in southern Haiti that Hildebrand hoped may provide evidence for the extinction-triggering impact crater. Hildebrand realized that some samples Maurasse attribute to volcanism were actually evidence of an impact and set out to perform his own field work in Haiti.[79]
Upchurch argued that plant fossils offered important insight into the Cretaceous-Tertiary extinction event because of their role as primary producers at the base of the food chain, their different living requirements from animals can yield information regarding environmental changes, and their great abundance in ancient ecosystems contributes to a more abundant fossil record that is less likely to suffer from the sampling problems that afflict the fossil records of less common taxa like dinosaurs.[17]
Tsakas and David proposed that elevated levels of cosmic ray bombardment or ultraviolet radiation caused harmful mutations to accumulate in the dinosaur gene pool, rendering them susceptible to environmental changes that ultimate drove them extinct.[64]
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^Cite error: The named reference powell-volc-damage-94 was invoked but never defined (see the help page).
^Cite error: The named reference powell-volc-clues-98 was invoked but never defined (see the help page).
^Cite error: The named reference powell-acrimony-162-163 was invoked but never defined (see the help page).
^ abcCite error: The named reference powell-andback-171 was invoked but never defined (see the help page).
^ abCite error: The named reference powell-without-26 was invoked but never defined (see the help page).
^Cite error: The named reference powell-without-26-27 was invoked but never defined (see the help page).
^Cite error: The named reference powell-prediction7-62-63 was invoked but never defined (see the help page).
^Cite error: The named reference powell-hills-75-76 was invoked but never defined (see the help page).
^Cite error: The named reference powell-shock-78-79 was invoked but never defined (see the help page).
^ abcdCite error: The named reference powell-volc-irid-86 was invoked but never defined (see the help page).
^ abCite error: The named reference powell-hills-76 was invoked but never defined (see the help page).
^ abcdeCite error: The named reference powell-shock-80 was invoked but never defined (see the help page).
^ abCite error: The named reference powell-ejecta-111 was invoked but never defined (see the help page).
^Cite error: The named reference powell-acrimony-163 was invoked but never defined (see the help page).
^Cite error: The named reference powell-acrimony-163-164 was invoked but never defined (see the help page).
^Cite error: The named reference powell-andback-170 was invoked but never defined (see the help page).
^Cite error: The named reference powell-newspeak-34 was invoked but never defined (see the help page).
^Cite error: The named reference powell-prediction1-58-59 was invoked but never defined (see the help page).
^ abCite error: The named reference bent-biotic-382 was invoked but never defined (see the help page).
^Cite error: The named reference arch-fast-regress-673 was invoked but never defined (see the help page).
^Cite error: The named reference powell-forams-155 was invoked but never defined (see the help page).
^Cite error: The named reference powell-hills-76-77 was invoked but never defined (see the help page).
^Cite error: The named reference powell-hills-77 was invoked but never defined (see the help page).
^Cite error: The named reference powell-top-106-107 was invoked but never defined (see the help page).
^Cite error: The named reference powell-top-107 was invoked but never defined (see the help page).
^Cite error: The named reference powell-acrimony-164 was invoked but never defined (see the help page).
^Cite error: The named reference powell-acrimony-165 was invoked but never defined (see the help page).
^Cite error: The named reference powell-ammonites-147 was invoked but never defined (see the help page).
^Cite error: The named reference powell-forams-152-153 was invoked but never defined (see the help page).
^Cite error: The named reference enbd-table13-1-249 was invoked but never defined (see the help page).
^Cite error: The named reference enbd-reason1-248 was invoked but never defined (see the help page).
^Cite error: The named reference powell-prediction3-59 was invoked but never defined (see the help page).
^Cite error: The named reference powell-manson-100 was invoked but never defined (see the help page).
^Cite error: The named reference powell-red-102 was invoked but never defined (see the help page).
