Editing Extinction event (section)

== The study of major extinction events ==

=== Breakthrough studies in the 1980s–1990s ===
[[File:LWA with Walt.JPG|thumb|[[Luis Walter Alvarez|Luis]] (left) and [[Walter Alvarez]] (right) at the [[Cretaceous–Paleogene boundary|K-Pg boundary]] in [[Gubbio, Italy]] in 1981. This team discovered geological evidence for an asteroid impact causing the K-Pg extinction, spurring a wave of public and scientific interest in mass extinctions and their causes]]
For much of the 20th century, the study of mass extinctions was hampered by insufficient data. Mass extinctions, though acknowledged, were considered mysterious exceptions to the prevailing [[Gradualism|gradualistic]] view of prehistory, where slow evolutionary trends define faunal changes. The first breakthrough was published in 1980 by a team led by [[Luis Walter Alvarez|Luis Alvarez]], who discovered trace metal evidence for an [[Impact event|asteroid impact]] at the end of the [[Cretaceous]] period. The [[Alvarez hypothesis]] for the [[End-Cretaceous extinction event|end-Cretaceous extinction]] gave mass extinctions, and [[Catastrophism#Current application|catastrophic]] explanations, newfound popular and scientific attention.<ref name="Alvarez">{{cite journal | vauthors = Alvarez LW, Alvarez W, Asaro F, Michel HV | date = June 1980 | title = Extraterrestrial cause for the cretaceous-tertiary extinction | journal = Science | volume = 208 | issue = 4448 | pages = 1095–1108 | pmid = 17783054 | doi = 10.1126/science.208.4448.1095 | bibcode = 1980Sci...208.1095A | s2cid = 16017767 | citeseerx = 10.1.1.126.8496 }}</ref>
[[File:Sepkoski 1997 extinction graphs.png|left|thumb|upright=1.4|Changes in diversity among genera and families, according to Sepkoski (1997). The "Big Five" mass extinctions are labelled with arrows, and taxa are segregated into Cambrian- (Cm), Paleozoic- (Pz), and Modern- (Md) type faunas.]]
Another landmark study came in 1982, when a paper written by [[David M. Raup]] and [[Jack Sepkoski]] was published in the journal ''[[Science (journal)|Science]]''.<ref name=Raup/> This paper, originating from a compendium of extinct marine animal [[Family (biology)|families]] developed by Sepkoski,<ref name=Sepkoski_1982>{{cite report |author=Sepkoski, J.J. Jr. |year=1982 |title=A compendium of fossil marine families |series=Milwaukee Public Museum Contributions in Biology and Geology |volume=51 |pages=1–125 |url=https://www.mpm.edu/sites/default/files/files%20and%20dox/C%26R/library/bio-geo/%23051%20MPM%20Contributions%20in%20Biology%20and%20Geology%20Number%2051.pdf }}</ref> identified five peaks of marine family extinctions that stand out among a backdrop of decreasing extinction rates through time. Four of these peaks were statistically significant: the [[Ashgillian]] ([[End-Ordovician extinction event|end-Ordovician]]), [[Late Permian]], [[Norian]] ([[End-Triassic extinction|end-Triassic]]), and [[Maastrichtian]] (end-Cretaceous). The remaining peak was a broad interval of high extinction smeared over the later half of the [[Devonian]], with its apex in the [[Frasnian]] stage.<ref name="Raup" />

Through the 1980s, Raup and Sepkoski continued to elaborate and build upon their extinction and origination data, defining a high-resolution [[biodiversity]] curve (the "Sepkoski curve") and successive [[evolutionary fauna]]s with their own patterns of diversification and extinction.<ref>{{Cite journal | author = Sepkoski, J.J. Jr. |year=1981 |title=A factor analytic description of the Phanerozoic marine fossil record |journal=Paleobiology |language=en |volume=7 |issue=1 |pages=36–53 |doi=10.1017/S0094837300003778 |bibcode=1981Pbio....7...36S |s2cid=133114885 |issn=0094-8373 |url=https://websites.pmc.ucsc.edu/~pkoch/EART_206/09-0120/Supplemental/Sepkoski%2081%20Pbio%207-36.pdf}}</ref><ref>{{cite journal | vauthors = Sepkoski JJ, Bambach RK, Raup DM, Valentine JW |year=1981 |title=Phanerozoic marine diversity and the fossil record |journal=Nature |language=en |volume=293 |issue=5832 |pages=435–437 |doi=10.1038/293435a0 |bibcode=1981Natur.293..435S |s2cid=4282371 |issn=1476-4687 |url= http://www.tomwbell.net/uploads/5/6/9/7/56976837/293435a0__1_.pdf}}</ref><ref>{{Cite book | vauthors = Sepkoski JJ |title=Geological Implications of Impacts of Large Asteroids and Comets on the Earth |date=1982-01-01 |chapter=Mass extinctions in the Phanerozoic oceans: A review |publisher=Geological Society of America |series=Geological Society of America Special Papers |volume=190 |pages=283–290 |language=en |id=Special Paper 190 |doi=10.1130/SPE190-p283 |isbn=0-8137-2190-3  |chapter-url=https://pubs.geoscienceworld.org/gsa/books/book/350/chapter/3796461/Mass-extinctions-in-the-Phanerozoic-oceans-A}}</ref><ref>{{Cite journal | vauthors = Sepkoski JJ |year=1984 |title=A kinetic model of Phanerozoic taxonomic diversity. III. Post-Paleozoic families and mass extinctions |journal=Paleobiology |language=en |volume=10 |issue=2 |pages=246–267 |doi=10.1017/S0094837300008186 |bibcode=1984Pbio...10..246S |s2cid=85595559 |issn=0094-8373 |url=https://www.cambridge.org/core/product/identifier/S0094837300008186/type/journal_article|url-access=subscription }}</ref><ref name=Sepkoski_1986>{{cite book | vauthors = Sepkoski JJ |year=1986 | chapter = Phanerozoic overview of mass extinction | title = Patterns and Processes in the History of Life |series=Dahlem Workshop Reports |pages=277–295 | veditors = Raup DM, Jablonski D |place=Berlin & Heidelberg, DE |publisher=Springer Berlin Heidelberg |language=en |doi=10.1007/978-3-642-70831-2_15 |isbn=978-3-642-70833-6 | chapter-url=http://link.springer.com/10.1007/978-3-642-70831-2_15 |access-date=2022-08-14 }}</ref><ref>{{cite journal | vauthors = Sepkoski JJ | year = 1989 | title = Periodicity in extinction and the problem of catastrophism in the history of life | journal = Journal of the Geological Society | volume = 146 | issue = 1 | pages = 7–19 | pmid = 11539792 | doi = 10.1144/gsjgs.146.1.0007 | bibcode = 1989JGSoc.146....7S | s2cid = 45567004 }}</ref> Though these interpretations formed a strong basis for subsequent studies of mass extinctions, Raup and Sepkoski also proposed a more controversial idea in 1984: a 26-million-year periodic pattern to mass extinctions.<ref name=Raup1984/> Two teams of [[astronomer]]s linked this to a hypothetical [[brown dwarf]] in the distant reaches of the [[Solar System]], inventing the "[[Nemesis hypothesis]]", which has been strongly disputed by other astronomers.

Around the same time, Sepkoski began to devise a compendium of marine animal [[genera]], which would allow researchers to explore extinction at a finer taxonomic resolution. He began to publish preliminary results of this in-progress study as early as 1986, in a paper that identified 29 extinction intervals of note.<ref name=Sepkoski_1986/> By 1992, he also updated his 1982 family compendium, finding minimal changes to the diversity curve despite a decade of new data.<ref name="Sepkoski_1992">{{cite report | vauthors = Sepkoski Jr JJ |year=1992 |title=A compendium of fossil marine animal families |edition=2nd |series=Milwaukee Public Museum Contributions in Biology and Geology |volume=83 |pages=1–156 |pmid=11542296 |url=https://www.mpm.edu/sites/default/files/files%20and%20dox/C%26R/library/bio-geo/%23083%20MPM%20Contributions%20in%20Biology%20and%20Geology%20Number%2083.pdf }}</ref><ref>{{cite journal | vauthors = Sepkoski JJ | year = 1993 | title = Ten years in the library: New data confirm paleontological patterns | journal = Paleobiology | volume = 19 | issue = 1 | pages = 43–51 | pmid = 11538041 | doi = 10.1017/S0094837300012306 | bibcode = 1993Pbio...19...43S | s2cid = 44295283 }}</ref> In 1996, Sepkoski published another paper that tracked marine genera extinction (in terms of net diversity loss) by stage, similar to his previous work on family extinctions. The paper filtered its sample in three ways: all genera (the entire unfiltered sample size), multiple-interval genera (only those found in more than one stage), and "well-preserved" genera (excluding those from groups with poor or understudied fossil records). Diversity trends in marine animal families were also revised based on his 1992 update.<ref name="Sepkoski_1996">{{cite book | vauthors = Sepkoski JJ |chapter=Patterns of Phanerozoic Extinction: A Perspective from Global Data Bases |year=1996 |title = Global Events and Event Stratigraphy in the Phanerozoic |pages=35–51 | veditors = Walliser OH |place=Berlin & Heidelberg, DE |publisher=Springer Berlin Heidelberg |language=en |doi=10.1007/978-3-642-79634-0_4 |isbn=978-3-642-79636-4 | chapter-url=http://link.springer.com/10.1007/978-3-642-79634-0_4 |access-date=2022-08-14}}</ref>

Revived interest in mass extinctions led many other authors to re-evaluate geological events in the context of their effects on life.<ref>{{cite journal | vauthors = Jablonski D | date = August 1991 | title = Extinctions: A paleontological perspective | journal = Science | volume = 253 | issue = 5021 | pages = 754–757 | pmid = 17835491 | doi = 10.1126/science.253.5021.754 | bibcode = 1991Sci...253..754J }}</ref> A 1995 paper by [[Michael Benton]] tracked extinction and origination rates among both marine and continental (freshwater & terrestrial) families, identifying 22 extinction intervals and no periodic pattern.<ref>{{cite journal | vauthors = Benton MJ | date = April 1995 | title = Diversification and extinction in the history of life | journal = Science | volume = 268 | issue = 5207 | pages = 52–58 | pmid = 7701342 | doi = 10.1126/science.7701342 | bibcode = 1995Sci...268...52B | url = http://doc.rero.ch/record/14806/files/PAL_E1962.pdf }}</ref> Overview books by O.H. Walliser (1996) and [[Anthony Hallam|A. Hallam]] and P.B. Wignall (1997) summarized the new extinction research of the previous two decades.<ref>{{Cite book | veditors = Walliser OH |year=1996 |title=Global Events and Event Stratigraphy in the Phanerozoic: Results of the International Interdisciplinary Cooperation in the IGCP-Project 216 "Global Biological Events in Earth History" |publisher=Springer Berlin Heidelberg |isbn=978-3-642-79636-4 |location=Berlin, Heidelberg |language=en |doi=10.1007/978-3-642-79634-0 |url=http://link.springer.com/10.1007/978-3-642-79634-0}}</ref><ref>{{Cite book | vauthors = Hallam A, Wignall PB |title=Mass Extinctions and Their Aftermath  |publisher=Oxford University Press |year=1997 |location=Oxford}}</ref> One chapter in the former source lists over 60 geological events that could conceivably be considered global extinctions of varying sizes.<ref>{{cite book | vauthors = Barnes CR, Hallam A, Kaljo D, Kauffman EG, Walliser OH |year=1996 | chapter = Global Event Stratigraphy |title = Global Events and Event Stratigraphy in the Phanerozoic |pages=319–333 |place=Berlin & Heidelberg, DE |publisher=Springer Berlin Heidelberg | doi = 10.1007/978-3-642-79634-0_16 |isbn=978-3-642-79636-4 }}</ref> These texts, and other widely circulated publications in the 1990s, helped to establish the popular image of mass extinctions as a "big five" alongside many smaller extinctions through prehistory.

=== New data on genera: Sepkoski's compendium ===
[[File:Bambach 2006 extinction graphs.png|thumb|350x350px|Major [[Phanerozoic]] extinctions tracked via proportional genera extinctions by Bambach (2006)]]
Though Sepkoski died in 1999, his marine genera compendium was formally published in 2002. This prompted a new wave of studies into the dynamics of mass extinctions.<ref name="Sepkoski_2002">{{Cite journal | vauthors = Sepkoski Jr JJ |date=2002 | veditors = Jablonski D, Foote M |title=A Compendium of Fossil Marine Animal Genera |url=https://www.biodiversitylibrary.org/item/40634#page/5/mode/1up |journal=Bulletins of American Paleontology |volume=363 |pages=1–560}}</ref> These papers utilized the compendium to track origination rates (the rate that new species appear or [[Speciation|speciate]]) parallel to extinction rates in the context of geological stages or substages.<ref name="Foote_2000">{{Cite journal | vauthors = Foote M |date=2000 |title=Origination and extinction components of taxonomic diversity: General problems |url= https://www.cambridge.org/core/journals/paleobiology/article/abs/origination-and-extinction-components-of-taxonomic-diversity-general-problems/7FE12B43106FC20C9AFC4031F72A56F9 |journal=Paleobiology |language=en |volume=26 |issue=S4 |pages=74–102 |doi=10.1017/S0094837300026890 |bibcode=2000Pbio...26S..74F |s2cid=53341052 |issn=0094-8373|url-access=subscription }}</ref> A review and re-analysis of Sepkoski's data by Bambach (2006) identified 18 distinct mass extinction intervals, including 4 large extinctions in the [[Cambrian]]. These fit Sepkoski's definition of extinction, as short substages with large diversity loss and overall high extinction rates relative to their surroundings.<ref name="Bambach_2006">{{Cite journal | vauthors = Bambach RK |date= May 2006 |title=Phanerozoic Biodiversity Mass Extinctions |journal=Annual Review of Earth and Planetary Sciences |language=en |volume=34 |issue=1 |pages=127–155 |doi=10.1146/annurev.earth.33.092203.122654 |bibcode= 2006AREPS..34..127B |issn=0084-6597}}</ref>

Bambach et al. (2004) considered each of the "Big Five" extinction intervals to have a different pattern in the relationship between origination and extinction trends. Moreover, background extinction rates were broadly variable and could be separated into more severe and less severe time intervals. Background extinctions were least severe relative to the origination rate in the middle Ordovician-early Silurian, late Carboniferous-Permian, and Jurassic-recent. This argues that the Late Ordovician, end-Permian, and end-Cretaceous extinctions were statistically significant outliers in biodiversity trends, while the Late Devonian and end-Triassic extinctions occurred in time periods that were already stressed by relatively high extinction and low origination.<ref>{{Cite journal | vauthors = Bambach RK, Knoll AH, Wang SC |date=2004 |title=Origination, extinction, and mass depletions of marine diversity |url=https://www.cambridge.org/core/journals/paleobiology/article/abs/origination-extinction-and-mass-depletions-of-marine-diversity/15BF4851C6E3C95D8486926A87ECD9B3B3 |journal=Paleobiology |language=en |volume=30 |issue=4 |pages=522–542 |doi=10.1666/0094-8373(2004)030<0522:OEAMDO>2.0.CO;2 |bibcode=2004Pbio...30..522B |s2cid=17279135 |issn=0094-8373}}</ref>

Computer models run by Foote (2005) determined that abrupt pulses of extinction fit the pattern of prehistoric biodiversity much better than a gradual and continuous background extinction rate with smooth peaks and troughs. This strongly supports the utility of rapid, frequent mass extinctions as a major driver of diversity changes. Pulsed origination events are also supported, though to a lesser degree that is largely dependent on pulsed extinctions.<ref>{{Cite journal | vauthors = Foote M |date=2005 |title=Pulsed origination and extinction in the marine realm |journal=Paleobiology |volume=31 |issue=1 |pages=6–20 |doi=10.1666/0094-8373(2005)031<0006:POAEIT>2.0.CO;2|bibcode=2005Pbio...31....6F |s2cid=53469954 |url=http://doc.rero.ch/record/14957/files/PAL_E2104.pdf }}</ref>

Similarly, Stanley (2007) used extinction and origination data to investigate turnover rates and extinction responses among different evolutionary faunas and taxonomic groups. In contrast to previous authors, his diversity simulations show support for an overall exponential rate of biodiversity growth through the entire Phanerozoic.<ref>{{Cite journal | vauthors = Stanley SM |date=2007 |title=Memoir 4: An Analysis of the History of Marine Animal Diversity |journal=Paleobiology |language=en |volume=33 |issue=S4 |pages=1–55 |doi=10.1017/S0094837300019217 |bibcode=2007Pbio...33Q...1S |s2cid=90130435 |issn=0094-8373}}</ref>

=== Tackling biases in the fossil record ===
[[File:Signor Lipps.gif|thumb|252x252px|An illustration of the [[Signor–Lipps effect|Signor-Lipps effect]], a geological bias that posits that increased fossil sampling would help to better constrain the exact time when an organism truly goes extinct.]]
As data continued to accumulate, some authors began to re-evaluate Sepkoski's sample using methods meant to account for [[sampling bias]]es. As early as 1982, a paper by Phillip W. Signor and [[Jere H. Lipps]] noted that the true sharpness of extinctions was diluted by the incompleteness of the fossil record.<ref>Signor III, P. W. and Lipps, J. H. (1982) "[http://specialpapers.gsapubs.org/content/190/291.full.pdf Sampling bias, gradual extinction patterns, and catastrophes in the fossil record]", in ''Geological implications of impacts of large asteroids and comets on the Earth'' (ed. L. T. Silver and P. H. Schultz), Geological Society of America Special Publication, vol. 190, pp. 291–296.</ref> This phenomenon, later called the [[Signor–Lipps effect|Signor-Lipps effect]], notes that a species' true extinction must occur after its last fossil, and that origination must occur before its first fossil. Thus, species that appear to die out just prior to an abrupt extinction event may instead be a victim of the event, despite an apparent gradual decline looking at the fossil record alone. A model by Foote (2007) found that many geological stages had artificially inflated extinction rates due to Signor-Lipps "backsmearing" from later stages with extinction events.<ref name="Foote_2007">{{Cite journal | vauthors = Foote M |date=2007 |title=Extinction and quiescence in marine animal genera |url=https://www.cambridge.org/core/product/identifier/S0094837300023794/type/journal_article |journal=Paleobiology |language=en |volume=33 |issue=2 |pages=261–272 |doi=10.1666/06068.1 |s2cid=53402257 |issn=0094-8373|url-access=subscription }}</ref>
[[File:Foote 2007 Kocsis 2019 extinction graphs.png|left|thumb|502x502px|Estimated extinction rates among genera through time. From Foote (2007), top, and Kocsis et al. (2019), bottom]]
Other biases include the difficulty in assessing taxa with high turnover rates or restricted occurrences, which cannot be directly assessed due to a lack of fine-scale temporal resolution. Many paleontologists opt to assess diversity trends by randomized sampling and [[Rarefaction (ecology)|rarefaction]] of fossil abundances rather than raw temporal range data, in order to account for all of these biases. But that solution is influenced by biases related to sample size. One major bias in particular is the "[[Pull of the recent]]", the fact that the fossil record (and thus known diversity) generally improves closer to the modern day. This means that biodiversity and abundance for older geological periods may be underestimated from raw data alone.<ref name="Foote_2000" /><ref name="Foote_2007" /><ref name="Alroy_2008" />

[[John Alroy|Alroy]] (2010) attempted to circumvent sample size-related biases in diversity estimates using a method he called "[[shareholder]] quorum subsampling" (SQS). In this method, fossils are sampled from a "collection" (such as a time interval) to assess the relative diversity of that collection. Every time a new species (or other [[taxon]]) enters the sample, it brings over all other fossils belonging to that species in the collection (its "[[Share (finance)|share]]" of the collection). For example, a skewed collection with half its fossils from one species will immediately reach a sample share of 50% if that species is the first to be sampled. This continues, adding up the sample shares until a "coverage" or "[[quorum]]" is reached, referring to a pre-set desired sum of share percentages. At that point, the number of species in the sample are counted. A collection with more species is expected to reach a sample quorum with more species, thus accurately comparing the relative diversity change between two collections without relying on the biases inherent to sample size.<ref name="Alroy_2010">{{Cite journal | vauthors = Alroy J |date=2010 |title=Fair Sampling of Taxonomic Richness and Unbiased Estimation of Origination and Extinction Rates |url=http://dx.doi.org/10.1017/s1089332600001819 |journal=The Paleontological Society Papers |volume=16 |pages=55–80 |doi=10.1017/s1089332600001819 |issn=1089-3326|url-access=subscription }}</ref>

Alroy also elaborated on three-timer algorithms, which are meant to counteract biases in estimates of extinction and origination rates. A given taxon is a "three-timer" if it can be found before, after, and within a given time interval, and a "two-timer" if it overlaps with a time interval on one side. Counting "three-timers" and "two-timers" on either end of a time interval, and sampling time intervals in sequence, can together be combined into equations to predict extinction and origination with less bias.<ref name="Alroy_2010" /> In subsequent papers, Alroy continued to refine his equations to improve lingering issues with precision and unusual samples.<ref>{{Cite journal | vauthors = Alroy J |date=2014 |title=Accurate and precise estimates of origination and extinction rates |url=https://www.cambridge.org/core/product/identifier/S0094837300001871/type/journal_article |journal=Paleobiology |language=en |volume=40 |issue=3 |pages=374–397 |doi=10.1666/13036 |s2cid=53125415 |issn=0094-8373|url-access=subscription }}</ref><ref>{{Cite journal | vauthors = Alroy J |date=2015 |title=A more precise speciation and extinction rate estimator |url=https://www.cambridge.org/core/product/identifier/S0094837315000263/type/journal_article |journal=Paleobiology |language=en |volume=41 |issue=4 |pages=633–639 |doi=10.1017/pab.2015.26 |bibcode=2015Pbio...41..633A |s2cid=85842940 |issn=0094-8373|url-access=subscription }}</ref>

McGhee et al. (2013), a paper thatprimarily focused on ecological effects of mass extinctions, also published new estimates of extinction severity based on Alroy's methods. Many extinctions were significantly more impactful under these new estimates, though some were less prominent.<ref name="McGhee_2013">{{Cite journal | vauthors = McGhee Jr GR, Clapham ME, Sheehan PM, Bottjer DJ, Droser ML |date= January 2013 |title=A new ecological-severity ranking of major Phanerozoic biodiversity crises |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |language=en |volume=370 |pages=260–270 |doi=10.1016/j.palaeo.2012.12.019 |bibcode= 2013PPP...370..260M |issn=0031-0182}}</ref>

Stanley (2016) was another paper that attempted to remove two common errors in previous estimates of extinction severity. The first error was the unjustified removal of "singletons", genera unique to only a single time slice. Their removal would mask the influence of groups with high turnover rates or lineages cut short early in their diversification. The second error was the difficulty in distinguishing background extinctions from brief mass extinction events within the same short time interval. To circumvent this issue, background rates of diversity change (extinction/origination) were estimated for stages or substages without mass extinctions, and then assumed to apply to subsequent stages with mass extinctions. For example, the [[Santonian]] and [[Campanian]] stages were each used to estimate diversity changes in the [[Maastrichtian]] prior to the K-Pg mass extinction. Subtracting background extinctions from extinction tallies had the effect of reducing the estimated severity of the six sampled mass extinction events. This effect was stronger for mass extinctions that occurred in periods with high rates of background extinction, like the Devonian.<ref name="Stanley_2016">{{Cite journal | vauthors = Stanley SM |date= October 2016 |title=Estimates of the magnitudes of major marine mass extinctions in earth history |journal= Proceedings of the National Academy of Sciences|language=en |volume=113 |issue=42 |pages= E6325–E6334 |doi=10.1073/pnas.1613094113 |issn=0027-8424 |pmc=5081622 |pmid=27698119|bibcode= 2016PNAS..113E6325S |doi-access= free }}</ref>