Late Ordovician mass extinction

Marine extinction intensity during Phanerozoic

%

Millions of years ago

(H)

K–Pg

Tr–J

P–Tr

Cap

Late D

O–S

The blue graph shows the apparent percentage (not the absolute number) of marine animal genera becoming extinct during any given time interval. It does not represent all marine species, just those that are readily fossilized. The labels of the traditional "Big Five" extinction events and the more recently recognised Capitanian mass extinction event are clickable links; see Extinction event for more details. (source and image info)

The Late Ordovician mass extinction (LOME), sometimes known as the end-Ordovician mass extinction or the Ordovician-Silurian extinction, is the first of the "big five" major mass extinction events in Earth's history, occurring roughly 445 million years ago (Ma).^[1] It is often considered to be the second-largest known extinction event, in terms of the percentage of genera that became extinct.^[2][3] Extinction was global during this interval, eliminating 49–60% of marine genera and nearly 85% of marine species.^[4] Under most tabulations, only the Permian-Triassic mass extinction exceeds the Late Ordovician mass extinction in biodiversity loss. The extinction event abruptly affected all major taxonomic groups and caused the disappearance of one third of all brachiopod and bryozoan families, as well as numerous groups of conodonts, trilobites, echinoderms, corals, bivalves, and graptolites.^[5][6] Despite its taxonomic severity, the Late Ordovician mass extinction did not produce major changes to ecosystem structures compared to other mass extinctions, nor did it lead to any particular morphological innovations. Diversity gradually recovered to pre-extinction levels over the first 5 million years of the Silurian period.^{[7][8][9][10]}

The Late Ordovician mass extinction is traditionally considered to occur in two distinct pulses.

Some researchers have proposed the existence of a third distinct pulse of the mass extinction during the early Rhuddanian, evidenced by a negative carbon isotope excursion and a pulse of anoxia into shelf environments amidst already low background oxygen levels. Others, however, have argued that Rhuddanian anoxia was simply part of the second pulse, which according to this view was longer and more drawn out than most authors suggest.[18]

Impact on life

Ecological impacts

The Late Ordovician mass extinction followed the Great Ordovician Biodiversification Event (GOBE), one of the largest surges of increasing biodiversity in the geological and biological history of the Earth.^[19] At the time of the extinction, most complex multicellular organisms lived in the sea, and the only evidence of life on land are rare spores from small early land plants.

At the time of the extinction, around 100 marine

Following such a major loss of diversity, Silurian communities were initially less complex and broader niched.[1] Nonetheless, in South China, warm-water benthic communities with complex trophic webs thrived immediately following LOME.^[22] Highly endemic faunas, which characterized the Late Ordovician, were replaced by faunas that were amongst the most cosmopolitan in the Phanerozoic, biogeographic patterns that persisted throughout most of the Silurian.^[1] LOME had few of the long-term ecological impacts associated with the Permian–Triassic and Cretaceous–Paleogene extinction events.^[7][9] Furthermore, biotic recovery from LOME proceeded at a much faster rate than it did after the Permian-Triassic extinction.^[23] Nevertheless, a large number of taxa disappeared from the Earth over a short time interval, eliminating and altering the relative diversity and abundance of certain groups.^[1] The Cambrian-type evolutionary fauna nearly died out, and was unable to rediversify after the extinction.^[10]

Biodiversity changes in marine invertebrates

Brachiopods

Brachiopod diversity and composition was strongly affected, with the Cambrian-type

The brachiopod survival intervals following the second pulse spanned the terminal Hirnantian to the middle Rhuddanian, after which the recovery interval began and lasted until the early Aeronian.[27] Overall, the brachiopod recovery in the late Rhuddanian was rapid.^[28] Brachiopod survivors of the mass extinction tended to be endemic to one palaeoplate or even one locality in the survival interval in the earliest Silurian, though their ranges geographically expanded over the course of the biotic recovery.^[29] The region around what is today Oslo was a hotbed of atrypide rediversification.^[30] Brachiopod recovery consisted mainly of the reestablishment of cosmopolitan brachiopod taxa from the Late Ordovician.^[31] Progenitor taxa that arose following the mass extinction displayed numerous novel adaptations for resisting environmental stresses.^[32] Although some brachiopods did experience the Lilliput effect in response to the extinction, this phenomenon was not particularly widespread compared to other mass extinctions.^[33]

Trilobites

Trilobites were hit hard by both phases of the extinction, with about 70% of genera and 50% of families going extinct between the Katian and Silurian. The extinction disproportionately affected deep water species and groups with fully planktonic larvae or adults. The order Agnostida was completely wiped out, and the formerly diverse Asaphida survived with only a single genus, Raphiophorus.^[34][35][10] A cool-water trilobite assemblage, the Mucronaspis fauna, coincides with the Hirnantia brachiopod fauna in the timing of its expansion and demise.^[1][26] Trilobite faunas after the extinction were dominated by families that appeared in the Ordovician and survived LOME, such as Encrinuridae and Odontopleuridae.^[36]

Bryozoans

Over a third of bryozoan genera went extinct, but most families survived the extinction interval and the group as a whole recovered in the Silurian. The hardest-hit subgroups were the cryptostomes and trepostomes, which never recovered the full extent of their Ordovician diversity. Bryozoan extinctions started in coastal regions of Laurentia, before high extinction rates shifted to Baltica by the end of the Hirnantian.^[37][10][1] Bryozoan biodiversity loss appears to have been a prolonged process which partially preceded the Hirnantian extinction pulses. Extinction rates among Ordovician bryozoan genera were actually higher in the early and late Katian, and origination rates sharply dropped in the late Katian and Hirnantian.^[38]

Echinoderms

About 70% of crinoid genera died out. Early studies of crinoid biodiversity loss by Jack Sepkoski overestimated crinoid biodiversity losses during LOME.^[39] Most extinctions occurred in the first pulse. However, they rediversified quickly in tropical areas and reacquired their pre-extinction diversity not long into the Silurian. Many other echinoderms became very rare after the Ordovician, such as the cystoids, edrioasteroids, and other early crinoid-like groups.^[10][1]

Sponges

The cause of the glaciation is heavily debated. The

This incurred a shift in the location of bottom water formation, shifting from low latitudes, characteristic of greenhouse conditions, to high latitudes, characteristic of icehouse conditions, which was accompanied by increased deep-ocean currents and oxygenation of the bottom water. An opportunistic fauna briefly thrived there, before anoxic conditions returned. The breakdown in the oceanic circulation patterns brought up nutrients from the abyssal waters. Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions.

However, not all studies agree that cooling and glaciation caused LOMEI-1. One study suggests that the first pulse began not during the rapid Hirnantian ice cap expansion but in an interval of deglaciation following it.^[66]

Anoxia and euxinia

Another heavily-discussed factor in the Late Ordovician mass extinction is

sulfate-reducing bacteria

(bottom) could instead have been responsible for the excursion without contributing to anoxia.

Some geologists have argued that anoxia played a role in the first extinction pulse, though this hypothesis is controversial. In the early Hirnantian, shallow-water sediments throughout the world experience a large positive excursion in the δ³⁴S ratio of buried pyrite. This ratio indicates that shallow-water pyrite which formed at the beginning of the glaciation had a decreased proportion of ³²S, a common lightweight isotope of sulfur. ³²S in the seawater could hypothetically be used up by extensive deep-sea pyrite deposition.^[69] The Ordovician ocean also had very low levels of sulfate, a nutrient which would otherwise resupply ³²S from the land. Pyrite forms most easily in anoxic and euxinic environments, while better oxygenation encourages the formation of gypsum instead.^[67] As a result, anoxia and euxinia would need to be common in the deep sea to produce enough pyrite to shift the δ³⁴S ratio.^[70][71]

Thallium isotope ratios can also be used as indicators of anoxia. A major positive ε²⁰⁵Tl excursion in the late Katian, just before the Katian-Hirnantian boundary, likely reflects a global enlargement of oxygen minimum zones. During the late Katian, thallium isotopic perturbations indicating proliferation of anoxic waters notably preceded the appearance of other geochemical indicators of the expansion of anoxia.^[72]

A more direct proxy for anoxic conditions is Fe_HR/Fe_T. This ratio describes the comparative abundance of highly reactive iron compounds which are only stable without oxygen. Most geological sections corresponding to the beginning of the Hirnantian glaciation have Fe_HR/Fe_T below 0.38, indicating oxygenated waters.^[70] However, higher Fe_HR/Fe_T values are known from a few deep-water early Hirnantian sequences found in China^[71] and Nevada.^[70] Elevated Fe_Py/Fe_HR values have also been found in association with LOMEI-1,^[71] including ones above 0.8 that are tell-tale indicators of euxinia.^[70]

Glaciation could conceivably trigger anoxic conditions, albeit indirectly. If continental shelves are exposed by falling sea levels, then organic surface runoff flows into deeper oceanic basins. The organic matter would have more time to leach out phosphate and other nutrients before being deposited on the seabed. Increased phosphate concentration in the seawater would lead to eutrophication and then anoxia. Deep-water anoxia and euxinia would impact deep-water benthic fauna, as expected for the first pulse of extinction. Chemical cycle disturbances would also steepen the chemocline, restricting the habitable zone of planktonic fauna which also go extinct in the first pulse. This scenario is congruent with both organic carbon isotope excursions and general extinction patterns observed in the first pulse.^[67]

However, data supporting deep-water anoxia during the glaciation contrasts with more extensive evidence for well-oxygenated waters.

Black shales, which are indicative of an anoxic environment, become very rare in the early Hirnantian compared to surrounding time periods. Although early Hirnantian black shales can be found in a few isolated ocean basins (such as the Yangtze platform of China), from a worldwide perspective these correspond to local events.^[50] Some Chinese sections record an early Hirnantian increase in the abundance of Mo-98, a heavy isotope of molybdenum. This shift can correspond to a balance between minor local anoxia^[73] and well-oxygenated waters on a global scale.^[74] Other trace elements point towards increased deep-sea oxygenation at the start of the glaciation.^[75][76] Oceanic current modelling suggest that glaciation would have encouraged oxygenation in most areas, apart from the Paleo-Tethys ocean.^[77] Devastation of the Dicranograptidae-Diplograptidae-Orthograptidae (DDO) graptolite fauna, which was well adapted to anoxic conditions, further suggests that LOMEI-1 was associated with increased oxygenation of the water column and not the other way around.^[78]

Deep-sea anoxia is not the only explanation for the δ³⁴S excursion of pyrite.

sulfate-reducing microbes living in the seabed. Sulfate-reducing microbes prioritize ³²S during anaerobic respiration, leaving behind heavier isotopes. A bloom of sulfate-reducing microbes can quickly account for the δ³⁴S excursion in marine sediments without a corresponding decrease in oxygen.^[68]

A few studies have proposed that the first extinction pulse did not begin with the Hirnantian glaciation, but instead corresponds to an interglacial period or other warming event. Anoxia would be the most likely mechanism of extinction in a warming event, as evidenced by other extinctions involving warming.[79]^[80][81] However, this view of the first extinction pulse is controversial and not widely accepted.^[50][82]

Late Hirnantian anoxia

The late Hirnantian experienced a dramatic increase in the abundance of black shales. Coinciding with the retreat of the Hirnantian glaciation, black shale expands out of isolated basins to become the dominant oceanic sediment at all latitudes and depths. The worldwide distribution of black shales in the late Hirnantian is indicative of a global anoxic event,^[50] which has been termed the Hirnantian ocean anoxic event (HOAE).^[83][17] Corresponding to widespread anoxia are δ³⁴S_CAS,^[84][85] δ⁹⁸Mo,^[74][73] δ²³⁸U,^[83][86][17] and εNd(t) excursions found in many different regions.^[87] At least in European sections, late Hirnantian anoxic waters were originally ferruginous (dominated by ferrous iron) before gradually becoming more euxinic.^[67] In the Yangtze Sea, located on the western margins of the South China microcontinent, the second extinction pulse occurred alongside intense euxinia which spread out from the middle of the continental shelf.^[88][71] Mercury loading in South China during LOMEI-2 was likely related to euxinia.^[89] However, some evidence suggests that the top of the water column in the Ordovician oceans remained well oxygenated even as the seafloor became deoxygenated.^[90] On a global scale, euxinia was probably one or two orders of magnitude more prevalent than in the modern day. Global anoxia may have lasted more than 3 million years, persisting through the entire Rhuddanian stage of the Silurian period. This would make the Hirnantian-Rhuddanian anoxia one of the longest-lasting anoxic events in geologic time.^[17]

The cause of the Hirnantian-Rhuddanian anoxic event is uncertain. Like most global anoxic events, an increased supply of nutrients (such as

oceanic stratification through an input of freshwater from melting glaciers. This would be more reasonable if the anoxic event coincided with the end of glaciation, as supported by most other studies.^[50] However, oceanic models argue that marine currents would recover too quickly for freshwater disruptions to have a meaningful effect on nutrient cycles. Retreating glaciers could expose more land to weathering, which would be a more sustained source of phosphates flowing into the ocean.^[77] There is also evidence implicating volcanism as a contributor to Late Hirnantian anoxia.^[94]

There were few clear patterns of extinction associated with the second extinction pulse. Every region and marine environment experienced the second extinction pulse to some extent. Many taxa which survived or diversified after the first pulse were finished off in the second pulse. These include the

graptolites and warm-water reef denizens were less affected.^[10][1][17] Sediments from China and Baltica seemingly show a more gradual replacement of the Hirnantia fauna after glaciation.^[95] Although this suggests that the second extinction pulse may have been a minor event at best, other paleontologists maintain that an abrupt ecological turnover accompanied the end of glaciation.^[26] There may be a correlation between the relatively slow recovery after the second extinction pulse, and the prolonged nature of the anoxic event which accompanied it.^[83][17] On the other hand, the occurrence of euxinic pulses similar in magnitude to LOMEI-2 during the Katian without ensuing biological collapses has caused some researchers to question whether euxinia alone could have been LOMEI-2's driver.^[96]

Early Rhuddanian anoxia

Deposition of black graptolite shales continued to be common into the earliest Rhuddanian, indicating that anoxia persisted well into the Llandovery. A sharp reduction in the average size of many organisms, likely attributable to the Lilliput effect, and the disappearance of many relict taxa from the Ordovician indicate a third extinction interval linked to an expansion of anoxic conditions into shallower shelf environments, particularly in Baltica. This sharp decline in dissolved oxygen concentrations was likely linked to a period of global warming documented by a negative carbon isotope excursion preserved in Baltican sediments.^[18]

Other potential factors

Metal poisoning

mercury concentrations during the lower late Katian, the Katian-Hirnantian boundary, and the late Hirnantian.^[97]

The toxic metals may have killed life forms in lower trophic levels of the food chain, causing a decline in population, and subsequently resulting in starvation for the dependent higher feeding life forms in the chain.^[98][99]

Gamma-ray burst

A minority hypothesis to explain the first burst has been proposed by Philip Ball,

extinct

than species dwelling in deep water, also consistent with the hypothetical effects of a galactic gamma-ray burst.

A gamma-ray burst could also explain the rapid expansion of glaciers, since the high energy rays would cause ozone, a greenhouse gas, to dissociate and its dissociated oxygen atoms to then react with nitrogen to form nitrogen dioxide, a darkly-coloured aerosol which cools the planet.^[106][102] It would also cohere with the major δ13C isotopic excursion indicating increased sequestration of carbon-12 out of the atmosphere, which would have occurred as a result of nitrogen dioxide, formed after the reaction of nitrogen and oxygen atoms dissociated by the gamma-ray burst, reacting with hydroxyl and raining back down to Earth as nitric acid, precipitating large quantities of nitrates that would have enhanced wetland productivity and sequestration of carbon dioxide.^[107][101] Although the gamma-ray burst hypothesis is consistent with some patterns at the onset of extinction, there is no unambiguous evidence that such a nearby gamma-ray burst ever happened.^[16]

Volcanism

Though more commonly associated with greenhouse gases and global warming, volcanoes may have cooled the planet and precipitated glaciation by discharging sulphur into the atmosphere.^[54] This is supported by a positive uptick in pyritic Δ³³S values, a geochemical signal of volcanic sulphur discharge, coeval with LOMEI-1.^[108]

More recently, in May 2020, a study suggested the first pulse of mass extinction was caused by volcanism which induced

global warming and anoxia, rather than cooling and glaciation.^[109][81] Higher resolution of species diversity patterns in the Late Ordovician suggest that extinction rates rose significantly in the early or middle Katian stage, several million years earlier than the Hirnantian glaciation. This early phase of extinction is associated with large igneous province (LIP) activity, possibly that of the Alborz LIP of northern Iran,^[110] as well as a warming phase known as the Boda event.^{[111][112][113]} However, other research still suggests the Boda event was a cooling event instead.^[114]

Increased volcanic activity during the early late Katian and around the Katian-Hirnantian boundary is also implied by heightened mercury concentrations relative to total organic carbon.[97]^[89] Marine bentonite layers associated with the subduction of the Junggar Ocean underneath the Yili Block have been dated to the late Katian, close to the Katian-Hirnantian boundary.^[115]

Volcanic activity could also provide a plausible explanation for anoxia during the first pulse of the mass extinction. A volcanic input of phosphorus, which was insufficient to enkindle persistent anoxia on its own, may have triggered a positive feedback loop of phosphorus recycling from marine sediments, sustaining widespread marine oxygen depletion over the course of LOMEI-1.^[11] Also, the weathering of nutrient-rich volcanic rocks emplaced during the middle and late Katian likely enhanced the reduction in dissolved oxygen.^[89] Intense volcanism also fits in well with the attribution of euxinia as the main driver of LOMEI-2; sudden volcanism at the Ordovician-Silurian boundary is suggested to have supplied abundant sulphur dioxide, greatly facilitating the development of euxinia.^[116]

Other papers have criticised the volcanism hypothesis, claiming that volcanic activity was relatively low in the Ordovician and that superplume and LIP volcanic activity is especially unlikely to have caused the mass extinction at the end of the Ordovician.^[2] A 2022 study argued against a volcanic cause of LOME, citing the lack of mercury anomalies and the discordance between deposition of bentonites and redox changes in drillcores from South China straddling the Ordovician-Silurian boundary.^[117] Mercury anomalies at the end of the Ordovician relative to total organic carbon, or Hg/TOC, that some researchers have attributed to large-scale volcanism have been reinterpreted by some to be flawed because the main mercury host in the Ordovician was sulphide, and thus Hg/TS should be used instead;^[118] Hg/TS values show no evidence of volcanogenic mercury loading,^[119] a finding bolstered by ∆¹⁹⁹Hg measurements much higher than would be expected for volcanogenic mercury input.^[118]

Asteroid impact

A 2023 paper points to the Deniliquin multiple-ring feature in southeastern Australia, which has been dated to around the start of LOMEI-1, for initiating the intense Hirnantian glaciation and the first pulse of the extinction event. According to the paper, it still requires further research to test the idea.^[57][120]

See also

• Global catastrophic risk
• Near-Earth supernova
• Anoxic event
• Late Devonian extinction
• Capitanian mass extinction event
• Permian–Triassic extinction event
• Triassic–Jurassic extinction event
• Cretaceous–Paleogene extinction event
• Andean-Saharan glaciation

Sources

1. ^
   doi:10.1016/j.gr.2012.12.021
   .
2. ^
   doi:10.1111/let.12252
   . Retrieved 23 October 2022.
3. ^ Marshall, Michael (24 May 2010). "The history of ice on Earth". New Scientist. Retrieved 12 April 2018.
4. ProQuest 1440071324
   .
5. ISBN 978-3-540-75915-7
   .
6. ^ ^{a b} Sole, R. V.; Newman, M. (2002). "The earth system: biological and ecological dimensions of global environment change". Encyclopedia of Global Environmental Change, Volume Two: Extinctions and Biodiversity in the Fossil Record. John Wiley & Sons. pp. 297–391.
7. ^
   ISSN 0091-7613
   .
8. ISSN 0091-7613
   .
9. ^
   S2CID 128870184
   .
10. ^
    ISSN 0084-6597
    .
11. ^
    doi:10.1038/s43247-022-00412-x
    .
12. ^ ^{a b} "Causes of the Ordovician Extinction". Archived from the original on 2008-05-09.
13. ^
    PMID 22511717
    .
14. ^
    PMID 27122567
    .
15. ^
    doi:10.1111/j.1502-3931.2002.tb00081.x
    . Retrieved 26 December 2022.
16. ^
    S2CID 129788917
    .
17. ^
    S2CID 215750045
    .
18. ^
    doi:10.1016/j.palaeo.2013.12.018
    . Retrieved 15 November 2022.
19. ^
    doi:10.1016/j.palaeo.2010.08.001
    .
20. S2CID 32520208
    .
21. S2CID 17279135
    .
22. S2CID 251480863
    . Retrieved 16 April 2023.
23. S2CID 140634879
    . Retrieved 25 May 2023.
24. S2CID 129370611
    .
25. PMID 28954854
    .
26. ^
    S2CID 225549860
    .
27. doi:10.1002/(SICI)1099-1034(199911/12)34:4<321::AID-GJ809>3.0.CO;2-I
    . Retrieved 16 April 2023.
28. doi:10.1016/S0031-0182(02)00507-2
    . Retrieved 10 August 2023.
29. doi:10.1016/j.palaeo.2012.01.009
    . Retrieved 16 April 2023.
30. S2CID 238707360
    . Retrieved 25 May 2023.
31. doi:10.1016/j.palaeo.2017.12.025
    . Retrieved 23 November 2022.
32. S2CID 129323463
    . Retrieved 16 April 2023.
33. doi:10.1016/j.palaeo.2009.11.020
    . Retrieved 15 January 2023.
34. S2CID 89379920
    .
35. doi:10.4095/132187
    .
36. doi:10.1016/j.earscirev.2022.104035
    .
37. ISSN 1502-3931
    .
38. ISSN 1502-3931
    .
39. S2CID 85903232
    . Retrieved 25 May 2023.
40. ISBN 9780660199306
    .
41. S2CID 134827223
    . Retrieved 14 March 2023.
42. doi:10.1016/j.jop.2023.03.005
    .
43. PMID 26538179
    . Retrieved 14 March 2023.
44. S2CID 54525645
    .
45. S2CID 258672970
    . Retrieved 11 December 2023.
46. S2CID 28224399
    . Retrieved 26 December 2022.
47. ^ Seth A. Young, Matthew R. Saltzman, William I. Ausich, André Desrochers, and Dimitri Kaljo, "Did changes in atmospheric CO₂ coincide with latest Ordovician glacial–interglacial cycles?", Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 296, No. 3–4, 15 October 2010, Pages 376–388.
48. ^ ^{a b} Jeff Hecht, High-carbon ice age mystery solved, New Scientist, 8 March 2010 (retrieved 30 June 2014)
49. ISSN 1752-0908
    . Retrieved 18 October 2022.
50. ^
    doi:10.1130/B30812.1
    . Retrieved 12 August 2023.
51. doi:10.1130/B36257.1
    . Retrieved 19 October 2022.
52. doi:10.1016/S0031-0182(99)00046-2
    . Retrieved 12 August 2023.
53. doi:10.1016/j.palaeo.2010.04.010
    . Retrieved 25 May 2023.
54. ^
    ISSN 0091-7613
    .
55. S2CID 244803446
    . Retrieved 21 October 2022.
56. ^
    ISSN 0091-7613
    .
57. ^
    S2CID 257273196
    . Retrieved 25 May 2023.
58. PMID 36572674
    .
59. S2CID 209381464
    . Retrieved 22 October 2022.
60. PMID 20696937
    .
61. ISSN 0091-7613
    . Retrieved 10 September 2023.
62. ISSN 0024-1164
    . Retrieved 11 December 2023 – via Wiley Online Library.
63. PMID 28483875
    .
64. ^ "Archived copy" (PDF). Archived from the original (PDF) on 2011-07-27. Retrieved 2009-07-22.{{cite web}}: CS1 maint: archived copy as title (link) IGCP meeting September 2004 reports pp 26f
65. S2CID 129101399
    .
66. PMID 25174941
    .
67. ^
    ISSN 0012-821X
    .
68. ^
    ISSN 0012-821X
    . Retrieved 12 August 2023.
69. S2CID 134586047
    .
70. ^
    ISSN 0012-821X
    . Retrieved 12 August 2023.
71. ^
    S2CID 135039656
    . Retrieved 12 August 2023.
72. PMID 36399571
    . Retrieved 12 August 2023.
73. ^
    ISSN 0031-0182
    .
74. ^
    ISSN 0009-2541
    .
75. ISSN 0921-8181
    .
76. S2CID 218930512
    .
77. ^
    S2CID 6896844
    .
78. ISSN 0024-1164
    . Retrieved 10 September 2023.
79. PMID 25174941
    .
80. S2CID 134603654
    .
81. ^
    doi:10.1130/G47377.1
    .
82. doi:10.1130/G47946C.1
    .
83. ^
    PMID 29784792
    .
84. ISSN 2576-604X
    .
85. doi:10.1130/G25477A.1
    .
86. S2CID 248681972
    . Retrieved 25 May 2023.
87. S2CID 210259392
    .
88. PMID 35915216
    .
89. ^
    PMID 35338189
    .
90. S2CID 240358402
    . Retrieved 22 October 2022.
91. ISSN 0031-0182
    .
92. S2CID 202191446
    .
93. S2CID 219934128
    .
94. S2CID 258402989
    . Retrieved 10 September 2023.
95. S2CID 134266940
    .
96. S2CID 264988361
    . Retrieved 11 December 2023 – via Elsevier Science Direct.
97. ^
    PMID 30816186
    .
98. ^ Katz, Cheryl (2015-09-11). "New Theory for What Caused Earth's Second-Largest Mass Extinction". National Geographic News. Archived from the original on September 13, 2015. Retrieved 2015-09-12.
99. PMID 26305681
    .
100. ISSN 1476-4687
     .
101. ^
     S2CID 6150230
     . Retrieved 21 August 2022.
102. ^
     S2CID 13124815
     .
103. ^ "Ray burst is extinction suspect". BBC. April 6, 2005. Retrieved 2008-04-30.
104. S2CID 11942132
     .
105. S2CID 226975864
     . Retrieved 21 October 2022.
106. S2CID 11199820
     . Retrieved 22 October 2022.
107. S2CID 2046052
     . Retrieved 22 October 2022.
108. S2CID 218540475
     . Retrieved 14 August 2023.
109. ^ Hall, Shannon (10 June 2020). "Familiar Culprit May Have Caused Mysterious Mass Extinction - A planet heated by giant volcanic eruptions drove the earliest known wipeout of life on Earth". The New York Times. Retrieved 15 June 2020.
110. S2CID 247653339
     . Retrieved 19 October 2022.
111. PMID 30910963
     .
112. S2CID 210698603
     .
113. ISSN 0012-8252
     .
114. ISSN 0031-0182
     . Retrieved 10 September 2023.
115. S2CID 249003502
     . Retrieved 26 December 2022.
116. S2CID 258235044
     . Retrieved 11 December 2023 – via Elsevier Science Direct.
117. S2CID 251029419
     . Retrieved 26 December 2022.
118. ^
     S2CID 133689085
     . Retrieved 29 March 2023.
119. S2CID 255019480
     . Retrieved 12 August 2023.
120. ^ Glikson, Andrew (9 August 2023). "New evidence suggests the world's largest known asteroid impact structure is buried deep in southeast Australia". theconversation.com. The Conversation US, Inc. Retrieved 2 September 2023. Hidden traces of Earth's early history

Further reading

• Gradstein, Felix M.; Ogg, James G.; Smith, Alan G. (2004). A Geological Time Scale 2004 (3rd ed.). Cambridge University Press: Cambridge University Press.
  ISBN 9780521786737
  .
• ISBN 9780191588396
  .
• Webby, Barry D.; Paris, Florentin; Droser, Mary L.; Percival, Ian G, eds. (2004). The great Ordovician biodiversification event. New York: Columbia University Press.
  ISBN 9780231501637
  .

External links

• Jacques Veniers, "The end-Ordovician extinction event": abstract of Hallam and Wignall, 1997.