Editing Ordovician (section)

==End of the period==
{{Main|Ordovician–Silurian extinction events}}

[[File:Anji Biota.jpg|thumb|The [[Anji Biota]] (Wenchang Formation, [[Zhejiang]] Province, [[China]]) preserves abundant and diverse [[Hexactinellid|glass sponges]] and graptolites as well as rare examples of other marine animals (such as the eurypterid [[Archopterus]]) living at a depth of several hundred metres. It is dated to just after the [[Late_Ordovician_mass_extinction|Hirnantian mass extinction]] at the end of the Ordovician period.<ref>{{Cite journal |last1=Wang |first1=Han |last2=Braddy |first2=Simon J. |last3=Botting |first3=Joseph |last4=Zhang |first4=Yuandong |date=2023 |title=The first documentation of an Ordovician eurypterid (Chelicerata) from China |url=https://www.cambridge.org/core/journals/journal-of-paleontology/article/first-documentation-of-an-ordovician-eurypterid-chelicerata-from-china/2F0762857D48061467555E2C91387957 |journal=Journal of Paleontology |language=en |volume=97 |issue=3 |pages=606–611 |doi=10.1017/jpa.2023.21 |issn=0022-3360}}</ref>]]

The Ordovician came to a close in a series of [[extinction event]]s that, taken together, comprise the second largest of the five major extinction events in [[History of Earth|Earth's history]] in terms of percentage of [[genus|genera]] that became extinct. The only larger one was the [[Permian–Triassic extinction event]].

The extinctions occurred approximately 447–444 million years ago and mark the boundary between the Ordovician and the following [[Silurian]] Period. At that time all complex multicellular organisms lived in the sea, and about 49% of genera of fauna disappeared forever; [[brachiopods]] and [[bryozoans]] were greatly reduced, along with many [[trilobite]], [[conodont]] and [[Graptolithina|graptolite]] families.

The most commonly accepted theory is that these events were triggered by the onset of cold conditions in the late Katian, followed by an [[ice age]], in the Hirnantian faunal stage, that ended the long, stable [[greenhouse]] conditions typical of the Ordovician.

The ice age was possibly not long-lasting. Oxygen [[isotopes]] in fossil brachiopods show its duration may have been only 0.5 to 1.5 million years.<ref name="Stanley1999b">{{cite book |title=Earth System History |last=Stanley |first=Steven M. |year=1999 |publisher=W.H. Freeman and Company |location=New York |isbn=978-0-7167-2882-5 |pages=358, 360 }}</ref> Other researchers (Page et al.) estimate more temperate conditions did not return until the late Silurian.

The [[late Ordovician glaciation]] event was preceded by a fall in atmospheric carbon dioxide (from 7000 ppm to 4400 ppm).<ref name="Young 2010">{{cite journal | last1 = Young | first1 = Seth A. | last2 = Saltzman | first2 = Matthew R. | last3 = Ausich | first3 = William I. | last4 = Desrochers | first4 = André | last5 = Kaljo | first5 = Dimitri | year = 2010 | title = Did changes in atmospheric CO2 coincide with latest Ordovician glacial–interglacial cycles? | journal = Palaeogeography, Palaeoclimatology, Palaeoecology | volume = 296 | issue = 3–4| pages = 376–388 | doi=10.1016/j.palaeo.2010.02.033| bibcode = 2010PPP...296..376Y }}</ref><ref name="Hecht 2010">Jeff Hecht, [https://www.newscientist.com/article/dn18618-highcarbon-ice-age-mystery-solved.html High-carbon ice age mystery solved] {{Webarchive|url=https://web.archive.org/web/20150423082538/http://www.newscientist.com/article/dn18618-highcarbon-ice-age-mystery-solved.html |date=2015-04-23 }}, ''[[New Scientist]]'', 8 March 2010 (retrieved 30 June 2014)</ref> The dip may have been caused by a burst of volcanic activity that deposited new silicate rocks, which draw CO<sub>2</sub> out of the air as they erode.<ref name="Hecht 2010" /> Another possibility is that [[bryophyte]]s and lichens, which colonized land in the middle to late Ordovician, may have increased weathering enough to draw down {{CO2}} levels.<ref name="Porado-etal-2016"/> The drop in {{CO2}} selectively affected the shallow seas where most organisms lived. It has also been suggested that shielding of the sun's rays from the proposed Ordovician ring system, which also caused the [[Ordovician meteor event]], may have also led to the glaciation.<ref name=":0" /> As the southern supercontinent [[Gondwana]] drifted over the South Pole, ice caps formed on it, which have been detected in Upper Ordovician rock strata of [[North Africa]] and then-adjacent northeastern South America, which were south-polar locations at the time. 

As glaciers grew, the sea level dropped, and the vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches. When they returned, they carried diminished founder populations that lacked many whole families of organisms. They then withdrew again with the next pulse of glaciation, eliminating biological diversity with each change.<ref>Emiliani, Cesare. (1992). ''Planet Earth : Cosmology, Geology, & the Evolution of Life & the Environment'' (Cambridge University Press) p. 491</ref> Species limited to a single epicontinental sea on a given landmass were severely affected.<ref name="Stanley1999" /> Tropical lifeforms were hit particularly hard in the first wave of extinction, while cool-water species were hit worst in the second pulse.<ref name="Stanley1999" />

Those species able to adapt to the changing conditions survived to fill the ecological niches left by the extinctions. For example, there is evidence the oceans became more deeply oxygenated during the glaciation, allowing unusual benthic organisms (Hirnantian fauna) to colonize the depths. These organisms were cosmopolitan in distribution and present at most latitudes.{{sfn|Torsvik|Cocks|2017|p=112-113}}

At the end of the second event, melting glaciers caused the sea level to rise and stabilise once more. The rebound of life's diversity with the permanent re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving Orders. Recovery was characterized by an unusual number of "Lazarus taxa", disappearing during the extinction and reappearing well into the Silurian, which suggests that the taxa survived in small numbers in [[Refugium (population biology)|refugia]].{{sfn|Torsvik|Cocks|2017|pp=122-123}}

An alternate extinction hypothesis suggested that a ten-second [[gamma-ray burst]] could have destroyed the [[ozone layer]] and exposed terrestrial and marine surface-dwelling life to deadly ultraviolet [[radiation]] and initiated global cooling.<ref name="Melott2006">{{cite journal |last=Melott |first=Adrian |year=2004 |title=Did a gamma-ray burst initiate the late Ordovician mass extinction? |journal=International Journal of Astrobiology |volume=3 |issue= 1|pages=55&ndash;61 |doi=10.1017/S1473550404001910 |bibcode=2004IJAsB...3...55M|arxiv = astro-ph/0309415 |display-authors=etal|hdl=1808/9204 |s2cid=13124815 }}</ref>

Recent work considering the [[sequence stratigraphy]] of the Late Ordovician argues that the mass extinction was a single protracted episode lasting several hundred thousand years, with abrupt changes in water depth and sedimentation rate producing two pulses of last occurrences of species.<ref>{{Cite journal|doi = 10.1111/pala.12188|title = The stratigraphy of mass extinction|journal = Palaeontology|volume = 58|issue = 5|pages = 903–924|year = 2015|last1 = Holland|first1 = Steven M|last2 = Patzkowsky|first2 = Mark E|s2cid = 129522636|doi-access = free}}</ref>