View source for Proterozoic

{{Short description|Geologic eon, 2500–539 million years ago}}
{{use dmy dates|date=August 2024}}
{{Infobox geologic timespan
| name                         = Proterozoic
| color                        = Proterozoic
| top_bar                      = all time
| time_start                   = {{period start|proterozoic}}
| time_end                     = {{period end|proterozoic}}
| time_end_prefix              =
| time_end_uncertainty         = {{period end error|proterozoic|sign=no}}
| image_map                    =
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| image_art                    = Proterozoic collage.png
| caption_art                  = From left to right: Four main Proterozoic events: [[Great Oxidation Event]] and subsequent [[Huronian glaciation]]; First [[eukaryotes]], like [[red algae]]; [[Snowball Earth]] in [[Cryogenian]] period; [[Ediacaran biota]]<ref>{{cite AV media |title=Smithsonian National Museum |website=[[flickr]] |medium=photo album |url=https://www.flickr.com/photos/ideonexus/albums/72157603838941938 }}</ref>
<!--Chronology-->  
| timeline                     = Eons
| former_subdivisions          =
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<!--Etymology-->
| chrono_name                  =
| strat_name                   =
| name_formality               = Formal
| name_accept_date             = 
| alternate_spellings          =
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<!--Usage Information--> 
| celestial_body               = earth
| usage                        = Global ([[International Commission on Stratigraphy|ICS]])
| timescales_used              = ICS Time Scale
| formerly_used_by             =
| not_used_by                  =
<!--Definition-->
| chrono_unit                  = Eon
| strat_unit                   = Eonothem
| proposed_by                  = 
| timespan_formality           = Formal
| lower_boundary_def           = Defined Chronometrically
| lower_gssa_accept_date       = {{period start ratification|proterozoic}}
| upper_boundary_def        = Appearance of the [[Trace fossil|Ichnofossil]] ''[[Treptichnus pedum]]''
| upper_gssp_location       = [[Fortune Head|Fortune Head section]], [[Newfoundland]], [[Canada]]
| upper_gssp_coords         = {{Coord|47.0762|N|55.8310|W|display=inline}}
| upper_gssp_accept_date    = {{period end ratification|proterozoic}}
}}
The '''Proterozoic''' ({{IPAc-en|ipa|ˌ|p|r|oʊ|t|ər|ə|ˈ|z|oʊ|ɪ|k|,_|ˌ|p|r|ɒ|t|-|,_|-|ər|oʊ|-|,_|-|t|r|ə|-|,_|-|t|r|oʊ|-}} {{respell|PROH|tər|ə|ZOH|ik|,_|PROT-|,_|-ər|oh-|,_|-trə-|,_|-|troh-}})<ref>{{cite dictionary |title=Proterozoic – definition of Proterozoic in English |dictionary=[[Oxford English Dictionary]] |via=[[OxfordDictionaries.com]] |url=https://www.oxforddictionaries.com/definition/english/Proterozoic |access-date=2016-01-20 |archive-url=https://web.archive.org/web/20120724122625/http://oxforddictionaries.com/definition/english/Proterozoic |archive-date=July 24, 2012 }}</ref><ref>{{MerriamWebsterDictionary|Proterozoic}}</ref><ref>{{dictionary.com|Proterozoic}}</ref> is the third of the four [[eon (geology)|geologic eon]]s of [[Earth's history]], spanning the time interval from {{period start|proterozoic}} to {{period end|proterozoic}} Ma,{{ref icc}} and is the longest eon of Earth's [[geologic time scale]]. It is preceded by the [[Archean]] and followed by the [[Phanerozoic]], and is the most recent part of the [[Precambrian]] "supereon".

The Proterozoic is subdivided into three [[era (geology)|geologic era]]s (from oldest to youngest): the [[Paleoproterozoic]], [[Mesoproterozoic]] and [[Neoproterozoic]].<ref>{{cite web |last1=Speer |first1=Brian |title=The Proterozoic eon |place=Berkeley, CA |website=[[University of California Museum of Paleontology]] (ucmp.berkeley.edu) |url=http://www.ucmp.berkeley.edu/precambrian/proterozoic.php }}</ref> It covers the time from the appearance of free [[oxygen]] in [[Earth's atmosphere]] to just before the proliferation of [[complex life]] on the Earth during the [[Cambrian Explosion]]. The name ''Proterozoic'' combines two words of [[Greek language|Greek]] origin: {{lang|grc-Latn|protero-}} meaning "former, earlier", and {{lang|grc-Latn|-zoic}}, meaning "of life".<ref>{{cite dictionary |title=Proterozoic, adj. and n. |date=June 2021 |dictionary=[[Oxford English Dictionary|OED]] Online |publisher=Oxford University Press |url=https://www.eod.com/view/Entry/237724?redirectedFrom=Proterozoic |access-date=25 June 2021 |archive-url=https://web.archive.org/web/20210625221324/https://www.eod.com/view/Entry/237724?redirectedFrom=Proterozoic |archive-date=25 June 2021 }}</ref>
 
Well-identified events of this eon were the [[Great Oxidation Event|transition to an oxygenated atmosphere]] during the Paleoproterozoic; the evolution of [[eukaryotes]] via [[symbiogenesis]]; several global [[glaciation]]s, which produced the 300&nbsp;million years-long [[Huronian glaciation]] (during the [[Siderian]] and [[Rhyacian]] periods of the Paleoproterozoic) and the hypothesized [[Snowball Earth]] (during the [[Cryogenian]] period in the late Neoproterozoic); and the [[Ediacaran]] period (635–538.8&nbsp;[[million years ago|Ma]]), which was characterized by the [[evolution]] of abundant soft-bodied [[multicellular organism]]s such as [[sponge]]s, [[algae]], [[cnidarian]]s, [[bilaterian]]s and the sessile [[Ediacaran biota]] (some of which had evolved [[sexual reproduction]]) and provides the first obvious [[fossil]] evidence of [[history of life|life on Earth]].

==The Proterozoic record==
The geologic record of the Proterozoic Eon is more complete than that for the preceding [[Archean]] Eon. In contrast to the deep-water deposits of the Archean, the Proterozoic features many [[stratum|strata]] that were laid down in extensive shallow [[Inland sea (geology)|epicontinental seas]]; furthermore, many of those rocks are less [[Metamorphic rock|metamorphosed]] than Archean rocks, and many are unaltered.<ref name=Stanley>{{cite book | last=Stanley | first=Steven M. | year=1999 | title=Earth System History | location=New York, NY | publisher=W.H. Freeman and Company | isbn=978-0-7167-2882-5 }}</ref>{{rp|315}} Studies of these rocks have shown that the eon continued the massive [[continent]]al accretion that had begun late in the Archean Eon. The Proterozoic Eon also featured the first definitive [[supercontinent cycle]]s and {{clarify span|text=wholly modern|explain=Compared to partially modern? What is (geologically) modern?|date=April 2023}} mountain building activity ([[orogeny]]).<ref name=Stanley/>{{rp|315–318, 329–332}}

There is evidence that the first known glaciations occurred during the Proterozoic. The first began shortly after the beginning of the Proterozoic Eon, and evidence of at least four during the Neoproterozoic Era at the end of the Proterozoic Eon, possibly climaxing with the hypothesized [[Snowball Earth]] of the [[Sturtian]] and [[Marinoan]] glaciations.<ref name=Stanley/>{{rp|320–321, 325}}

==The accumulation of oxygen==
{{Main|Great Oxygenation Event|Neoproterozoic Oxygenation Event}}
One of the most important events of the Proterozoic was the [[oxygen catastrophe|accumulation of oxygen]] in the Earth's atmosphere. Though oxygen is believed to have been released by [[photosynthesis]] as far back as the Archean Eon, it could not build up to any significant degree until mineral sinks of unoxidized [[sulfur]] and [[iron]] had been exhausted. Until roughly 2.3&nbsp;Ga, oxygen was probably only 1% to 2% of its current level.<ref name=Stanley/>{{rp|323}} The [[banded iron formation]]s, which provide most of the world's [[iron ore]], are one mark of that mineral sink process. Their accumulation ceased after 1.9&nbsp;Ga, after the iron in the oceans had all been [[oxidized]].<ref name=Stanley/>{{rp|324}}

[[Red bed]]s, which are colored by [[hematite]], indicate an increase in atmospheric oxygen 2&nbsp;Ga. Such massive iron oxide formations are not found in older rocks.<ref name=Stanley/>{{rp|324}} The oxygen buildup was probably due to two factors: Exhaustion of the chemical sinks, and an increase in [[carbon sequestration]], which sequestered [[organic compound]]s that would have otherwise been oxidized by the atmosphere.<ref name=Stanley/>{{rp|325}}

The first surge in atmospheric oxygen at the beginning of the Proterozoic is called the [[Great Oxygenation Event]], or alternately the [[Oxygen Catastrophe]] – to reflect the mass extinction of almost all life on Earth, which at the time was virtually all [[obligate anaerobe|obligate anaerobic]]. A second, later surge in oxygen concentrations is called the [[Neoproterozoic Oxygenation Event]],<ref name=Shields-Zhou2011>{{cite journal |last1=Shields-Zhou |first1=Graham |last2=Och |first2=Lawrence |title=The case for a Neoproterozoic Oxygenation Event: Geochemical evidence and biological consequences |journal=GSA Today |date=March 2011 |volume=21 |issue=3 |pages=4–11 |doi=10.1130/GSATG102A.1 |bibcode=2011GSAT...21c...4S |url=https://discovery.ucl.ac.uk/id/eprint/1354478/1/1354478_Shields-Zhou%2520and%2520Och_2011.pdf }}</ref> occurred during the Middle and Late Neoproterozoic<ref name="Och2012">{{cite journal |last1=Och |first1=Lawrence M. |last2=Shields-Zhou |first2=Graham A. |title=The Neoproterozoic oxygenation event: Environmental perturbations and biogeochemical cycling |journal=Earth-Science Reviews |date=January 2012 |volume=110 |issue=1–4 |pages=26–57 |doi=10.1016/j.earscirev.2011.09.004 |bibcode=2012ESRv..110...26O }}</ref> and drove the rapid evolution of multicellular life towards the end of the era.<ref>{{cite journal |last1=Canfield |first1=Donald Eugene |last2=Poulton |first2=Simon W. |last3=Narbonne |first3=Guy M. |date=5 January 2007 |title=Late-Neoproterozoic deep-ocean oxygenation and the rise of animal life |journal=Science |volume=315 |issue=5808 |pages=92–95 |doi=10.1126/science.1135013 |pmid=17158290 |s2cid=24761414 |doi-access=free |bibcode=2007Sci...315...92C }}</ref><ref name=EdiacaranOxygenationIronIsotopes>{{cite journal |last1=Fan |first1=Haifeng |last2=Zhu |first2=Xiangkun |last3=Wen |first3=Hanjie |last4=Yan |first4=Bin |last5=Li |first5=Jin |last6=Feng |first6=Lianjun |title=Oxygenation of Ediacaran Ocean recorded by iron isotopes |journal=Geochimica et Cosmochimica Acta |date=September 2014 |volume=140 |pages=80–94 |doi=10.1016/j.gca.2014.05.029 |bibcode=2014GeCoA.140...80F }}</ref>

==Subduction processes==
The Proterozoic Eon was a very tectonically active period in the Earth's history. Oxygen changed the chemistry allowing for extensive geological changes. Volcanism was also extensive resulting in more geologic changes.

The late Archean Eon to Early Proterozoic Eon corresponds to a period of increasing crustal recycling, suggesting [[subduction]]. Evidence for this increased subduction activity comes from the abundance of old granites originating mostly after 2.6&nbsp;[[billion years ago|Ga]].<ref name=Kearey>{{cite book |last1=Kearey |first1=P. |last2=Klepeis |first2=K. |last3=Vine |first3=F. |year=2008 |title=Precambrian Tectonics and the Supercontinent Cycle |series=Global Tectonics |edition=Third |pages=361–377 }}</ref>

The occurrence of [[eclogite]] (a type of [[metamorphic rock]] created by high pressure, >&nbsp;1&nbsp;GPa), is explained using a model that incorporates subduction. The lack of eclogites that date to the Archean Eon suggests that conditions at that time did not favor the formation of high grade metamorphism and therefore did not achieve the same levels of subduction as was occurring in the Proterozoic Eon.<ref>{{cite journal |last=Bird |first=P. |year=2003 |title=An updated digital model of plate boundaries |journal=Geochemistry, Geophysics, Geosystems |volume=4 |issue=3 |page=1027 |doi=10.1029/2001GC000252 |doi-access=free |bibcode=2003GGG.....4.1027B }}</ref>

As a result of remelting of [[basalt]]ic [[oceanic crust]] due to subduction, the cores of the first continents grew large enough to withstand the crustal recycling processes.

The long-term tectonic stability of those [[craton]]s is why we find [[continental crust]] ranging up to a few billion years in age.<ref>{{cite book |last=Mengel |first=F. |year=1998 |title=Proterozoic History |series=Earth System: History and variablility |volume=2 }}</ref> It is believed that 43% of modern continental crust was formed in the Proterozoic, 39% formed in the Archean, and only 18% in the [[Phanerozoic]].<ref name=Kearey/> Studies by Condie (2000)<ref>{{cite journal |last=Condie |first=K. |year=2000 |title=Episodic continental growth models: Afterthoughts and extensions |journal=Tectonophysics |volume=322 |issue=1 |pages=153–162 |doi=10.1016/S0040-1951(00)00061-5 |bibcode=2000Tectp.322..153C }}</ref> and  Rino et al. (2004)<ref>{{cite journal |last1=Rino |first1=Shuji |last2=Komiya |first2=Tsuyoshi |last3=Windley |first3=Brian F. |last4=Katayama |first4=Ikuo |last5=Motoki |first5=Akihisa |last6=Hirata |first6=Takafumi |date=August 2004 |title=Major episodic increases of continental crustal growth determined from zircon ages of river sands; implications for mantle overturns in the Early Precambrian |journal=Physics of the Earth and Planetary Interiors |volume=146 |issue=1–2 |pages=369–394 |doi=10.1016/j.pepi.2003.09.024|bibcode=2004PEPI..146..369R |s2cid=140166194 }}</ref> suggest that crust production happened episodically. By isotopically calculating the ages of Proterozoic granitoids it was determined that there were several episodes of rapid increase in continental crust production. The reason for these pulses is unknown, but they seemed to have decreased in magnitude after every period.<ref name=Kearey/>

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==Supercontinent tectonic history==

<gallery>
File:Paleoglobe NO 1590 mya-vector-colors.svg|[[Columbia (supercontinent)|Columbia]], about 1,590 Ma
File:Rodinia 900Ma.jpg|[[Rodinia]], about 900&nbsp;Ma
File:Pannotia.svg|[[Pannotia]], 545&nbsp;Ma (disputed{{clarify|date=June 2023}}), centered on South Pole
File:Gondwana 420 Ma.png|[[Gondwana]] 420&nbsp;Ma, centered on South Pole
</gallery>

Evidence of collision and rifting between continents raises the question as to what exactly were the movements of the Archean cratons composing Proterozoic continents. [[Paleomagnetism|Paleomagnetic]] and geochronological dating mechanisms have allowed the deciphering of Precambrian Supereon tectonics. It is known that tectonic processes of the Proterozoic Eon resemble greatly the evidence of tectonic activity, such as [[orogenic belt]]s or [[ophiolite]] complexes, we see today. Hence, most geologists would conclude that the Earth was active at that time. It is also commonly accepted that during the Precambrian, the Earth went through several supercontinent breakup and rebuilding cycles ([[Wilson cycle]]).<ref name=Kearey/>

In the late Proterozoic (most recent), the dominant supercontinent was [[Rodinia]] (~1000–750&nbsp;Ma). It consisted of a series of continents attached to a central craton that forms the core of the North American Continent called [[Laurentia]]. An example of an orogeny (mountain building processes) associated with the construction of Rodinia is the [[Grenville orogeny]] located in Eastern North America. Rodinia formed after the breakup of the supercontinent [[Columbia (supercontinent)|Columbia]] and prior to the assemblage of the supercontinent [[Gondwana]] (~500&nbsp;Ma).<ref>{{cite journal |last1=Condie |first1=K. C. |last2=O'Neill |first2=C. |year=2011 |title=The Archean-Proterozoic boundary: 500&nbsp;my of tectonic transition in Earth history |journal=[[American Journal of Science]] |volume=310 |issue=9 |pages=775–790 |doi=10.2475/09.2010.01 |doi-access=free |bibcode=2010AmJS..310..775C|s2cid=128469935 }}</ref> The defining orogenic event associated with the formation of Gondwana was the collision of Africa, South America, Antarctica and Australia forming the [[Pan-African orogeny]].<ref>{{cite book |last=Huntly |first=C.|year=2002 |title=The Mozambique Belt, Eastern Africa: Tectonic evolution of the Mozambique Ocean and Gondwana amalgamation |publisher=[[The Geological Society of America]] }}</ref>

[[Columbia (supercontinent)|Columbia]] was dominant in the early-mid Proterozoic and not much is known about continental assemblages before then. There are a few plausible models that explain tectonics of the early Earth prior to the formation of Columbia, but the current most plausible hypothesis is that prior to Columbia, there were only a few independent cratons scattered around the Earth (not necessarily a supercontinent, like Rodinia or Columbia).<ref name=Kearey/>

==Life==
{{Life timeline}}
{{also|Symbiogenesis|Boring Billion|Avalon Explosion}}
The Proterozoic can be roughly divided into seven biostratigraphic zones which correspond to informal time periods. The first was the Labradorian, lasting from 2.0–1.65&nbsp;[[giga annum|Ga]]. It was followed by the Anabarian, which lasted from 1.65–1.2&nbsp;Ga and was itself followed by the Turukhanian from 1.2–1.03&nbsp;Ga. The Turukhanian was succeeded by the Uchuromayan, lasting from 1.03–0.85&nbsp;Ga, which was in turn succeeded by the Yuzhnouralian, lasting from 0.85–0.63&nbsp;Ga. The final two zones were the Amadeusian, spanning the first half of the Ediacaran from 0.63–0.55&nbsp;Ga, and the Belomorian, spanning from 0.55–0.542&nbsp;Ga.<ref>{{Cite journal |last=Sergeev |first=V.N. |date=September 2009 |title=The distribution of microfossil assemblages in Proterozoic rocks |journal=[[Precambrian Research]] |volume=173 |issue=1–4 |pages=212–222 |doi=10.1016/j.precamres.2009.04.002 |bibcode=2009PreR..173..212S }}</ref>

The emergence of advanced single-celled [[eukaryotes]] began after the [[Oxygen Catastrophe]].<ref>{{cite journal |last1=Fakhraee |first1=Mojtaba |last2=Tarhan |first2=Lidya G. |last3=Reinhard |first3=Christopher T. |last4=Crowe |first4=Sean A. |last5=Lyons |first5=Timothy W. |last6=Planavsky |first6=Noah J. |date=May 2023 |title=Earth's surface oxygenation and the rise of eukaryotic life: Relationships to the Lomagundi positive carbon isotope excursion revisited |journal=Earth-Science Reviews |volume=240 |article-number=104398 |s2cid=257761993 |doi=10.1016/j.earscirev.2023.104398 |doi-access=free |bibcode=2023ESRv..24004398F }}</ref> This may have been due to an increase in the oxidized [[nitrate]]s that eukaryotes use, as opposed to [[cyanobacteria]].<ref name=Stanley/>{{rp|325}} It was also during the Proterozoic that the first [[symbiosis|symbiotic]] relationships between [[mitochondria]] (found in nearly all eukaryotes) and [[chloroplast]]s (found in [[plant]]s and some [[protist]]s only) and their hosts evolved.<ref name=Stanley/>{{rp|321–322}}

By the late Palaeoproterozoic, eukaryotic organisms had become moderately biodiverse.<ref>{{cite journal |last1=Miao |first1=Lanyun |last2=Moczydłowska |first2=Małgorzata |last3=Zhu |first3=Shixing |last4=Zhu |first4=Maoyan |title=New record of organic-walled, morphologically distinct microfossils from the late Paleoproterozoic Changcheng Group in the Yanshan Range, North China |journal=Precambrian Research |date=February 2019 |volume=321 |pages=172–198 |doi=10.1016/j.precamres.2018.11.019 |bibcode=2019PreR..321..172M }}</ref> The blossoming of eukaryotes such as [[acritarch]]s did not preclude the expansion of cyanobacteria – in fact, [[stromatolites]] reached their greatest abundance and diversity during the Proterozoic, peaking roughly 1.2&nbsp;Ga.<ref name=Stanley/>{{rp|321–323}}

The earliest [[fossil]]s possessing features typical of [[Fungus|fungi]] date to the [[Paleoproterozoic]] Era, some 2.4&nbsp;Ga; these multicellular [[benthic]] organisms had filamentous structures capable of [[anastomosis]].<ref>{{cite journal|last1=Bengtson|first1=Stefan|last2=Rasmussen|first2=Birger|last3=Ivarsson |first3=Magnus |last4=Muhling |first4=Janet |last5=Broman |first5=Curt |last6=Marone |first6=Federica |last7=Stampanoni |first7=Marco |last8=Bekker |first8=Andrey |display-authors=6 |date=24 April 2017 |title=Fungus-like mycelial fossils in 2.4&nbsp;billion-year-old vesicular basalt |journal=[[Nature Ecology & Evolution]] |volume=1 |issue=6 |page=141 |doi=10.1038/s41559-017-0141 |pmid=28812648 |bibcode=2017NatEE...1..141B |hdl=20.500.11937/67718 |hdl-access=free |s2cid=25586788 }}</ref>

The [[Viridiplantae]] evolved sometime in the Palaeoproterozoic or Mesoproterozoic, according to molecular data.<ref>{{Cite journal |last1=Yang |first1=Zhiping |last2=Ma |first2=Xiaoya |last3=Wang |first3=Qiuping |last4=Tian |first4=Xiaolin |last5=Sun |first5=Jingyan |last6=Zhang |first6=Zhenhua |last7=Xiao |first7=Shuhai |last8=De Clerck |first8=Olivier |last9=Leliaert |first9=Frederik |last10=Zhong |first10=Bojian |display-authors=6 |date=11 September 2023 |title=Phylotranscriptomics unveil a Paleoproterozoic-Mesoproterozoic origin and deep relationships of the Viridiplantae |journal=[[Nature Communications]] |volume=14 |issue=1 |page=5542 |doi=10.1038/s41467-023-41137-5 |issn=2041-1723 |pmc=10495350 |pmid=37696791 |bibcode=2023NatCo..14.5542Y }}</ref>

Eukaryote fossils from before the Cryogenian are sparse, and there seems to be low and relatively constant rates of species appearance, change, and extinction. This contrasts with the Ediacaran and early Cambrian periods, in which the quantity and variety of speciations, changes, and extinctions exploded.<ref name="Tang, Zheng, Zhang, Fan et all 2024">{{cite journal |last=Tang |first=Qing |last2=Zheng |first2=Wentao |last3=Zhang |first3=Shuhan |last4=Fan |first4=Junxuan |last5=Riedman |first5=Leigh Anne |last6=Hou |first6=Xudong |last7=Muscente |first7=A. D. |last8=Bykova |first8=Natalia |last9=Sadler |first9=Peter M. |last10=Wang |first10=Xiangdong |last11=Zhang |first11=Feifei |last12=Yuan |first12=Xunlai |last13=Zhou |first13=Chuanming |last14=Wan |first14=Bin |last15=Pang |first15=Ke |last16=Ouyang |first16=Qing |last17=McKenzie |first17=N. Ryan |last18=Zhao |first18=Guochun |last19=Shen |first19=Shuzhong |last20=Xiao |first20=Shuhai |display-authors=4 |title=Quantifying the global biodiversity of Proterozoic eukaryotes |journal=Science |volume=386 |issue=6728 |date=2024-12-20 |issn=0036-8075 |doi=10.1126/science.adm9137 |page=}}</ref>

Classically, the boundary between the Proterozoic and the [[Phanerozoic]] eons was set at the base of the Cambrian [[period (geology)|Period]] when the first fossils of animals, including [[trilobite]]s and [[archaeocyatha|archeocyathids]], as well as the animal-like ''[[Caveasphaera]]'', appeared. In the second half of the 20th century, a number of fossil forms have been found in Proterozoic rocks, particularly in ones from the Ediacaran, proving that multicellular life had already become widespread tens of millions of years before the [[Cambrian explosion|Cambrian Explosion]] in what is known as the [[Avalon explosion|Avalon Explosion]].<ref name="OnTheEveOfAnimalRadiation">{{cite journal |last1=Xiao |first1=Shuhai |last2=Laflamme |first2=Marc |date=January 2009 |title=On the eve of animal radiation: phylogeny, ecology and evolution of the Ediacara biota |journal=[[Trends in Ecology & Evolution]] |volume=24 |issue=1 |pages=31–40 |doi=10.1016/j.tree.2008.07.015 |pmid=18952316 |bibcode=2009TEcoE..24...31X }}</ref> Nonetheless, the upper boundary of the Proterozoic has remained fixed at the base of the [[Cambrian]], which is currently placed at 538.8&nbsp;Ma.
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==See also==
* [[Boring Billion]]
* [[Timeline of natural history]]
* [[Volyn biota]]

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==References==
{{reflist|25em}}

==External links==
{{commons category}}
* {{cite web |title=Proterozoic eon |website=Palaeos.com |url=http://www.palaeos.com/Proterozoic/Proterozoic.htm |archive-url=https://web.archive.org/web/20070702223746/http://www.palaeos.com/Proterozoic/Proterozoic.htm |archive-date=2 July 2007 }}
* {{cite web |title=Proterozoic |department=chronostratigraphy scale |url=https://ghkclass.com/ghkC.html?proterozoic }}

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[[Category:Proterozoic| ]]
[[Category:Precambrian geochronology|*03]]