Editing Cambrian (section)

== Oceanic life ==
{{Life timeline}}
{{Main|Cambrian explosion}}
The Cambrian explosion was a period of rapid multicellular growth. Most animal life during the Cambrian was aquatic. Trilobites were once assumed to be the dominant life form at that time,<ref>{{cite web|url=http://www.humboldt.edu/natmus/lifeThroughTime/Cambrian.web/index.html|title=Cambrian |date = 28 October 2012|last = Paselk|first = Richard|work = Natural History Museum|publisher=Humboldt State University}}</ref> but this has proven to be incorrect. Arthropods were by far the most dominant animals in the ocean, but trilobites were only a minor part of the total arthropod diversity. What made them so apparently abundant was their heavy armor reinforced by calcium carbonate (CaCO<sub>3</sub>), which fossilized far more easily than the fragile [[chitin]]ous exoskeletons of other arthropods, leaving numerous preserved remains.<ref>{{Cite book|title=3 Evolving Respiratory Systems as a Cause of the Cambrian Explosion – Out of Thin Air: Dinosaurs, Birds, and Earth's Ancient Atmosphere – The National Academies Press|doi=10.17226/11630|year=2006|isbn=978-0-309-10061-8|last1=Ward|first1=Peter}}</ref>

The period marked a steep change in the diversity and composition of Earth's [[biosphere]]. The [[Ediacaran biota]] suffered a mass extinction at the start of the Cambrian Period, which corresponded with an increase in the abundance and complexity of burrowing behaviour. This behaviour had a [[Cambrian substrate revolution|profound and irreversible effect on the substrate]] which transformed the [[seabed]] ecosystems. Before the Cambrian, the sea floor was covered by [[microbial mat]]s. By the end of the Cambrian, burrowing animals had destroyed the mats in many areas through bioturbation. As a consequence, many of those organisms that were dependent on the mats became extinct, while the other species adapted to the changed environment that now offered new ecological niches.<ref>{{cite web|url=http://www.sciencenews.org/view/feature/id/48630/title/As_the_worms_churn|title=As the worms churn|first=Sid|last= Perkins|date=23 October 2009|archive-url = https://web.archive.org/web/20091025115251/http://www.sciencenews.org/view/feature/id/48630/title/As_the_worms_churn |archive-date = 25 October 2009|work = ScienceNews}}</ref> Around the same time there was a seemingly rapid appearance of representatives of all the mineralized [[phylum|phyla]], including the [[Bryozoa]],<ref>{{cite journal |last1=Zhang |first1=Zhiliang |last2=Zhang |first2=Zhifei |last3=Ma |first3=J. |last4=Taylor |first4=P. D. |last5=Strotz |first5=L. C. |last6=Jacquet |first6=S. M. |last7=Skovsted |first7=C. B. |last8=Chen |first8=F. |last9=Han |first9=J. |last10=Brock |first10=G. A. |year=2021 |title=Fossil evidence unveils an early Cambrian origin for Bryozoa |journal=Nature |volume=599 |issue=7884 |pages=251–255 |doi=10.1038/s41586-021-04033-w |pmid=34707285 |pmc=8580826 |bibcode=2021Natur.599..251Z |s2cid=240073948 }}</ref> which were once thought to have only appeared in the Lower Ordovician.<ref name=Taylor2013>{{Cite journal|doi=10.1666/13-029|title=Reinterpretation of the Cambrian 'bryozoan' ''Pywackia'' as an octocoral|year=2013|last1=Taylor|first1=P.D. |last2= Berning|first2=B.|last3=Wilson|first3=M.A.|journal=Journal of Paleontology|volume=87|issue=6|pages=984–990|bibcode=2013JPal...87..984T |s2cid=129113026|url=https://zenodo.org/record/907861}}</ref> However, many of those phyla were represented only by stem-group forms; and since mineralized phyla generally have a benthic origin, they may not be a good proxy for (more abundant) non-mineralized phyla.<ref name=Budd2000>{{cite journal|last1=Budd |first1=G. E.|last2=Jensen |first2=S.|year=2000|title=A critical reappraisal of the fossil record of the bilaterian phyla|volume=75 |issue=2 |pages=253–95|journal=Biological Reviews of the Cambridge Philosophical Society|doi=10.1111/j.1469-185X.1999.tb00046.x|pmid=10881389|s2cid=39772232}}</ref>

[[File:Margaretia dorus Reconstruction.png|thumb|150px|left|A reconstruction of ''[[Margaretia|Margaretia dorus]]'' from the [[Burgess Shale]], which were once believed to be [[green algae]], but are now understood to represent [[hemichordate]]s<ref>{{Cite journal|doi=10.1186/s12915-016-0271-4|pmid=27383414|pmc=4936055|title=Cambrian suspension-feeding tubicolous hemichordates|journal=BMC Biology|volume=14|pages=56|year=2016|last1=Nanglu|first1=Karma|last2=Caron|first2=Jean-Bernard|last3=Conway Morris|first3=Simon|last4=Cameron|first4=Christopher B. |doi-access=free }}</ref>]]

While the early Cambrian showed such diversification that it has been named the Cambrian Explosion, this changed later in the period, when there occurred a sharp drop in biodiversity. About 515 Ma, the number of species going extinct exceeded the number of new species appearing. Five million years later, the number of genera had dropped from an earlier peak of about 600 to just 450. Also, the [[speciation]] rate in many groups was reduced to between a fifth and a third of previous levels. 500 Ma, oxygen levels fell dramatically in the oceans, leading to [[hypoxia (environmental)|hypoxia]], while the level of poisonous [[hydrogen sulfide]] simultaneously increased, causing another extinction. The later half of Cambrian was surprisingly barren and showed evidence of several rapid extinction events; the [[stromatolite]]s which had been replaced by reef building sponges known as [[Archaeocyatha]], returned once more as the archaeocyathids became extinct. This declining trend did not change until the [[Great Ordovician Biodiversification Event]].<ref>{{Cite web |url=http://www.nuffieldfoundation.org/sites/default/files/the-ordovician-explosion-651.doc |title=The Ordovician: Life's second big bang |access-date=10 February 2013 |archive-url=https://web.archive.org/web/20181009201136/http://www.nuffieldfoundation.org/sites/default/files/the-ordovician-explosion-651.doc |archive-date=9 October 2018 |url-status=dead }}</ref><ref>{{cite web|url=https://www.newscientist.com/article/dn19916-oxygen-crash-led-to-cambrian-mass-extinction.html|title=Oxygen crash led to Cambrian mass extinction|first=Michael|last=Marshall}}</ref>

Marine life lived under low and fluctuating levels of [[oxygen]] in the ocean. During upwellings of [[Anoxic waters|anoxic]] deep ocean waters into shallow marine environments could push organisms over the edge into mass extinctions, leading ultimately to increased [[biodiversity]].<ref name="Pruss-2024" />

[[File:Artistic reconstruction of the Cambrian (Drumian) Marjum biota.png|thumb|250px|Artistic reconstruction of [[Marjum Formation|Marjum biota]], including various arthropods ([[trilobite]]s, [[Hymenocarina|hymenocarines]], and [[Radiodonta|radiodonts]]), sponges, echinoderms, and various other groups ]]
Some Cambrian organisms ventured onto land, producing the trace fossils ''[[Protichnites]]'' and ''[[Climactichnites]]''. Fossil evidence suggests that [[euthycarcinoid]]s, an extinct group of arthropods, produced at least some of the ''Protichnites''.{{sfnm|1a1=Collette|1a2=Hagadorn|1y=2010|2a1=Collette|2a2=Gass|2a3=Hagadorn|2y=2012}} Fossils of the track-maker of ''Climactichnites'' have not been found; however, fossil trackways and resting traces suggest a large, [[slug]]-like [[mollusc]].{{sfnm|1a1=Yochelson|1a2=Fedonkin|1y=1993|2a1=Getty|2a2=Hagadorn|2y=2008}}

In contrast to later periods, the Cambrian fauna was somewhat restricted; free-floating organisms were rare, with the majority living on or close to the sea floor;<ref name=Munnecke2010>{{Cite journal| last1 = Munnecke | first1 = A.| last2 = Calner | first2 = M.| last3 = Harper | first3 = D. A. T.| author-link3 = David Harper (palaeontologist)| last4 = Servais | first4 = T.| title = Ordovician and Silurian sea-water chemistry, sea level, and climate: A synopsis| journal = Palaeogeography, Palaeoclimatology, Palaeoecology| volume = 296| issue = 3–4| pages = 389–413| year = 2010| doi = 10.1016/j.palaeo.2010.08.001| bibcode = 2010PPP...296..389M}}</ref> and mineralizing animals were rarer than in future periods, in part due to the unfavourable [[ocean chemistry]].<ref name=Munnecke2010/>

Many modes of preservation are unique to the Cambrian, and some preserve soft body parts, resulting in an abundance of {{lang|de|[[Lagerstätte]]n}}. These include [[Sirius Passet]],<ref>{{cite journal |doi=10.1111/let.12174|title=The Sirius Passet Lagerstätte: Silica death masking opens the window on the earliest matground community of the Cambrian explosion|journal=Lethaia|volume=49|issue=4|pages=631–643|year=2016|last1=Strang|first1=Katie M.|last2=Armstrong|first2=Howard A.|last3=Harper|first3=David A. T.|last4=Trabucho-Alexandre|first4=João P.|doi-access=free}}</ref><ref>{{Cite journal |last1=Nielsen |first1=Morten Lunde |last2=Lee |first2=Mirinae |last3=Ng |first3=Hong Chin |last4=Rushton |first4=Jeremy C. |last5=Hendry |first5=Katharine R. |last6=Kihm |first6=Ji-Hoon |last7=Nielsen |first7=Arne T. |last8=Park |first8=Tae-Yoon S. |last9=Vinther |first9=Jakob |last10=Wilby |first10=Philip R. |date=2022-01-01 |title=Metamorphism obscures primary taphonomic pathways in the early Cambrian Sirius Passet Lagerstätte, North Greenland |journal=Geology |language=en |volume=50 |issue=1 |pages=4–9 |doi=10.1130/G48906.1 |bibcode=2022Geo....50....4N |issn=0091-7613|doi-access=free }}</ref> the Sinsk Algal Lens,<ref name="SinskAlgalLens">{{cite journal |last1=Ivantsov |first1=Andrey Yu. |last2=Zhuravlev |first2=Andrey Yu. |last3=Leguta |first3=Anton V. |last4=Krassilov |first4=Valentin A. |last5=Melnikova |first5=Lyudmila M. |last6=Ushatinskaya |first6=Galina T. |date=2 May 2005 |title=Palaeoecology of the Early Cambrian Sinsk biota from the Siberian Platform |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018204005784#! |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=220 |issue=1–2 |pages=69–88 |doi=10.1016/j.palaeo.2004.01.022 |bibcode=2005PPP...220...69I |access-date=12 November 2022}}</ref> the [[Maotianshan Shales]],<ref>{{cite journal |last1=MacKenzie |first1=Lindsay A. |last2=Hofmann |first2=Michael H. |last3=Junyuan |first3=Chen |last4=Hinman |first4=Nancy W. |date=15 February 2015 |title=Stratigraphic controls of soft-bodied fossil occurrences in the Cambrian Chengjiang Biota Lagerstätte, Maotianshan Shale, Yunnan Province, China |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018214005641 |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=420 |pages=96–115 |doi=10.1016/j.palaeo.2014.11.006 |bibcode=2015PPP...420...96M |access-date=12 November 2022}}</ref> the [[Emu Bay Shale]],<ref>{{cite journal |last1=Paterson |first1=John R. |last2=García-Bellido |first2=Diego C. |last3=Jago |first3=James B. |last4=Gehling |first4=James G. |last5=Lee |first5=Michael S. Y. |last6=Edgecombe |first6=Gregory D. |date=10 November 2015 |title=The Emu Bay Shale Konservat-Lagerstätte: a view of Cambrian life from East Gondwana |url=https://pubs.geoscienceworld.org/jgs/article/173/1/1/144811/The-Emu-Bay-Shale-Konservat-Lagerstatte-a-view-of |journal=[[Journal of the Geological Society]] |volume=173 |issue=1 |pages=1–11 |doi=10.1144/jgs2015-083 |s2cid=130614466 |access-date=12 November 2022}}</ref> and the Burgess Shale.<ref name=Butterfield1990>{{cite journal |jstor = 2400788 |author = Butterfield, N.J. |journal = [[Paleobiology (journal)|Paleobiology]] |volume = 16 |issue = 3 |pages = 272–286 |year = 1990 |title=Organic Preservation of Non-Mineralizing Organisms and the Taphonomy of the Burgess Shale |doi = 10.1017/S0094837300009994 |bibcode = 1990Pbio...16..272B |s2cid = 133486523}}</ref><ref name="Page2008">{{cite journal |year= 2008 |doi = 10.1130/G24991A.1 |title = Ubiquitous Burgess Shale–style "clay templates" in low-grade metamorphic mudrocks |last1 = Page |first1 = Alex |last2 = Gabbott |first2 = Sarah |last3 = Wilby |first3 = Phillip R. |last4 = Zalasiewicz  |first4 = Jan A. |journal = [[Geology (journal)|Geology]] |volume = 36 |issue = 11 |pages = 855–858 |bibcode = 2008Geo....36..855P }}</ref><ref name=OrrEtAl1998>{{cite journal | doi = 10.1126/science.281.5380.1173 | pmid = 9712577 | author = Orr, Patrick J. | author2 = Briggs, Derek E. G. | author2-link = Briggs, Derek E. G. | author3 = Kearns, Stuart L. | journal = [[Science (journal)|Science]] | volume = 281 | issue = 5380 | pages = 1173–5 | year = 1998 | title = Cambrian Burgess Shale Animals Replicated in Clay Minerals | bibcode=1998Sci...281.1173O}}</ref>