Editing Fossil (section)

== Stromatolites ==
{{Main|Stromatolite}}
{{Further|Earliest known life forms}}
[[File:Stromatolites Cochabamba.jpg|thumb|right| Lower Proterozoic [[stromatolite]]s from [[Bolivia]], South America]]

Stromatolites are layered [[Accretion (geology)|accretionary]] [[structure]]s formed in shallow water by the trapping, binding and cementation of sedimentary grains by [[biofilm]]s of [[microorganism]]s, especially [[cyanobacteria]].<ref>{{cite journal| last = Riding | year = 2007 | first = R. | title = The term stromatolite: towards an essential definition | journal = Lethaia | volume = 32 | issue = 4 | pages = 321–330 | url = http://www3.interscience.wiley.com/journal/119935443/abstract | archive-url = https://archive.today/20150502101712/http://www3.interscience.wiley.com/journal/119935443/abstract | url-status=dead | archive-date = 2 May 2015 | doi = 10.1111/j.1502-3931.1999.tb00550.x}}</ref> Stromatolites provide some of the most ancient fossil records of life on Earth, dating back more than 3.5&nbsp;billion years ago.<ref>{{cite web| url=http://www.fossilmuseum.net/Tree_of_Life/Stromatolites.htm| title=Stromatolites, the Oldest Fossils| access-date=4 March 2007| archive-url=https://web.archive.org/web/20070309173949/http://www.fossilmuseum.net/Tree_of_Life/Stromatolites.htm| archive-date=9 March 2007| url-status=live}}</ref>

Stromatolites were much more abundant in Precambrian times. While older, [[Archean]] fossil remains are presumed to be [[colony (biology)|colonies]] of [[cyanobacteria]], younger (that is, [[Proterozoic]]) fossils may be [[prehistoric|primordial]] forms of the [[Eukaryota|eukaryote]] [[Chlorophyta|chlorophytes]] (that is, [[green algae]]). One [[genus]] of stromatolite very common in the [[geologic time scale|geologic record]] is ''[[Collenia]]''. The earliest stromatolite of confirmed microbial origin dates to 2.724&nbsp;billion years ago.<ref name=Lepot2008>{{Cite journal
| doi = 10.1038/ngeo107
| volume = 1
| pages = 118–21
| last1 = Lepot
| first1 = Kevin
| first2=Karim |last2=Benzerara |first3=Gordon E. |last3=Brown |first4=Pascal |last4=Philippot
| title = Microbially influenced formation of 2.7 billion-year-old stromatolites
| journal = [[Nature Geoscience]]
| year = 2008
| issue=2
|bibcode = 2008NatGe...1..118L }}</ref>

A 2009 discovery provides strong evidence of microbial stromatolites extending as far back as 3.45&nbsp;billion years ago.<ref name=Allwood2009>{{Cite journal
| last1 = Allwood
| first1 = Abigail C.
|first2=John P. |last2=Grotzinger |first3=Andrew H. |last3=Knoll |first4=Ian W. |last4=Burch |first5=Mark S. |last5=Anderson |first6=Max L. |last6=Coleman |first7=Isik |last7=Kanik
| title = Controls on development and diversity of Early Archean stromatolites
| journal = Proceedings of the National Academy of Sciences
| year = 2009
| doi = 10.1073/pnas.0903323106
| volume = 106
| pages = 9548–9555
| issue = 24
|bibcode = 2009PNAS..106.9548A
| pmid=19515817
| pmc=2700989| doi-access = free
}}</ref><ref>{{cite book |title=Cradle of life: the discovery of earth's earliest fossils |first=J. William |last=Schopf |publisher=Princeton University Press |location=Princeton, N.J |year=1999 |url=https://books.google.com/books?id=YJHBAolcIk8C&pg=PA87 |pages=87–89 |isbn=978-0-691-08864-8 |access-date=11 October 2018 |archive-date=17 March 2023 |archive-url=https://web.archive.org/web/20230317062709/https://books.google.com/books?id=YJHBAolcIk8C&pg=PA87 |url-status=live }}</ref>

Stromatolites are a major constituent of the fossil record for life's first 3.5&nbsp;billion years, peaking about 1.25&nbsp;billion years ago.<ref name=Allwood2009 /> They subsequently declined in abundance and diversity,<ref>{{cite journal| year=1982 | title=Precambrian conical stromatolites from California and Sonora | author=McMenamin, M. A. S. | journal=Bulletin of the Southern California Paleontological Society | volume=14 | issue=9&10 | pages=103–105 }}</ref> which by the start of the Cambrian had fallen to 20% of their peak. The most widely supported explanation is that stromatolite builders fell victims to grazing creatures (the [[Cambrian substrate revolution]]), implying that sufficiently complex organisms were common over 1&nbsp;billion years ago.<ref name="McNamara1996DatingOriginAnimals">{{cite journal
 | author = McNamara, K.J.
 | title = Dating the Origin of Animals
 | journal = Science
 | volume = 274
 | pages = 1993–1997
 | date = 20 December 1996
 | doi = 10.1126/science.274.5295.1993f
 | issue = 5295
 | bibcode = 1996Sci...274.1993M
 | doi-access = free
 }}</ref><ref name="AwramikStromatoliteDiversityMetazoanAppearance">{{cite journal
 | author = Awramik, S.M.
 | title = Precambrian columnar stromatolite diversity: Reflection of metazoan appearance
 | journal = Science
 | volume = 174
 | pages = 825–827
 | date = 19 November 1971
 | doi=10.1126/science.174.4011.825
 | pmid = 17759393
 | issue=4011
|bibcode = 1971Sci...174..825A | s2cid = 2302113
 }}</ref><ref name="Bengtson2002OriginsOfPredation">{{Cite encyclopedia
 | author=Bengtson, S.
 | year=2002
 | chapter=Origins and early evolution of predation
 | encyclopedia=The Paleontological Society Papers
 | volume=8
 | title=The fossil record of predation
 | editor=Kowalewski, M. |editor2=Kelley, P.H.
 | pages=289–317
 | publisher=The Paleontological Society
 | chapter-url=http://www.nrm.se/download/18.4e32c81078a8d9249800021552/Bengtson2002predation.pdf |archive-url=https://web.archive.org/web/20080910205539/http://www.nrm.se/download/18.4e32c81078a8d9249800021552/Bengtson2002predation.pdf |archive-date=2008-09-10 |url-status=live
 | access-date=29 December 2014
}}</ref>

The connection between grazer and stromatolite abundance is well documented in the younger [[Ordovician]] [[evolutionary radiation]]; stromatolite abundance also increased after the [[Ordovician-Silurian extinction events|end-Ordovician]] and [[Permian extinction|end-Permian extinctions]] decimated marine animals, falling back to earlier levels as marine animals recovered.<ref name="SheehanHarris2004ResurgenceAfterOrdovicianExtinction">{{cite journal
| title=Microbialite resurgence after the Late Ordovician extinction
| journal=Nature
| volume=430
| pages=75–78
| year=2004
| doi=10.1038/nature02654
| author1=Sheehan, P.M. |author2=Harris, M.T.
| pmid=15229600
| issue=6995
| bibcode=2004Natur.430...75S
| s2cid=4423149
}}</ref> Fluctuations in [[metazoan]] population and diversity may not have been the only factor in the reduction in stromatolite abundance. Factors such as the chemistry of the environment may have been responsible for changes.<ref name="Riding2006">{{cite journal | url=http://www.robertriding.com/pdf/riding2006mc.pdf | title=Microbial carbonate abundance compared with fluctuations in metazoan diversity over geological time | author=Riding R | journal=Sedimentary Geology | date=March 2006 | volume=185 | issue=3–4 | pages=229–38 | doi=10.1016/j.sedgeo.2005.12.015 | bibcode=2006SedG..185..229R | access-date=9 December 2011 | archive-url=https://web.archive.org/web/20120426041343/http://www.robertriding.com/pdf/riding2006mc.pdf | archive-date=26 April 2012 | url-status=dead }}</ref>

While [[prokaryote|prokaryotic]] cyanobacteria themselves reproduce asexually through cell division, they were instrumental in priming the environment for the [[timeline of evolution|evolutionary development]] of more complex [[eukaryote|eukaryotic]] organisms. Cyanobacteria (as well as [[extremophile]] [[Gammaproteobacteria]]) are thought to be largely responsible for increasing the amount of [[oxygen]] in the primeval Earth's [[atmosphere]] through their continuing [[photosynthesis]]. Cyanobacteria use [[water]], [[carbon dioxide]] and [[sunlight]] to create their food. A layer of [[mucus]] often forms over mats of cyanobacterial cells. In modern microbial mats, debris from the surrounding habitat can become trapped within the mucus, which can be cemented by the calcium carbonate to grow thin laminations of [[limestone]]. These laminations can accrete over time, resulting in the banded pattern common to stromatolites. The domal morphology of biological stromatolites is the result of the vertical growth necessary for the continued infiltration of sunlight to the organisms for photosynthesis. Layered spherical growth structures termed [[oncolite]]s are similar to stromatolites and are also known from the [[fossil record]]. [[Thrombolite]]s are poorly laminated or non-laminated clotted structures formed by cyanobacteria common in the fossil record and in modern sediments.<ref name=Lepot2008 />

The Zebra River Canyon area of the Kubis platform in the deeply dissected Zaris Mountains of southwestern [[Namibia]] provides an extremely well exposed example of the thrombolite-stromatolite-metazoan reefs that developed during the Proterozoic period, the stromatolites here being better developed in updip locations under conditions of higher current velocities and greater sediment influx.<ref>{{cite journal |author1=Adams, E. W. |author2=Grotzinger, J. P. |author3=Watters, W. A. |author4=Schröder, S. |author5=McCormick, D. S. |author6=Al-Siyabi, H. A. |year=2005 |title=Digital characterization of thrombolite-stromatolite reef distribution in a carbonate ramp system (terminal Proterozoic, Nama Group, Namibia) |journal=AAPG Bulletin |volume=89 |issue=10 |pages=1293–1318 |doi=10.1306/06160505005 |bibcode=2005BAAPG..89.1293A |url=http://www.wellesley.edu/Astronomy/Wwatters/adams%20etal%20-%20digital%20models%20nama%20reefs%20-%20aapg%20bull%202005.pdf |access-date=8 December 2011 |archive-url=https://web.archive.org/web/20160307215400/http://wellesley.edu/Astronomy/wwatters/adams%20etal%20-%20digital%20models%20nama%20reefs%20-%20aapg%20bull%202005.pdf |archive-date=7 March 2016 |url-status=dead }}</ref>