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===Acritarchs=== [[File:Acritarch from the Weng'an biota.jpg|thumb| {{center|Acritarch from the Weng'an biota<br />c. 570–609 [[MYA (unit)|mya]] {{hsp}}<ref>{{cite journal |doi = 10.1144/jgs2016-142|title = The Weng'an Biota (Doushantuo Formation): An Ediacaran window on soft-bodied and multicellular microorganisms|year = 2017|last1 = Cunningham|first1 = John A.|last2 = Vargas|first2 = Kelly|last3 = Yin|first3 = Zongjun|last4 = Bengtson|first4 = Stefan|last5 = Donoghue|first5 = Philip C. J.|journal = Journal of the Geological Society|volume = 174|issue = 5|pages = 793–802|bibcode = 2017JGSoc.174..793C|doi-access = free|hdl = 1983/d874148a-f20e-498a-97d2-379b3feaa18a|hdl-access = free}}</ref> }}]] [[Acritarch]]s, Greek for ''confused origins'',<ref>definition of [http://dictionary.reference.com/browse/acritarch acritarch] at [[dictionary.com]]</ref> are organic-walled microfossils, known from about {{Ma|2000}} to the present. Acritarchs are not a specific biological taxon, but rather a group with uncertain or unknown affinities.<ref name=Evitt1963>{{cite journal |doi = 10.1073/pnas.49.3.298|title = A Discussion and Proposals Concerning Fossil Dinoflagellates, Hystrichospheres, and Acritarchs, Ii|year = 1963|last1 = Evitt|first1 = W. R.|journal = Proceedings of the National Academy of Sciences|volume = 49|issue = 3|pages = 298–302|pmid = 16591055|pmc = 299818|bibcode = 1963PNAS...49..298E|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1111/j.1469-185X.1993.tb01241.x|title = Acritarchsa Review|year = 1993|last1 = Martin|first1 = Francine|journal = Biological Reviews|volume = 68|issue = 4|pages = 475–537|s2cid = 221527533}}</ref><ref>{{cite journal |doi = 10.1016/0034-6667(94)00148-D|title = Review of biological affinities of Paleozoic acid-resistant, organic-walled eukaryotic algal microfossils (Including "acritarchs")|year = 1995|last1 = Colbath|first1 = G.Kent|last2 = Grenfell|first2 = Hugh R.|journal = Review of Palaeobotany and Palynology|volume = 86|issue = 3–4|pages = 287–314| bibcode=1995RPaPa..86..287C }}</ref> Most commonly they are composed of thermally altered acid insoluble carbon compounds ([[kerogen]]). While the [[biological classification|classification]] of acritarchs into [[form taxon|form genera]] is entirely artificial, it is not without merit, as the form taxa show traits similar to those of genuine [[taxon|taxa]] — for example the '[[Cambrian explosion|explosion]]' in the [[Cambrian]] and the [[mass extinction]] at the [[Permian-Triassic extinction event|end]] of the [[Permian]]. Acritarch diversity reflects major ecological events such as the appearance of predation and the [[Cambrian explosion]]. Precambrian marine diversity was dominated by acritarchs. They underwent a boom around {{Ma|1000}}, increasing in abundance, diversity, size, complexity of shape, and especially size and number of spines. Their increasingly spiny forms in the last 1 billion years may indicate an increased need for defence against predation.<ref>{{Cite book| author=Bengtson, S. | year=2002 | contribution=Origins and early evolution of predation | title=The fossil record of predation. The Paleontological Society Papers 8 | editor=Kowalewski, M. |editor2=Kelley, P.H. | pages=289–317 | publisher=The Paleontological Society | url=http://www.nrm.se/download/18.4e32c81078a8d9249800021552/Bengtson2002predation.pdf | format = Free full text| access-date=2007-12-01}}</ref> Acritarchs may include the remains of a wide range of quite different kinds of organisms—ranging from the egg cases of small [[metazoan]]s to resting cysts of many kinds of [[chlorophyta]] (green algae). It is likely that most acritarch species from the [[Paleozoic]] represent various stages of the life cycle of algae that were ancestral to the [[dinoflagellates]].<ref>{{Cite journal |last1=Colbath|first1= G.Kent|last2=Grenfell|first2= Hugh R.|date= 1995|title= Review of biological affinities of Paleozoic acid-resistant, organic-walled eukaryotic algal microfossils (including "acritarchs")|journal =Review of Palaeobotany and Palynology|volume =86|issue =3–4|pages =287–314|doi=10.1016/0034-6667(94)00148-d|bibcode= 1995RPaPa..86..287C|issn =0034-6667}}</ref> The nature of the organisms associated with older acritarchs is generally not well understood, though many are probably related to unicellular marine [[alga]]e. In theory, when the biological source (taxon) of an acritarch does become known, that particular microfossil is removed from the acritarchs and classified with its proper group. Acritarchs were most likely [[eukaryote]]s. While archaea, bacteria and cyanobacteria ([[prokaryotes]]) usually produce simple fossils of a very small size, eukaryotic unicellular fossils are usually larger and more complex, with external morphological projections and ornamentation such as spines and hairs that only eukaryotes can produce; as most acritarchs have external projections (e.g., hair, spines, thick cell membranes, etc.), they are predominantly eukaryotes, although simple eukaryote acritarchs also exist.<ref>{{Cite journal| doi = 10.1038/463885a| pmid = 20164911| year = 2010| last1 = Buick | first1 = R. .| title = Early life: Ancient acritarchs| volume = 463| issue = 7283| pages = 885–886| journal = Nature |bibcode = 2010Natur.463..885B | doi-access = free}}</ref> Acritarchs are found in sedimentary rocks from the present back into the [[Archean]].<ref>{{cite journal | title=MONTENARI, M. & LEPPIG, U. (2003): The Acritarcha: their classification morphology, ultrastructure and palaeoecological/palaeogeographical distribution. | journal=Paläontologische Zeitschrift | year=2003 | volume=77 | pages=173–194 | doi=10.1007/bf03004567| s2cid=127238427 }}</ref> They are typically isolated from siliciclastic sedimentary rocks using [[hydrofluoric acid]] but are occasionally extracted from carbonate-rich rocks. They are excellent candidates for index fossils used for dating rock formations in the [[Palaeozoic|Paleozoic]] Era and when other fossils are not available. Because most acritarchs are thought to be marine (pre-Triassic), they are also useful for palaeoenvironmental interpretation. The Archean and earliest [[Proterozoic]] microfossils termed "acritarchs" may actually be prokaryotes. The earliest eukaryotic acritarchs known (as of 2020) are from between 1950 and 2150 million years ago.<ref>{{cite journal |last1=Yin |first1=Leiming |title=Microfossils from the Paleoproterozoic Hutuo Group, Shanxi, North China: Early evidence for eukaryotic metabolism |journal=Precambrian Research |volume=342 |pages=105650 |date=Feb 2020 |doi=10.1016/j.precamres.2020.105650|bibcode=2020PreR..342j5650Y |doi-access=free }}</ref> Recent application of [[atomic force microscopy]], [[confocal microscopy]], [[Raman spectroscopy]], and other analytic techniques to the study of the ultrastructure, life history, and systematic affinities of mineralized, but originally organic-walled microfossils,<ref>{{cite journal |doi = 10.1073/pnas.142310299|title = Atomic force microscopy of Precambrian microscopic fossils|year = 2002|last1 = Kempe|first1 = A.|last2 = Schopf|first2 = J. W.|last3 = Altermann|first3 = W.|last4 = Kudryavtsev|first4 = A. B.|last5 = Heckl|first5 = W. M.|journal = Proceedings of the National Academy of Sciences|volume = 99|issue = 14|pages = 9117–9120|pmid = 12089337|pmc = 123103|bibcode = 2002PNAS...99.9117K|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1016/j.precamres.2005.07.002|title = Focussed ion beam preparation and in situ nanoscopic study of Precambrian acritarchs|year = 2005|last1 = Kempe|first1 = A.|last2 = Wirth|first2 = R.|last3 = Altermann|first3 = W.|last4 = Stark|first4 = R.|last5 = Schopf|first5 = J.|last6 = Heckl|first6 = W.|journal = Precambrian Research|volume = 140|issue = 1–2|pages = 36–54|bibcode = 2005PreR..140...36K}}</ref><ref>{{cite journal |doi = 10.1016/j.precamres.2005.05.006|title = Combined micro-Fourier transform infrared (FTIR) spectroscopy and micro-Raman spectroscopy of Proterozoic acritarchs: A new approach to Palaeobiology|year = 2005|last1 = Marshall|first1 = C.|last2 = Javaux|first2 = E.|last3 = Knoll|first3 = A.|last4 = Walter|first4 = M.|journal = Precambrian Research|volume = 138|issue = 3–4|pages = 208–224|bibcode = 2005PreR..138..208M}}</ref><ref>{{cite journal |doi = 10.4202/app.2008.0060|title = Spore-Like Bodies in Some Early Paleozoic Acritarchs: Clues to Chlorococcalean Affinities|year = 2009|last1 = Kaźmierczak|first1 = Józef|last2 = Kremer|first2 = Barbara|journal = Acta Palaeontologica Polonica|volume = 54|issue = 3|pages = 541–551|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1666/09-134.1|title = Confocal laser scanning microscopy and Raman imagery of the late Neoproterozoic Chichkan microbiota of South Kazakhstan|year = 2010|last1 = Schopf|first1 = J. William|last2 = Kudryavtsev|first2 = Anatoliy B.|last3 = Sergeev|first3 = Vladimir N.|journal = Journal of Paleontology|volume = 84|issue = 3|pages = 402–416| bibcode=2010JPal...84..402S |s2cid = 130041483}}</ref> have shown some acritarchs are fossilized [[microalgae]]. In the end, it may well be, as Moczydłowska et al. suggested in 2011, that many acritarchs will, in fact, turn out to be algae.<ref>{{cite journal |doi = 10.1111/j.1475-4983.2011.01054.x|title = Proterozoic phytoplankton and timing of Chlorophyte algae origins|year = 2011|last1 = Moczydłowska|first1 = Małgorzata|last2 = Landing|first2 = ED|last3 = Zang|first3 = Wenlong|last4 = Palacios|first4 = Teodoro|journal = Palaeontology|volume = 54|issue = 4|pages = 721–733| bibcode=2011Palgy..54..721M |doi-access = free}}</ref><ref name=Chamberlain2016>{{cite journal |doi = 10.3390/geosciences6040057|title = A Mineralized Alga and Acritarch Dominated Microbiota from the Tully Formation (Givetian) of Pennsylvania, USA|year = 2016|last1 = Chamberlain|first1 = John|last2 = Chamberlain|first2 = Rebecca|last3 = Brown|first3 = James|journal = Geosciences|volume = 6|issue = 4|page = 57|bibcode = 2016Geosc...6...57C|doi-access = free}} [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref> [[File:Microfossil Morphologies-1.png|thumb|upright=1.3|Three main types of [[Archean]] cell morphologies]]
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