Editing Microfossil

{{Short description|Fossil that requires the use of a microscope to see}}
{{Use British English|date=August 2021}}
{{Use dmy dates|date=August 2021}}
[[File:Cochleatina.jpg|thumb|upright=1.35| {{center|An enigmatic [[carbonaceous microfossil]],<br />''[[Cochleatina|Cochleatina canilovica]]'', from the [[Ediacaran|Late Ediacaran]]{{hsp}}<ref>{{Cite journal|last1=Slater|first1=Ben J.|last2=Harvey|first2=Thomas H. P.|last3=Bekker|first3=Andrey|last4=Butterfield|first4=Nicholas J.|title=Cochleatina: an enigmatic Ediacaran–Cambrian survivor among small carbonaceous fossils (SCFs)|journal=Palaeontology|year=2020|volume=63|issue=5|language=en|pages=733–752|doi=10.1111/pala.12484|bibcode=2020Palgy..63..733S |issn=1475-4983|doi-access=free}}</ref>}}]]

A '''microfossil''' is a [[fossil]] that is generally between 0.001&nbsp;mm and 1&nbsp;mm in size,<ref>{{cite web|last1 = Drewes|first1 = Charlie|title = Discovering Devonian Microfossils|url = http://www.eeob.iastate.edu/faculty/DrewesC/htdocs/microfossilsABLE.doc|publisher=Iowa State University|access-date = 4 March 2017}}</ref> the visual study of which requires the use of light or electron [[microscopy]]. A fossil which can be studied with the naked eye or low-powered magnification, such as a hand lens, is referred to as a [[macrofossil]].

Microfossils are a common feature of the [[geologic timescale|geological record]], from the [[Precambrian]] to the [[Holocene]]. They are most common in deposits of [[ocean|marine]] environments, but also occur in brackish water, fresh water and terrestrial [[sedimentary]] deposits. While every [[Kingdom (biology)|kingdom]] of [[prehistoric life|life]] is represented in the microfossil record, the most abundant forms are [[protist skeletons]] or [[microbial cyst]]s from the [[Chrysophyta]], [[Pyrrhophyta]], [[Sarcodina]], [[acritarch]]s and [[chitinozoa]]ns, together with [[pollen]] and [[spores]] from the [[vascular plant]]s.

==Overview==
{{paleontology}}

A microfossil is a descriptive term applied to fossilized plants and animals whose size is just at or below the level at which the fossil can be analyzed by the naked eye. A commonly applied cutoff point between "micro" and [[macrofossil|"macro" fossils]] is 1&nbsp;mm. Microfossils may either be complete (or near-complete) organisms in themselves (such as the marine plankters [[foraminifera]] and [[coccolithophore]]s) or component parts (such as small teeth or [[palynology|spores]]) of larger animals or plants. Microfossils are of critical importance as a reservoir of [[Paleoclimatology|paleoclimate]] information, and are also commonly used by [[biostratigraphy|biostratigraphers]] to assist in the correlation of rock units.

Microfossils are found in rocks and sediments as the microscopic remains of what were once life forms such as plants, animals, fungus, protists, bacteria and archaea. Terrestrial microfossils include [[pollen]] and [[spore]]s. Marine microfossils found in [[marine sediment]]s are the most common microfossils. Everywhere in the oceans, microscopic [[Marine protists|protist organisms]] multiply prolifically, and many grow [[Protist shell|tiny skeletons]] which readily fossilise. These include [[foraminifera]], [[dinoflagellate]]s and [[radiolarian]]s. [[Palaeontologist]]s (geologists who study fossils) are interested in these microfossils because they can use them to determine how environments and climates have changed in the past, and where oil and gas can be found today.<ref name=Campbell2006>Campbell, Hamish (12 Jun 2006) [https://teara.govt.nz/en/fossils/page-7#:~:text=Microfossils%20are%20the%20remains%20of,use%20microscopes%20to%20study%20them. "Fossils - Microfossils"], ''Te Ara - the Encyclopedia of New Zealand". Accessed 11 May 2021.</ref>

Some microfossils are formed by [[colonial organism]]s such as [[Bryozoa]] (especially the [[Cheilostomata]]), which have relatively large [[colony (biology)|colonies]] but are classified by fine skeletal details of the small individuals of the colony. As another example, many fossil [[genus|genera]] of [[Foraminifera]], which are protists are known from shells (called [[Test (biology)|tests]]) that were as big as coins, such as the genus ''[[Nummulites]]''.

In 2017, fossilized [[microorganism]]s, or microfossils, were discovered in [[hydrothermal vent]] [[precipitates]] in the [[Nuvvuagittuq Greenstone Belt|Nuvvuagittuq Belt]] of Quebec, Canada that may be as old as 4.28 billion years old, the [[Earliest known life forms|oldest record of life on Earth]], suggesting "an almost instantaneous emergence of life" (in a geological time-scale), after [[Origin of water on Earth#History of water on Earth|ocean formation 4.41 billion years ago]], and not long after the [[Age of the Earth|formation of the Earth]] 4.54 billion years ago.<ref name="NAT-20170301">{{cite journal |author=Dodd, Matthew S. |author2=Papineau, Dominic |author3=Grenne, Tor |author4=slack, John F. |author5=Rittner, Martin |author6=Pirajno, Franco |author7=O'Neil, Jonathan |author8=Little, Crispin T. S. |title=Evidence for early life in Earth's oldest hydrothermal vent precipitates|journal=Nature |volume=543 |issue=7643 |pages=60–64 |date=2 March 2017 | doi=10.1038/nature21377|pmid=28252057 |bibcode=2017Natur.543...60D |url=http://eprints.whiterose.ac.uk/112179/1/ppnature21377_Dodd_for%20Symplectic.pdf |doi-access=free }}</ref><ref name="NYT-20170301">{{cite news |last=Zimmer |first=Carl |author-link=Carl Zimmer |title=Scientists Say Canadian Bacteria Fossils May Be Earth's Oldest |url=https://www.nytimes.com/2017/03/01/science/earths-oldest-bacteria-fossils.html |date=1 March 2017 |work=[[New York Times]] |access-date=2 March 2017 }}</ref><ref name="BBC-20170301">{{cite web |last=Ghosh |first=Pallab |title=Earliest evidence of life on Earth 'found |url=https://www.bbc.co.uk/news/science-environment-39117523 |work=[[BBC News]] |date=1 March 2017 |access-date=2 March 2017}}</ref><ref name="4.3b oldest">{{cite news |last1=Dunham |first1=Will |title=Canadian bacteria-like fossils called oldest evidence of life |url=http://ca.reuters.com/article/topNews/idCAKBN16858B?sp=true |archive-url=https://web.archive.org/web/20170302114728/http://ca.reuters.com/article/topNews/idCAKBN16858B?sp=true |url-status=dead |archive-date=2 March 2017 |date=1 March 2017 |publisher=[[Reuters]] |access-date=1 March 2017 }}</ref> Nonetheless, life may have started even earlier, at nearly 4.5 billion years ago, as claimed by some researchers.<ref name="PHY-20180820">{{cite web |author=Staff |title=A timescale for the origin and evolution of all of life on Earth |url=https://phys.org/news/2018-08-timescale-evolution-life-earth.html |date=20 August 2018 |work=[[Phys.org]] |access-date=20 August 2018 }}</ref><ref name="NAT-20180820">{{cite journal |last1=Betts |first1=Holly C. |last2=Putick |first2=Mark N. |last3=Clark |first3=James W. |last4=Williams |first4=Tom A. |last5=Donoghue |first5=Philip C.J. |last6=Pisani |first6=Davide |title=Integrated genomic and fossil evidence illuminates life's early evolution and eukaryote origin |date=20 August 2018 |journal=[[Nature (journal)|Nature]] |volume=2 |issue=10 |pages=1556–1562 |doi=10.1038/s41559-018-0644-x |pmid=30127539 |pmc=6152910 }}</ref>

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==Index fossils==
{{biomineralization sidebar}}

[[Index fossils]], also known as guide fossils, indicator fossils or dating fossils, are the fossilized remains or traces of particular plants or animals that are characteristic of a particular span of geologic time or environment, and can be used to identify and date the containing rocks. To be practical, index fossils must have a limited vertical time range, wide geographic distribution, and rapid evolutionary trends. Rock formations separated by great distances but containing the same index fossil species are thereby known to have both formed during the limited time that the species lived.

Index fossils were originally used to define and identify geologic units, then became a basis for defining [[geologic column|geologic periods]], and then for faunal stages and zones.

Species of [[micropaleontology|microfossils]] such as [[acritarchs]], [[chitinozoa]]ns, [[conodont]]s, [[dinoflagellate]] cysts, [[ostracods]], [[pollen]], [[spores]] and [[foraminifera]]ns are amongst the many species have been identified as index fossils that are widely used in [[biostratigraphy]]. Different fossils work well for sediments of different ages. To work well, the fossils used must be widespread geographically, so that they can be found in many different places. They must also be short lived as a species, so that the period of time during which they could be incorporated in the sediment is relatively narrow. The longer lived the species, the poorer the stratigraphic precision, so fossils that evolve rapidly.

Often biostratigraphic correlations are based on a [[faunal assemblage]], rather than an individual species — this allows greater precision as the time span in which all of the species in the assemblage existed together is narrower than the time spans of any of the members. Further, if only one species is present in a sample, it can mean either that (1) the strata were formed in the known fossil range of that organism; or (2) that the fossil range of the organism was incompletely known, and the strata extend the known fossil range. If the fossil is easy to preserve and easy to identify, more precise time estimating of the [[stratigraphic layer]]s is possible.

==Composition==
[[File:Deep Sea Drilling Project microfossils.jpg|thumb|upright=1.3| {{center|Microfossils from a [[deep sea sediment]] core}}]]

Microfossils can be classified by their composition as: (a) [[silicate|siliceous]], as in [[diatom]]s and [[radiolaria]], (b) [[calcareous]], as in [[coccolith]]s and [[foraminifera]], (c) [[Phosphate|phosphatic]], as in the study of some [[vertebrate]]s, or (d) [[organic matter|organic]], as in the [[pollen]] and [[spores]] studied in [[palynology]]. This division focuses on differences in the mineralogical and chemical composition of microfossil remains rather than on [[scientific classification|taxonomic]] or [[ecological]] distinctions.

* Siliceous microfossils: [[Silicate|Siliceous]] microfossils include [[diatoms]], [[radiolaria]]ns, [[silicoflagellate]]s, [[Ebriid|ebridians]], [[phytolith]]s, some [[scolecodonts]] (worm jaws), and [[sponge spicule]]s.
* Calcareous microfossils: [[Calcareous]] ([[Calcium carbonate|CaCO<sub>3</sub>]]) microfossils include [[coccolith]]s, [[foraminifera]], [[calcareous dinoflagellate cysts]], and [[ostracod]]s (seed shrimp).
* Phosphatic microfossils: [[Phosphate|Phosphatic]] microfossils include [[conodont]]s (tiny oral structures of an extinct chordate group), some [[scolecodonts]] (worm jaws), [[shark]] spines and teeth and other [[fish]] remains (collectively called [[ichthyolith]]s).
* Organic microfossils: The study of [[organic matter|organic]] microfossils is called [[palynology]]. Organic microfossils include [[pollen]], [[spores]], [[chitinozoa]]ns (thought to be the egg cases of marine invertebrates), [[scolecodonts]] (worm jaws), [[acritarchs]], [[dinoflagellate cysts]], and [[fungal]] remains.

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==Organic-walled==

===Palynomorphs===
{{further|Palynomorphs|Kerogen}}

===Pollen grain===
[[File:Trilete spores.png|thumb|upright=1.3| {{center|[[Late Silurian]] [[sporangium]] bearing [[trilete spore]]s provide the earliest evidence of life on land.<ref name=Gray1985>{{cite journal| author = Gray, J.| date = 1985| title = The Microfossil Record of Early Land Plants: Advances in Understanding of Early Terrestrialization, 1970&ndash;1984| journal = [[Philosophical Transactions of the Royal Society B]] | volume = 309| issue = 1138| pages = 167–195| doi  = 10.1098/rstb.1985.0077| last2 = Chaloner| first2 = W. G.| last3  = Westoll| first3 = T. S.| jstor=2396358| bibcode=1985RSPTB.309..167G| doi-access = free}}</ref><br /><small>Green: spore tetrad. Blue: spore with Y-shaped trilete mark.<br />Spores are about 30&ndash;35&nbsp;μm across</small>}}]]
{{see also|Pollen zone}}

[[Pollen]] has an outer sheath, called a  [[sporopollenin]], which affords it some resistance to the rigours of the fossilisation process that destroy weaker objects. It is produced in huge quantities. There is an extensive fossil record of pollen grains, often disassociated from their parent plant. The discipline of [[palynology]] is devoted to the study of pollen, which can be used both for biostratigraphy and to gain information about the abundance and variety of plants alive — which can itself yield important information about paleoclimates. Also, pollen analysis has been widely used for reconstructing past changes in vegetation and their associated drivers.<ref>{{cite journal |last=Franco-Gaviria |first=Felipe |display-authors=etal |title=The human impact imprint on modern pollen spectra of the Mayan lands |year=2018 |journal=[[Boletín de la Sociedad Geológica Mexicana]] |volume=70 |issue=1 |pages=61–78 |doi=10.18268/BSGM2018v70n1a4 |url=http://boletinsgm.igeolcu.unam.mx/bsgm/vols/epoca04/7001/%284%29Franco.pdf|doi-access=free }}</ref>
Pollen is first found in the [[fossil]] record in the late [[Devonian]] period,<ref name="palynology">{{Cite book| last = Traverse | first = Alfred | chapter = Devonian Palynology | pages=199–227 | title = Paleopalynology | volume = 28 |series = Topics in Geobiology, 28 | year =2007 | publisher = Springer | location = Dordrecht | isbn = 978-1-4020-6684-9 | doi = 10.1007/978-1-4020-5610-9_8 }}</ref><ref>{{cite journal |last1=Wang |first1=De-Ming |last2=Meng |first2=Mei-Cen |last3=Guo |first3=Yun |title=Pollen Organ Telangiopsis sp. of Late Devonian Seed Plant and Associated Vegetative Frond |year=2016 |journal=PLOS ONE |volume=11 |issue=1 |pages=e0147984 |doi=10.1371/journal.pone.0147984 |pmid=26808271 |pmc=4725745 |bibcode=2016PLoSO..1147984W |doi-access=free }}</ref> but at that time it is indistinguishable from spores.<ref name="palynology"/> It increases in abundance until the present day.

===Plant spores===
{{see also|Cryptospore}}

A [[spore]] is a unit of [[sexual reproduction|sexual]] or [[asexual reproduction]] that may be adapted for [[biological dispersal|dispersal]] and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the [[Biological life cycle|life cycles]] of many [[plant]]s, [[algae]], [[fungus|fungi]] and [[protozoa]].<ref>{{Cite web |url=http://tolweb.org/tree/home.pages/searchresults.html?cx=009557456284541951685%3A50nf_5tpvuq&cof=FORID%3A9&ie=UTF-8&q=spore&sa=Search |title=Tree of Life Web Project |access-date=5 February 2018 |archive-url=https://web.archive.org/web/20180205184645/http://tolweb.org/tree/home.pages/searchresults.html?cx=009557456284541951685%3A50nf_5tpvuq&cof=FORID%3A9&ie=UTF-8&q=spore&sa=Search |archive-date=5 February 2018 |url-status=live }}</ref> [[Bacterial spore]]s are not part of a sexual cycle but are resistant structures used for survival under unfavourable conditions.

===Fungal spores===

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===Chitinozoa===
[[File:Whole_chitinozoan_cropped.jpg|thumb| A [[Late Silurian]] chitinozoan from the [[Burgsvik beds]] showing its flask shape]]

[[Chitinozoa]] are a [[taxon]] of [[laboratory flask|flask]]-shaped, [[organic matter|organic]] walled [[marine biology|marine]] microfossils produced by an as-yet-unknown organism.<ref name=Mullins>{{cite journal| doi=10.1111/1475-4983.00131| title=A chitinozoan morphological lineage and its importance in Lower Silurian stratigraphy| author= Gary Lee Mullins| year=2000| journal=Palaeontology| volume=43| pages=359–373| issue=2| bibcode=2000Palgy..43..359M| doi-access=free}}</ref>

Common from the [[Ordovician]] to [[Devonian]] periods (i.e. the mid-Paleozoic), the millimetre-scale organisms are abundant in almost all types of [[marine sediment]] across the globe.<ref name=Jansonius1978>{{cite book | author = Jansonius, J. |author2=Jenkins, W.A.M.| year = 1978 | chapter = Chitinozoa | isbn = 0-444-00267-7 | title = Introduction to marine micropaleontology. | publisher = Elsevier, New York | pages = 341–357}}</ref> This wide distribution, and their rapid pace of evolution, makes them valuable [[biostratigraphic]] markers.

Their bizarre form has made [[scientific classification|classification]] and ecological reconstruction difficult.  Since their discovery in 1931, suggestions of [[protist]], [[plant]], and [[fungus|fungal]] affinities have all been entertained.  The organisms have been better understood as improvements in microscopy facilitated the study of their fine structure, and it has been suggested that they represent either the [[egg (biology)|eggs]] or juvenile stage of a marine animal.<ref name=Gabbott1998>{{cite journal | author = Gabbott, S.E. |author2=Aldridge, R.J. |author3=Theron, J.N. | year = 1998 | title = Chitinozoan chains and cocoons from the Upper Ordovician Soom Shale lagerstatte, South Africa; implications for affinity | journal = Journal of the Geological Society | volume = 155 | issue = 3 | pages = 447–452 | doi = 10.1144/gsjgs.155.3.0447| bibcode = 1998JGSoc.155..447G|s2cid=129236534 }}</ref> However, recent research has suggested that they represent the [[Test (biology)|test]] of a group of protists with uncertain affinities.<ref name=Liang2020>{{Cite journal|last1=Liang|first1=Yan|last2=Hints|first2=Olle|last3=Tang|first3=Peng|last4=Cai|first4=Chenyang|last5=Goldman|first5=Daniel|last6=Nõlvak|first6=Jaak|last7=Tihelka|first7=Erik|last8=Pang|first8=Ke|last9=Bernardo|first9=Joseph|last10=Wang|first10=Wenhui|date=2020-12-01|title=Fossilized reproductive modes reveal a protistan affinity of Chitinozoa|journal=Geology|language=en|volume=48|issue=12|pages=1200–1204|doi=10.1130/G47865.1|bibcode=2020Geo....48.1200L|issn=0091-7613|doi-access=free}}</ref>

The ecology of chitinozoa is also open to speculation; some may have floated in the water column, where others may have attached themselves to other organisms.  Most species were particular about their living conditions, and tend to be most common in specific paleoenvironments. Their abundance also varied with the seasons.

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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]]

===Archean cells===
{{see also|Archean life in the Barberton Greenstone Belt}}

Cells can be preserved in the [[rock record]] because their cell walls are made of proteins which convert to the organic material [[kerogen]] as the cell breaks down after death. Kerogen is [[insoluble]] in mineral [[acids]], [[base (chemistry)|base]]s, and [[organic solvents]].<ref>{{cite journal|last=Philp|first=R.P.|author2=Calvin, M. |title=Possible origin for insoluble organic (kerogen) debris in sediments from insoluble cell-wall materials of algae and bacteria|journal=Nature|year=1976|volume=262|pages=134–136 |doi=10.1038/262134a0|issue=5564|bibcode = 1976Natur.262..134P |s2cid=42212699}}</ref> Over time, it is mineralised into [[graphite]] or graphite-like [[carbon]], or degrades into oil and gas hydrocarbons.<ref>{{cite journal|last=Tegelaar|first=E.W.|author2=deLeeuw, J.W.|author3=Derenne, S.|author4=Largeau, C. |title=A reappraisal of kerogen formation|journal=Geochimica et Cosmochimica Acta|year=1989|volume=53|issue=11|pages=3103–3106 |doi=10.1016/0016-7037(89)90191-9|bibcode=1989GeCoA..53.3103T}}</ref> There are three main types of cell morphologies. Though there is no established range of sizes for each type, spheroid microfossils can be as small as about 8&nbsp;[[micrometre]]s, filamentous microfossils have diameters typically less than 5&nbsp;micrometres and have a length that can range from tens of micrometres to 100&nbsp;micrometres, and spindle-like microfossils can be as long as 50&nbsp;micrometres.<ref name=Walsh>{{cite journal|last=Walsh|first=M.|title=Microfossils and possible microfossils from the early Archean Onverwacht Group, Barberton mountain land, South Africa |journal=[[Precambrian Research]]|year=1991|volume=54|issue=2–4|pages=271–293 |pmid=11540926 |doi=10.1016/0301-9268(92)90074-X}}</ref><ref>{{cite journal|author=Oehler, D.Z. |author2=Robert, F. |author3=Mostefaoui, S. |author4=Meibom, A. |author5=Selo, M. |author6=McKay, D.S.|title=Chemical Mapping of Proterozoic Organic Matter at Submicron Spatial Resolution|journal=Astrobiology|year=2006|volume=6|issue=6|pages=838–850 |pmid=17155884 |doi=10.1089/ast.2006.6.838|bibcode = 2006AsBio...6..838O |hdl=2060/20060028086 |hdl-access=free }}</ref>

{{clear}}

==Mineralised==
{{further|Shelled protists}}

===Siliceous===
[[Siliceous ooze]] is a type of biogenic [[pelagic sediment]] located on the [[Abyssal|deep]] [[ocean floor]]. Siliceous oozes are the least common of the deep sea sediments, and make up approximately 15% of the ocean floor.<ref>{{Citation|last1=Mulder|first1=Thierry|title=Progress in Deep-Sea Sedimentology|date=2011|work=Deep-Sea Sediments|pages=1–24|publisher=Elsevier|isbn=9780444530004 | doi = 10.1016/b978-0-444-53000-4.00001-9 |last2=Hüneke|first2=Heiko|last3=Van Loon|first3=A.J.}}</ref> Oozes are defined as sediments which contain at least 30% skeletal remains of pelagic microorganisms.<ref>{{Cite journal|last1=Bohrmann|first1=Gerhard|last2=Abelmann|first2=Andrea|last3=Gersonde|first3=Rainer|last4=Hubberten|first4=Hans|last5=Kuhn|first5=Gerhard|date=1994|title=Pure siliceous ooze, a diagenetic environment for early chert formation |journal=Geology|volume=22|issue=3|pages=207|doi=10.1130/0091-7613(1994)022<0207:psoade>2.3.co;2|bibcode=1994Geo....22..207B}}</ref> Siliceous oozes are largely composed of the silica based skeletons of microscopic marine organisms such as [[diatom]]s and [[radiolarian]]s. Other components of siliceous oozes near continental margins may include terrestrially derived silica particles and sponge spicules. Siliceous oozes are composed of skeletons made from opal silica [[SiO2|Si(O<sub>2</sub>)]], as opposed to [[calcareous ooze]]s, which are made from skeletons of calcium carbonate organisms (i.e. [[coccolithophore]]s). Silica (Si) is a bioessential element and is efficiently recycled in the marine environment through the [[silica cycle]].<ref>{{Cite journal|last=DeMaster|first=David J.|date= October 1981 |title=The supply and accumulation of silica in the marine environment |journal=Geochimica et Cosmochimica Acta|volume=45|issue=10|pages=1715–1732|doi=10.1016/0016-7037(81)90006-5 |bibcode=1981GeCoA..45.1715D}}</ref> Distance from land masses, water depth and ocean fertility are all factors that affect the opal silica content in seawater and the presence of siliceous oozes.

{|class="wikitable"
! colspan=8 |{{center|[[Siliceous ooze]]}}
|-
! mineral<br />forms
! protist<br />involved
!
! name of skeleton
! width=100px | typical size
! colspan=2 |
|-
| width=90px  rowspan=2 align=center | [[Silicon oxide|SiO<sub>2</sub>]]<br />[[silica]]<br />[[quartz]]<br />[[glass]]<br />[[opal]]<br />[[chert]]
| [[diatom]]
| style="background:#000000;"| [[File:Lyrella hennedy 1600x contrast invertion.jpg|90px]]
| [[frustule]]
| 0.002 to 0.2&nbsp;mm{{hsp}}<ref name="HasleSyvertsen1996">{{cite book|first1=Grethe R.|last1=Hasle|first2=Erik E. |last2=Syvertsen |first3=Karen A. |last3=Steidinger |first4=Karl|last4=Tangen|editor-first=Carmelo R.|editor-last=Tomas|title=Identifying Marine Diatoms and Dinoflagellates|chapter-url=https://books.google.com/books?id=KQxPtwonlqoC|access-date=2013-11-13|date=1996-01-25|publisher=Academic Press|isbn=978-0-08-053441-1|pages=5–385|chapter=Marine Diatoms}}</ref>
| [[File:Stephanopyxis grunowii.jpg|100px]] 
| diatom microfossil from 40 million years ago
|-
| [[radiolarian]]
| style="background:#000000;"| [[File:Calocycloma sp. - Radiolarian (32163186535).jpg|90px]]
| [[Test (biology)|test]] or shell
| 0.1 to 0.2&nbsp;mm{{hsp}}
| [[File:Radiolarian - Heliodiscus umbonatus (Ehr.), Haeckel (28187768550).jpg|100px]]
| elaborate silica shell of a radiolarian
|-
|}

<gallery mode=packed heights=160px style=float:left;>
File:Diatomaceous Earth BrightField.jpg|[[Diatomaceous earth]] is a soft, [[siliceous]], [[sedimentary rock]] made up of microfossils in the form of the [[frustule]]s (shells) of centric and pennate [[diatom]]s (click to magnify)
File:Detail, CSIRO ScienceImage 7632 SEM diatom (cropped).jpg|{{center|[[Centric diatom]]<br />(radial symmetry)}}
File:Pennate diatoms (3075304186).jpg|{{center|[[Pennate diatom]]<br />(bilateral symmetry)}}
</gallery>

{{multiple image
 | align = right
 | direction = horizontal
 | width1    = 195
 | image1    = The silicoflagellate Dictyocha fibula.png
 | alt1      = 
 | caption1  = {{center|[[Silicoflagellate]]}}
 | width2    = 130
 | image2    = Radiolarian - Podocyrtis ampla (29391267424).jpg
 | alt2      = 
 | caption2  = {{center|[[Radiolarian]]}}
}}

[[File:Phytolith 3.png|thumb|upright=1.3| {{center|Phytolith from a leaf of the tree<br />''[[Cornus controversa]]''<ref name=Ge2020>{{cite journal |doi = 10.1038/s41598-020-72547-w|title = Phytoliths in selected broad-leaved trees in China|year = 2020|last1 = Ge|first1 = Yong|last2 = Lu|first2 = Houyuan|last3 = Wang|first3 = Can|last4 = Gao|first4 = Xing|journal = Scientific Reports|volume = 10|issue = 1|page = 15577|pmid = 32968165|pmc = 7512002|bibcode = 2020NatSR..1015577G}}</ref> • <small>scale bar 20 μm</small>}}]]

{{clear left}}

[[Phytoliths]] (Greek for ''plant stones'') are rigid, microscopic structures made of [[silica]], found in some plant tissues and persisting after the decay of the plant. These plants take up silica from the soil, whereupon it is deposited within different intracellular and extracellular structures of the plant.  Phytoliths come in varying shapes and sizes. The term "phytolith" is sometimes used to refer to all mineral secretions by plants, but more commonly refers to siliceous plant remains.<ref name="Piperno, Dolores R. 2006">Piperno, Dolores R. (2006). Phytoliths: A Comprehensive Guide for Archaeologists and Paleoecologists. AltaMira Press {{ISBN|0759103852}}.</ref>

{{clear}}

===Calcareous===
The term ''calcareous'' can be applied to a fossil, sediment, or sedimentary rock which is formed from, or contains a high proportion of, [[calcium carbonate]] in the form of [[calcite]] or [[aragonite]]. Calcareous sediments ([[limestone]]) are usually deposited in shallow water near land, since the carbonate is precipitated by marine organisms that need land-derived nutrients. Generally speaking, the farther from land sediments fall, the less calcareous they are. Some areas can have interbedded calcareous sediments due to storms, or changes in ocean currents. [[Calcareous ooze]] is a form of calcium carbonate derived from planktonic organisms that accumulates on the [[sea floor]]. This can only occur if the ocean is shallower than the [[carbonate compensation depth]]. Below this depth, calcium carbonate begins to dissolve in the ocean, and only non-calcareous sediments are stable, such as [[siliceous ooze]] or [[pelagic red clay]].

{|class="wikitable"
! colspan=8 |{{center|[[Calcareous]] ooze}}
|-
! mineral<br />forms
! protist<br />involved
!
! name of skeleton
! width=100px | typical size
! colspan=2 |
|-
|  width=90px rowspan=3 align=center | [[Calcium carbonate|CaCO<sub>3</sub>]]<br />[[calcite]]<br />[[aragonite]]<br />[[limestone]]<br />[[marble]]<br />[[chalk]]
| [[foraminiferan]]
| style="background:#000000;"| [[File:Foram-globigerina hg.jpg|90px]]
| [[Foraminifera test|test]] or shell
| many under 1&nbsp;mm
| [[File:Globigerina.png|100px]]
| [[Calcified]] [[Foraminifera test|test]] of a planktic foraminiferan. There are about 10,000 living species of foraminiferans<ref name="adl2007">{{cite journal | last1 = Ald | first1 = S.M. | display-authors = etal | year = 2007 | title = Diversity, Nomenclature, and Taxonomy of Protists | journal = Syst. Biol. | volume = 56 | issue = 4 | pages = 684–689 | doi = 10.1080/10635150701494127 | pmid = 17661235 | doi-access = free }}</ref>
|-
| [[coccolithophore]]
| style="background:#000000;"| [[File:Coccolithus pelagicus 2.jpg|90px]]
| [[coccolith]]s
| under 0.1&nbsp;mm{{hsp}}<ref name=Moheimani2012>{{citation |journal=[[Algal Research]] |volume=1 |issue=2 |year=2012 |pages=120–133 |title=Bioremediation and other potential applications of coccolithophorid algae: A review. . Bioremediation and other potential applications of coccolithophorid algae: A review |first1=N.R. |last1=Moheimani |first2=J.P. |last2=Webb |first3= M.A. |last3=Borowitzka |doi=10.1016/j.algal.2012.06.002}}</ref>
| [[File:CSIRO ScienceImage 7202 SEM Coccolithophorid.jpg|100px]] 
| Coccolithophores are the largest global source of biogenic calcium carbonate, and significantly contribute to the global [[carbon cycle]].<ref>{{cite journal | last1 = Taylor | first1 = A.R. | last2 = Chrachri | first2 = A. | last3 = Wheeler | first3 = G. | last4 = Goddard | first4 = H. | last5 = Brownlee | first5 = C. | year = 2011 | title = A voltage-gated H+ channel underlying pH homeostasis in calcifying coccolithophores | journal = PLOS Biology | volume = 9 | issue = 6| page = e1001085 | doi = 10.1371/journal.pbio.1001085 | pmid = 21713028 | pmc = 3119654 | doi-access = free }}</ref> They are the main constituent of chalk deposits such as the [[white cliffs of Dover]].
|-
|}

<gallery mode=packed heights=160px style=float:left;>
File:Nanoplankton-fossil-sediment hg.jpg| {{center| Calcareous microfossils from marine sediment consisting mainly of star-shaped [[discoaster]] with a sprinkling of coccoliths}}
File:PSM V44 D483 Globigerina ooze.jpg|Illustration of a ''[[Globigerina]]'' ooze
File:FMIB 47660 Shells from Globigerina Ooze.jpeg|Shells ([[Test (biology)|tests]]), usually made of calcium carbonate, from a [[foraminifera]]l ooze on the deep ocean floor
</gallery>

[[File:Mesozoic benthic foraminifera.png|thumb| {{center|[[Mesozoic]] benthic foraminifera{{hsp}}<ref>{{cite journal |doi = 10.5194/jm-37-395-2018|title = New species of Mesozoic benthic foraminifera from the former British Petroleum micropalaeontology collection|year = 2018|last1 = Fox|first1 = Lyndsey R.|last2 = Stukins|first2 = Stephen|last3 = Hill|first3 = Tom|last4 = Bailey|first4 = Haydon W.|journal = Journal of Micropalaeontology|volume = 37|issue = 1|pages = 395–401|bibcode = 2018JMicP..37..395F|hdl = 10141/622407|hdl-access = free | doi-access=free }}</ref>}}]]

[[File:Oscillatoriopsis longa fossil.jpg|thumb|upright=0.8|center| {{center|[[Cyanobacteria]]l remains of an annulated tubular microfossil ''Oscillatoriopsis longa''{{hsp}}<ref>{{cite journal |doi = 10.1111/pala.12374|title = First record of Cyanobacteria in Cambrian Orsten deposits of Sweden|year = 2018|last1 = Castellani|first1 = Christopher|last2 = Maas|first2 = Andreas|last3 = Eriksson|first3 = Mats E.|last4 = Haug|first4 = Joachim T.|last5 = Haug|first5 = Carolin|last6 = Waloszek|first6 = Dieter|journal = Palaeontology|volume = 61|issue = 6|pages = 855–880| bibcode=2018Palgy..61..855C |doi-access = free}}</ref><br /><small>Scale bar: 100 μm</small>}}]]

{{clear}}

==Ostracods==
[[File:Ostracoda hg.jpg|thumb|{{center|Ostracod microfossil}}]]

[[Ostracod]]s are widespread crustaceans, generally small, sometimes known as ''seed shrimps''. They are flattened from side to side and protected with a calcareous or chitinous [[bivalve|bivalve-like]] shell. There are about 70,000 known species, 13,000 of which are [[extant taxon|extant]].<ref>{{cite book |author1=Richard C. Brusca  |author2=Gary J. Brusca  |name-list-style=amp |title=Invertebrates |year=2003 |publisher=[[Sinauer Associates]] |edition=2nd |isbn=978-0-87893-097-5 }}</ref> Ostracods are typically about {{convert|1|mm|in|abbr=on}} in size, though they can range from {{convert|0.2|to|30|mm|in|abbr=on|3}}, with some species such as  ''[[Gigantocypris]]'' being too large to be regarded as microfossils.

{{clear}}

==Conodonts==
[[File:Conodonts from the Glen Dean formation (Chester) of the Illinois basin (1958) (20654535006).jpg|thumb| {{center|Conodont element found from the [[Cambrian]] to the end of the [[Triassic]]}}]]
{{see also|Conodont biostratigraphy}}

Conodonts (''cone tooth'' in Greek) are tiny, extinct jawless fish that resemble eels. For many years, they were known only from tooth-like microfossils found in isolation and now called conodont elements. The evolution of [[mineralized tissues]] has been a puzzle for more than a century. It has been hypothesized that the first mechanism of chordate tissue mineralization began either in the oral skeleton of conodont or the dermal skeleton of early [[agnathans]].<ref name=Shubin2009 /> Conodont elements are made of a phosphatic mineral, [[hydroxylapatite]].<ref>{{cite journal | doi = 10.1016/j.chemgeo.2006.03.004 | volume=233 | title=Chemical systematics of conodont apatite determined by laser ablation ICPMS | year=2006 | journal=Chemical Geology | pages=196–216 | last1 = Trotter | first1 = Julie A.| issue=3–4 | bibcode=2006ChGeo.233..196T }}</ref>

The element array constituted a feeding apparatus that is radically different from the jaws of modern animals. They are now termed "conodont elements" to avoid confusion. The three forms of teeth (i.e., coniform cones, ramiform bars, and pectiniform platforms) probably performed different functions. For many years, conodonts were known only from enigmatic tooth-like microfossils (200 micrometres to 5 millimetres in length) which occur commonly, but not always in isolation, and were not associated with any other fossil.<ref>{{cite web|last1=MIRACLE|title=Conodonts|url=http://www.ucl.ac.uk/GeolSci/micropal/conodont.html|access-date=26 August 2014}}</ref>

[[Conodonts]] are globally widespread in sediments.Their many forms are considered [[index fossil]]s, fossils used to define and identify geological periods and date strata. Conodonts elements can be used to estimate the temperatures rocks have been exposed to, which allows the thermal maturation levels of sedimentary rocks to be determined, which is important for [[hydrocarbon exploration]].<ref>[https://phys.org/news/2019-08-microfossils-extreme-global-environmental.html Study of microfossils maps extreme global warming and environmental change] ''Phys.org'', 7 August 2019.</ref><ref>{{cite journal |title = Recurrent biotic rebounds during the Early Triassic: Biostratigraphy and temporal size variation of conodonts from the Nanpanjiang Basin, South China|year = 2019|doi = 10.1144/jgs2019-065|last1 = Wu|first1 = Kui|last2 = Tian|first2 = Li|last3 = Liang|first3 = Lei|last4 = Metcalfe|first4 = Ian|last5 = Chu|first5 = Daoliang|last6 = Tong|first6 = Jinnan|journal = Journal of the Geological Society|volume = 176|issue = 6|pages = 1232–1246|bibcode = 2019JGSoc.176.1232W|s2cid = 202181855}}</ref>  Conodont [[tooth|teeth]] are the earliest vertebrate teeth found in the fossil record,<ref name=Shubin2009>{{cite book|last=Shubin|first=Neil|title=Your Inner Fish: A Journey into the 3.5 Billion Year History of the Human Body|year=2009|edition=reprint|publisher=Pantheon Books|location=New York|isbn=9780307277459|pages=85–86}}</ref> and some conodont teeth are the sharpest that have ever been recorded.<ref>[http://www.sci-news.com/paleontology/article00217.html Scientists Discover Sharpest Teeth in History] ''Sci-News.com'', 20 March 2012.</ref><ref>{{cite journal |title = The sharpest tools in the box? Quantitative analysis of conodont element functional morphology|year = 2012|doi = 10.1098/rspb.2012.0147|last1 = Jones|first1 = David|last2 = Evans|first2 = Alistair R.|last3 = Siu|first3 = Karen K. W.|last4 = Rayfield|first4 = Emily J.|last5 = Donoghue|first5 = Philip C. J.|journal = Proceedings of the Royal Society B: Biological Sciences|volume = 279|issue = 1739|pages = 2849–2854|pmid = 22418253|pmc = 3367778}}</ref>

{{clear}}

==Scolecodonts==
[[File:Scolecodonts from the Ordovician and Silurian.jpg|thumb|{{center|Scolecodonts<br />from the Ordovician and Silurian{{hsp}}<ref name=Hints2016 />}}]]

[[Scolecodont]]s (''worm jaws'' in Latin) are tiny jaws of [[polychaete]] [[annelid]]s of the order [[Eunicida]] - a diverse and abundant group of worms which has been inhabiting different marine environments in the past 500 million years. Composed of highly resistant organic substance, the scolecodonts are frequently found as fossils from the rocks as old as the late [[Cambrian]]. Since the worms themselves were soft-bodied and hence extremely rarely preserved in the fossil record, their jaws constitute the main evidence of polychaetes in the geological past, and the only way to restore the evolution of this important group of animals. Small size of scolecodonts, usually less than 1&nbsp;mm, puts them into a microfossil category. They are common by-product of conodont, [[chitinozoan]] and [[acritarch]] samples, but sometimes they occur in the sediments where other fossils are very rare or absent.<ref name=Hints2016>Hints, Olle (2016) [http://scolecodonts.net/ Scolecodonts — jaws of polychaete annelids] Institute of Geology at Tallinn University of Technology. [[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>

{{clear}}

==Cloudinids==
The [[cloudinid]]s were an early [[metazoan]] [[Family (biology)|family]] that lived in the late [[Ediacaran]] [[Period (geology)|period]] about 550 million years ago,<ref name="NYT-20200110">{{cite news|url=https://www.nytimes.com/2020/01/10/science/fossil-guts-intestines.html|title=Fossil Reveals Earth's Oldest Known Animal Guts - The find in a Nevada desert revealed an intestine inside a creature that looks like a worm made of a stack of ice cream cones.|last=Joel|first=Lucas|date=10 January 2020|work=[[The New York Times]]|access-date=10 January 2020}}</ref><ref name="NAT-20200110" /> and became extinct at the base of the [[Cambrian]].<ref name="Yang2016">{{cite journal|doi=10.1016/j.precamres.2016.09.016|title=Transitional Ediacaran–Cambrian small skeletal fossil assemblages from South China and Kazakhstan: Implications for chronostratigraphy and metazoan evolution|journal=Precambrian Research|volume=285|pages=202–215|year=2016|last1=Yang|first1=Ben|last2=Steiner|first2=Michael|last3=Zhu|first3=Maoyan|last4=Li|first4=Guoxiang|last5=Liu|first5=Jianni|last6=Liu|first6=Pengju|bibcode=2016PreR..285..202Y}}</ref> They formed small millimetre size conical fossils consisting of [[calcareous]] cones nested within one another; the appearance of the organism itself remains unknown. The name ''Cloudina'' honors [[Preston Cloud]].<ref name=Description>{{cite journal| doi=10.2475/ajs.272.8.752| author=Germs, G.J.B. | title=New shelly fossils from Nama Group, South West Africa| journal=American Journal of Science |date=October 1972 | volume=272 | pages=752–761| issue=8| bibcode=1972AmJS..272..752G}}</ref>  Fossils consist of a series of stacked vase-like [[calcite]] tubes, whose original mineral composition is unknown,<ref name="Porter2007">{{cite journal| journal=Science | date= 1 June 2007 |volume= 316 |issue=5829 |doi= 10.1126/science.1137284| title= Seawater Chemistry and Early Carbonate Biomineralization |author=Porter, S.M.| pages = 1302 | pmid= 17540895 | bibcode=2007Sci...316.1302P| s2cid= 27418253 }}</ref> Cloudinids comprise two genera: ''Cloudina'' itself is mineralized, whereas ''Conotubus'' is at best weakly mineralized, whilst sharing the same "funnel-in-funnel" construction.<ref>{{cite journal|doi=10.1130/G38157.1|title=The end of the Ediacaran: Two new exceptionally preserved body fossil assemblages from Mount Dunfee, Nevada, USA|journal=Geology|volume=44|issue=11|pages=911|year=2016|last1=Smith|first1=E.F.|last2=Nelson|first2=L.L.|last3=Strange|first3=M.A.|last4=Eyster|first4=A.E.|last5=Rowland|first5=S.M.|last6=Schrag|first6=D.P.|last7=MacDonald|first7=F.A.|bibcode=2016Geo....44..911S}}</ref>

Cloudinids had a wide geographic range, reflected in the present distribution of localities in which their fossils are found, and are an abundant component of some deposits. ''Cloudina'' is usually found in association with microbial [[stromatolites]], which are limited to shallow water, and it has been suggested that cloudinids lived embedded in the [[microbial mat]]s, growing new cones to avoid being buried by silt. However no specimens have been found embedded in mats, and their mode of life is still an unresolved question.

The [[Taxonomy (biology)|classification]] of the cloudinids has proved difficult: they were initially regarded as [[polychaete]] worms, and then as coral-like [[cnidarian]]s on the basis of what look like [[Budding|buds]] on some specimens. Current scientific opinion is divided between classifying them as polychaetes and regarding it as unsafe to classify them as members of any broader grouping. In 2020, a new study showed the presence of [[Nephrozoa]]n type [[Gastrointestinal tract|guts]], the oldest on record, supporting the [[bilateria]]n interpretation.<ref name="NAT-20200110">{{cite journal |author=Schiffbauer, James D. |display-authors=et al. |title=Discovery of bilaterian-type through-guts in cloudinomorphs from the terminal Ediacaran Period |date=10 January 2020 |journal=[[Nature Communications]] |volume=11 |number=205 |pages=205 |doi=10.1038/s41467-019-13882-z |pmid=31924764 |pmc=6954273 |bibcode=2020NatCo..11..205S }}</ref>

Cloudinids are important in the history of animal evolution for two reasons. They are among the earliest and most abundant of the [[small shelly fossils]] with [[Mineralization (biology)|mineralized]] [[skeleton]]s, and therefore feature in the debate about why such skeletons first appeared in the Late Ediacaran. The most widely supported answer is that their shells are a defense against predators, as some ''[[Cloudina]]'' specimens from China bear the marks of multiple attacks, which suggests they survived at least a few of them. The holes made by predators are approximately proportional to the size of the ''Cloudina'' specimens, and ''[[Sinotubulites]]'' fossils, which are often found in the same beds, have so far shown no such holes. These two points suggest that predators attacked in a selective manner, and the [[evolutionary arms race]] which this indicates is commonly cited as a cause of the [[Cambrian explosion]] of animal [[Biodiversity|diversity]] and complexity.

==Dinoflagellate cysts==
[[File:Dinocyst drawn by Ehrenberg in 1837.jpg|thumb| {{center|'''[[Dinocyst]]'''<br />as drawn by [[Christian Gottfried Ehrenberg|Ehrenberg]] in 1837}}]]
{{see also|Microbial cyst|Dinoflagellate#Dinoflagellate cysts}}

Some dinoflagellates produce [[Resting spore|resting stages]], called dinoflagellate cysts or [[dinocyst]]s, as part of their lifecycles. Dinoflagellates are mainly represented in the fossil record by these dinocysts, typically 15 to 100 micrometres in diameter, which accumulate in sediments as microfossils. Organic-walled dinocysts have resistant cell walls made out of [[dinosporin]]. There are also [[calcareous dinoflagellate cysts]] and [[siliceous dinoflagellate cysts]].

Dinocysts are produced by a proportion of [[dinoflagellate]]s as a [[Dormancy|dormant]], [[zygotic]] stage of their lifecycle. These dinocyst stages are known to occur in 84 of the 350 described freshwater dinoflagellate species, and in about 10% of the known marine species.<ref>{{cite journal | vauthors = Mertens KN, Rengefors K, Moestrup Ø, Ellegaard M |title = A review of recent freshwater dinoflagellate cysts: Taxonomy, phylogeny, ecology and palaeocology|journal = Phycologia|volume = 51|issue = 6|pages = 612–619|year = 2012 |doi = 10.2216/11-89.1 |s2cid = 86845462}}</ref><ref>{{cite journal | vauthors = Bravo I, Figueroa RI | title = Towards an Ecological Understanding of Dinoflagellate Cyst Functions | journal = Microorganisms | volume = 2 | issue = 1 | pages = 11–32 | date = January 2014 | pmid = 27694774 | pmc = 5029505 | doi = 10.3390/microorganisms2010011 | doi-access = free }}</ref> Dinocysts have a long geological record with geochemical markers suggest a presence that goes back to the [[Early Cambrian]].<ref>{{cite journal | vauthors = Moldowan JM, Talyzina NM | title = Biogeochemical evidence for dinoflagellate ancestors in the Early Cambrian | journal = Science | volume = 281 | issue = 5380 | pages = 1168–70 | date = August 1998 | pmid = 9712575 | doi = 10.1126/science.281.5380.1168 | bibcode = 1998Sci...281.1168M }}</ref>

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==Sponge spicules==
[[File:Sponge-spicule hg.jpg|thumb|300px| Six-pointed spicule from a [[siliceous]] [[glass sponge]]]]
{{main|Sponge spicule}}

[[Spicule (sponge)|Spicules]] are structural elements found in most [[Sea sponge|sponge]]s. They provide structural support and deter [[predator]]s.<ref>{{Cite journal|last1=Jones|first1=Adam C.|last2=Blum|first2=James E.|last3=Pawlik|first3=Joseph R.|date=2005-08-08|title=Testing for defensive synergy in Caribbean sponges: Bad taste or glass spicules?|url=https://www.sciencedirect.com/science/article/pii/S0022098105000882|journal=Journal of Experimental Marine Biology and Ecology|language=en|volume=322|issue=1|pages=67–81|doi=10.1016/j.jembe.2005.02.009|s2cid=85614908 |issn=0022-0981|url-access=subscription}}</ref> The meshing of many spicules serves as the sponge's [[skeleton]], providing structural support and defense against predators.

Smaller, [[microscopic]] spicules can become microfossils, and are referred to as ''microscleres''. Larger spicules visible to the naked eye are called ''megascleres''. Spicule can be [[Calcium carbonate|calcareous]], [[Biogenic silica|siliceous]], or composed of [[spongin]]. They are found in a range of symmetry types.

[[File:Demospongiae spicule diversity.png|thumb|400px|left| [[Scanning electron microscope]] images of various microscleres and megascleres of [[Demospongiae|demosponges]]]]

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==Freshwater sediments==
{{see also|Paleolimnology}}

==Marine sediments==
[[File:Distribution of sediment types on the seafloor.png|thumb|upright=1.8|right| {{center|'''Distribution of sediment types on the seafloor'''<br /> Within each colored area, the type of material shown is what dominates, although other materials are also likely to be present.<br /><small>For further information, [https://en.wikibooks.org/wiki/Historical_Geology/Marine_sediments see here]</small>}}]]
[[File:Carbonate-Silicate Cycle (Carbon Cycle focus).jpg|thumb|upright=1.8|right|{{center|[[Carbonate-silicate cycle]]}}]]

{{main|Marine sediments}}
{{further|Paleoceanography|Paleoclimatology|Marine isotope stage}}

Sediments at the bottom of the ocean have two main origins, terrigenous and biogenous.

[[Terrigenous sediment]]s account for about 45% of the total marine sediment, and originate in the erosion of [[Rock (geology)|rocks]] on land, transported by rivers and land runoff, windborne dust, volcanoes, or grinding by glaciers.

===Biogenous===
[[Biogenous sediment]]s account for the other 55% of the total sediment, and originate in the skeletal remains of [[#Marine protists|marine protists]] (single-celled plankton and benthos microorganisms). Much smaller amounts of precipitated minerals and meteoric dust can also be present. ''Ooze'', in the context of a marine sediment, does not refer to the consistency of the sediment but to its biological origin. The term ooze was originally used by [[John Murray (oceanographer)|John Murray]], the "father of modern oceanography", who proposed the term ''radiolarian ooze'' for the silica deposits of radiolarian shells brought to the surface during the [[Challenger expedition]].<ref>Thomson, Charles Wyville (2014) [https://books.google.com/books?id=zcFkAwAAQBAJ&q=radiolarian+ooze ''Voyage of the Challenger : The Atlantic''] Cambridge University Press, page235. 
{{ISBN|9781108074759}}.</ref> A ''biogenic ooze'' is a [[pelagic sediment]] containing at least 30 per cent from the skeletal remains of marine organisms.

* [[Diatomaceous earth]]
* [[Siliceous ooze]]
* [[Kerogen]]
** [[Alginite]]

===Lithified===
{{further|sedimentary rock}}

<gallery mode="packed" heights="160px" style="float:left;">
File:Coober Pedy Opal Doublet.jpg| Opal can include microfossil diatoms, radiolarians, [[silicoflagellate]]s and [[Ebriid|ebridians]]<ref name=Haq1998>Haq B.U. and Boersma A. (Eds.) (1998) [https://books.google.com/books?id=0XezCm7IwpUC&q=%22Introduction+to+Marine+Micropaleontology%22 ''Introduction to Marine Micropaleontology''] Elsevier. {{ISBN|9780080534961}}</ref>
File:MarmoCipollino FustoBasMassenzioRoma.jpg| Marble can contain microfossil foraminiferans, coccolithophores, [[calcareous nannoplankton]] and algae, [[ostracode]]s, [[pteropod]]s, calpionellids and [[bryozoa]]{{hsp}}<ref name=Haq1998 />|alt=Marble can contain microfossil foraminiferans, coccolithophores, calcareous nannoplankton and algae, ostracodes, pteropods, calpionellids and bryozoa
</gallery>

[[File:Marine sediment thickness (cropped).jpg|thumb|upright=1.8|right| {{center|Thickness of marine sediments}}]]

[[File:Ötzi the Iceman - Dagger 2.png|thumb|upright=1.6|left| Stone dagger of [[Ötzi|Ötzi the Iceman]] who lived during the [[Copper Age]]. The blade is made of [[chert]] containing radiolarians, calcispheres, calpionellids and a few sponge spicules. The presence of [[calpionellids]], which are extinct, was used to date this dagger.<ref>{{cite journal | last1 = Wierer | first1 = U. | last2 = Arrighi | first2 = S. | last3 = Bertola | first3 = S. | last4 = Kaufmann | first4 = G. | last5 = Baumgarten | first5 = B. | last6 = Pedrotti | first6 = A. | last7 = Pernter | first7 = P. | last8 = Pelegrin | first8 = J. | year = 2018 | title = The Iceman's lithic toolkit: Raw material, technology, typology and use | journal = PLOS ONE | volume = 13 | issue = 6| page = e0198292 | doi = 10.1371/journal.pone.0198292 | pmid = 29924811 | pmc = 6010222 | bibcode = 2018PLoSO..1398292W | doi-access = free }}</ref>]]

[[File:Marine-microfossils hg.jpg|thumb|upright=1|center| {{center|Some marine microfossils}}]]

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==Micropaleontology==
{{main|Micropaleontology}}

The study of microfossils is called [[micropaleontology]]. In micropaleontology, what would otherwise be distinct categories are grouped together based solely on their size, including microscopic organisms and minute parts of larger organisms. Numerous sediments have microfossils, which serve as significant [[biostratigraphic]], [[paleoenvironmental]], and paleoceanographic markers.<ref>{{cite book | last1=Armstrong | first1=Howard | last2=Brasier | first2=Martin | title=Microfossils | publication-place=New York, NY | date=2013 | isbn=978-1-118-68545-7 | oclc=904814387 | url=https://books.google.com/books?id=ULlUKMwizsAC&q=Microfossils}}</ref> Their widespread presence around the world and physical toughness makes microfossils important for biostratigraphy, while the manner in which they have reacted to environmental changes makes them helpful when reconstructing past environments.<ref>{{cite book | last=Martin | first=Ronald E. | title=Environmental Micropaleontology : the Application of Microfossils to Environmental Geology | publication-place=New York | date=2000 | isbn=978-1-4615-4167-7 | oclc=840285428 | url=https://books.google.com/books?id=QKbKBgAAQBAJ&q=Microfossils}}</ref>

==See also==
{{div col|colwidth=24em}}
* [[Biosignature]]
* [[Biostratigraphy]]
* [[Chemostratigraphy]]
* [[Gunflint microfossils]]
* [[Macrofossil]]
* [[Protists in the fossil record]]
* [[Protist shell]]
* [[Scale microfossils]]
* [[Small carbonaceous fossil]]
{{div col end}}

==References==
{{reflist}}

==Other sources==
* {{cite book | last=De Wever | first=Patrick | title=Marvelous microfossils : creators, timekeepers, architects | publication-place=Baltimore | date=2020 | isbn=978-1-4214-3674-6 | oclc=1148175375 |url=https://books.google.com/books?id=p1HaDwAAQBAJ }}

{{Commons category|Microfossils|Microfossil}}

[[Category:Microfossils]]