Editing Lichen (section)

== Taxonomy and classification ==
Lichens are classified by the fungal component. Lichen species are given the same scientific name ([[binomial name]]) as the fungus species in the lichen. Lichens are being integrated into the classification schemes for fungi. The alga bears its own scientific name, which bears no relationship to that of the lichen or fungus.<ref name="Kirk pp.378-81"/> There are about 20,000 identified lichen species,<ref name="Lücking et al. 2017">{{cite journal |last1=Lücking |first1=Robert |last2=Hodkinson |first2=Brendan P. |last3=Leavitt |first3=Steven D. |year=2017 |title=The 2016 classification of lichenized fungi in the Ascomycota and Basidiomycota–Approaching one thousand genera |journal=The Bryologist |volume=119 |issue=4 |pages=361–416 |doi=10.1639/0007-2745-119.4.361 |s2cid=90258634 |jstor=44250015}}</ref><ref name="Lücking Hodkinson Leavitt 2017">{{cite journal |last1=Lücking |first1=Robert |last2=Hodkinson | first2=Brendan P. |last3=Leavitt |first3=Steven D. |title=Corrections and amendments to the 2016 classification of lichenized fungi in the Ascomycota and Basidiomycota |journal=The Bryologist |volume=120 |issue=1 |year=2017 |doi=10.1639/0007-2745-120.1.058 |pages=58–69|s2cid=90363578 }}</ref> and taxonomists have estimated that the total number of lichen species (including those yet undiscovered) might be as high as 28,000.<ref name="Lücking et al. 2009">{{cite journal |last1=Lücking |first1=Robert |last2=Rivas-Plata |first2=E. |last3=Chavez |first3=J.L. |last4=Umaña |first4=L. |last5=Sipman |first5=H.J.M. |title=How many tropical lichens are there… really? |journal=Bibliotheca Lichenologica |year=2009 |volume=100 |pages=399–418}}</ref> Nearly 20% of known fungal species are associated with lichens.<ref name=ELWWL/>

"''Lichenized fungus''" may refer to the entire lichen, or to just the fungus. This may cause confusion without context. A particular fungus species may form lichens with different algae species, giving rise to what appear to be different lichen species, but which are still classified (as of 2014) as the same lichen species.<ref name=FSSD>{{cite web |title=Form and structure – ''Sticta'' and ''Dendriscocaulon'' |publisher=Australian National Botanic Gardens |url=https://www.anbg.gov.au/lichen/form-structure-sticta.html |access-date=18 September 2014 |archive-url=https://web.archive.org/web/20140428172500/http://www.anbg.gov.au/lichen/form-structure-sticta.html |archive-date=28 April 2014 |url-status=dead }}</ref>

Formerly, some lichen taxonomists placed lichens in their own division, the ''Mycophycophyta'', but this practice is no longer accepted because the components belong to separate [[biological classification|lineages]]. Neither the ascolichens nor the basidiolichens form [[monophyletic]] lineages in their respective fungal phyla, but they do form several major solely or primarily lichen-forming groups within each phylum.<ref name=Lutzoni_2004/> Even more unusual than basidiolichens is the fungus ''[[Geosiphon|Geosiphon pyriforme]]'', a member of the [[Glomeromycota]] that is unique in that it encloses a cyanobacterial symbiont inside its cells. ''[[Geosiphon]]'' is not usually considered to be a lichen, and its peculiar symbiosis was not recognized for many years. The genus is more closely allied to [[Mycorrhiza|endomycorrhizal]] genera. Fungi from [[Verrucariales]] also form marine lichens with the [[brown algae]] ''Petroderma maculiforme'',<ref>{{Cite journal |title=The intertidal marine lichen formed by the pyrenomycete fungus ''Verrucaria tavaresiae'' (Ascomycotina) and the brown alga ''Petroderma maculiforme'' (Phaeophyceae): thallus organization and symbiont interaction – NCBI |journal = American Journal of Botany|volume = 91|issue = 4|pages = 511–22|pmid = 21653406|year = 2004|last1 = Sanders|first1 = W. B.|last2 = Moe|first2 = R. L.|last3 = Ascaso|first3 = C.|doi = 10.3732/ajb.91.4.511|hdl = 10261/31799|hdl-access = free}}</ref> and have a symbiotic relationship with [[seaweed]] (such as [[Ascophyllum|rockweed]]) and ''[[Blidingia minima]]'', where the algae are the dominant components. The fungi is thought to help the rockweeds to resist desiccation when exposed to air.<ref>{{Cite web |url=http://website.nbm-mnb.ca/mycologywebpages/NaturalHistoryOfFungi/AlgalMutualisms.html |title=Mutualisms between fungi and algae – New Brunswick Museum |access-date=4 October 2018 |archive-url=https://web.archive.org/web/20180918084250/http://website.nbm-mnb.ca/mycologywebpages/NaturalHistoryOfFungi/AlgalMutualisms.html |archive-date=18 September 2018 |url-status=live }}</ref><ref>{{Cite journal |url=https://www.cambridge.org/core/journals/lichenologist/article/challenging-the-lichen-concept-turgidosculum-ulvae-verrucariaceae-represents-an-independent-photobiont-shift-to-a-multicellular-bladelike-alga/3F6C39EE053596364B0F5B23EE1F581C |title=Challenging the lichen concept: Turgidosculum ulvae – Cambridge |journal=The Lichenologist |volume=50 |issue=3 |pages=341–356 |access-date=7 October 2018 |archive-url=https://web.archive.org/web/20181007223526/https://www.cambridge.org/core/journals/lichenologist/article/challenging-the-lichen-concept-turgidosculum-ulvae-verrucariaceae-represents-an-independent-photobiont-shift-to-a-multicellular-bladelike-alga/3F6C39EE053596364B0F5B23EE1F581C |archive-date=7 October 2018 |url-status=live |doi=10.1017/S0024282918000117 |last1=Miller |first1=Kathy Ann |last2=Pérez-Ortega |first2=Sergio |date=May 2018 |s2cid=90117544 }}</ref> In addition, lichens can also use [[yellow-green algae]] (''[[Heterococcus]]'') as their symbiotic partner.<ref>{{Cite journal |title=Congruence of chloroplast – BMC Evolutionary Biology – BioMed Central |journal=BMC Evolutionary Biology |volume=13 |pages=39 |doi=10.1186/1471-2148-13-39 |pmid=23402662 |pmc=3598724 |year=2013 |last1=Rybalka |first1=N. |last2=Wolf |first2=M. |last3=Andersen |first3=R. A. |last4=Friedl |first4=T. |doi-access=free }}</ref>

Lichens independently emerged from fungi associating with algae and cyanobacteria multiple times throughout history.<ref>{{cite journal |last1=Lutzoni |first1=Francois |last2=Pagel |first2=Mark |last3=Reeb |first3=Valerie |title=Major fungal lineages are derived from lichen symbiotic ancestors |journal=Nature |date=21 June 2001 |volume=411 |issue=6840 |pages=937–940 |doi=10.1038/35082053|pmid=11418855 |bibcode=2001Natur.411..937L |s2cid=4414913 |url=https://www.semanticscholar.org/paper/f36413b98c70b197806bfb75444e49bf0777b1c8 }}</ref>
[[File:Lichen foliacé.jpg|thumb|Mycobiont]]

=== Fungi ===
The fungal component of a lichen is called the ''mycobiont''. The mycobiont may be an [[Ascomycete]] or [[Basidiomycete]].<ref name=UCLS/> The associated lichens are called either [[Ascomycota|ascolichens]] or [[basidiolichen]]s, respectively. Living as a [[Symbiosis|symbiont]] in a lichen appears to be a successful way for a fungus to derive essential nutrients, since about 20% of all fungal species have acquired this mode of life.<ref name=Hawksworth1988/>

Thalli produced by a given fungal symbiont with its differing partners may be similar,{{citation needed|date=September 2014}} and the secondary metabolites identical,{{citation needed|date=September 2014}} indicating{{citation needed|date=September 2014}} that the fungus has the dominant role in determining the morphology of the lichen. But the same mycobiont with different photobionts may also produce very different growth forms.<ref name=FSSD/> Lichens are known in which there is one fungus associated with two or even three algal species.

Although each lichen thallus generally appears homogeneous, some evidence seems to suggest that the fungal component may consist of more than one genetic individual of that species.{{citation needed|date=September 2014}}

Two or more fungal species can interact to form the same lichen.<ref name=Rikkinen1995/>

The following table lists the [[Order (biology)|orders]] and [[Family (biology)|families]] of fungi that include lichen-forming species.
{{Lichen family taxonomy}}

===Photobionts===
[[File:Bipartite and tripartite cyanolichens (10.3897-mycokeys.6.3869) Figure 1 lowerleft.jpg|thumb|Photobionts]]
The [[photosynthetic]] partner in a lichen is called a ''photobiont''. The photobionts in lichens come from a variety of simple [[prokaryotic]] and [[eukaryotic]] organisms. In the majority of lichens the photobiont is a green alga ([[Chlorophyta]]) or a [[Cyanobacteria|cyanobacterium]]. In some lichens both types are present; in such cases, the alga is typically the primary partner, with the cyanobacteria being located in cryptic pockets.<ref>{{cite web |last1=Luecking |first1=Robert |title=One Fungus - Two Lichens |url=https://www.fieldmuseum.org/blog/one-fungus-two-lichens |publisher=Field Museum of Natural History |access-date=25 February 2022 |date=25 February 2015}}</ref> Algal photobionts are called ''phycobionts'', while cyanobacterial photobionts are called ''cyanobionts''.<ref name=UNOLP/> About 90% of all known lichens have phycobionts, and about 10% have cyanobionts.<ref name=UNOLP/> Approximately 100 species of photosynthetic partners from 40<ref name=UNOLP/> genera and five distinct classes (prokaryotic: [[Cyanophyceae]]; eukaryotic: [[Trebouxiophyceae]], [[Phaeophyceae]], [[Chlorophyceae]]) have been found to associate with the lichen-forming fungi.<ref name=Friedl1996/>

Common [[algal]] photobionts are from the genera ''[[Trebouxia]]'', ''[[Trentepohlia (alga)|Trentepohlia]]'', ''[[Pseudotrebouxia]]'', or ''[[Myrmecia (alga)|Myrmecia]]''. ''Trebouxia'' is the most common genus of green algae in lichens, occurring in about 40% of all lichens. "Trebouxioid" means either a photobiont that is in the genus ''Trebouxia'', or resembles a member of that genus, and is therefore presumably a member of the class [[Trebouxiophyceae]].<ref name=ASLGPZ>{{cite web|url=http://www.lichens.lastdragon.org/faq/glossary3.html|title=Alan Silverside's Lichen Glossary (p-z), Alan Silverside|access-date=10 October 2014|archive-url=https://web.archive.org/web/20141031095023/http://www.lichens.lastdragon.org/faq/glossary3.html|archive-date=31 October 2014|url-status=live}}</ref> The second most commonly represented green alga genus is ''[[Trentepohlia (alga)|Trentepohlia]]''.<ref name=Dobson/> Overall, about 100 species of eukaryotes are known to occur as photobionts in lichens. All the algae are probably able to exist independently in nature as well as in the lichen.<ref name=Rikkinen1995/>

A "[[cyanolichen]]" is a lichen with a [[cyanobacteria|cyanobacterium]] as its main photosynthetic component (photobiont).<ref name=ASLGAF>{{cite web|url=http://www.lichens.lastdragon.org/faq/glossary1.html|title=Alan Silverside's Lichen Glossary (a-f), Alan Silverside|access-date=10 October 2014|archive-url=https://web.archive.org/web/20141031095019/http://www.lichens.lastdragon.org/faq/glossary1.html|archive-date=31 October 2014|url-status=live}}</ref> Most cyanolichen are also ascolichens, but a few basidiolichen like [[Dictyonema]] and [[Acantholichen]] have cyanobacteria as their partner.<ref>{{Cite book |url=https://books.google.com/books?id=AtOyDgAAQBAJ&q=The+cyanobionts+of+the+Dictyonema+and+related+basidiomycetes+Cora+Acantholichen&pg=PA255 |title=Modern Topics in the Phototrophic Prokaryotes: Environmental and Applied Aspects |access-date=4 October 2018 |archive-url=https://web.archive.org/web/20181004103837/https://books.google.no/books?id=AtOyDgAAQBAJ&pg=PA255&dq=The+cyanobionts+of+the+Dictyonema+and+related+basidiomycetes+Cora+Acantholichen&hl=no&sa=X&ved=0ahUKEwiE1LyW1-vdAhUI2qQKHQaeALAQ6AEIKDAA#v=onepage&q=The%20cyanobionts%20of%20the%20Dictyonema%20and%20related%20basidiomycetes%20Cora%20Acantholichen&f=false |archive-date=4 October 2018 |url-status=live |isbn=9783319462615 |last1=Hallenbeck |first1=Patrick C. |date=18 April 2017 |publisher=Springer }}</ref>

The most commonly occurring cyanobacterium [[genus]] is ''[[Nostoc]]''.<ref name=Rikkinen1995/> Other<ref name=Dobson/> common [[cyanobacterium]] photobionts are from ''[[Scytonema]]''.<ref name=UCLS/> Many cyanolichens are small and black, and have [[limestone]] as the substrate.{{citation needed|date=September 2014}} Another cyanolichen group, the [[jelly lichen]]s of the genera ''[[Collema]]'' or ''[[Leptogium]]'' are gelatinous and live on moist soils. Another group of large and [[foliose]] species including ''[[Peltigera]]'', ''[[Lobaria]]'', and ''[[Degelia]]'' are grey-blue, especially when dampened or wet. Many of these characterize the [[Lobarion]] communities of higher rainfall areas in western Britain, e.g., in the [[Celtic rain forest]]. Strains of cyanobacteria found in various cyanolichens are often closely related to one another.<ref name=SM/> They differ from the most closely related free-living strains.<ref name=SM>{{cite journal|title= Lichen Guilds Share Related Cyanobacterial Symbionts| doi=10.1126/science.1072961|pmid= 12130774|volume=297|issue= 5580|journal=Science|page=357|year=2002 | last1 = Rikkinen | first1 = J.| s2cid=35731669}}</ref>

The lichen association is a close symbiosis. It extends the ecological range of both partners but is not always obligatory for their growth and reproduction in natural environments, since many of the algal symbionts can live independently. A prominent example is the alga ''[[Trentepohlia (alga)|Trentepohlia]]'', which forms orange-coloured populations on tree trunks and suitable rock faces. Lichen propagules ([[Diaspore (botany)|diaspores]]) typically contain cells from both partners, although the fungal components of so-called "fringe species" rely instead on algal cells dispersed by the "core species".<ref name="RaiBergman2002"/>

The same cyanobiont species can occur in association with different fungal species as lichen partners.<ref>{{cite journal | last1 = O'Brien | first1 = H. | last2 = Miadlikowska | first2 = J. | last3 = Lutzoni | first3 = F. | year = 2005 | title = Assessing host specialization in symbiotic cyanobacteria associated with four closely related species of the lichen fungus ''Peltigera'' | journal = European Journal of Phycology | volume = 40 | issue = 4| pages = 363–378 | doi=10.1080/09670260500342647| bibcode = 2005EJPhy..40..363O | s2cid = 4094256 | doi-access = free }}</ref> The same phycobiont species can occur in association with different fungal species as lichen partners.<ref name=UNOLP/> More than one phycobiont may be present in a single thallus.<ref name=UNOLP/>

A single lichen may contain several algal [[genotype]]s.<ref>{{cite journal | last1 = Guzow-Krzeminska | first1 = B | year = 2006 | title = Photobiont ?exibility in thelichen Protoparmeliopsis muralis as revealed by ITS rDNA analyses | journal = Lichenologist | volume = 38 | issue = 5| pages = 469–476 | doi=10.1017/s0024282906005068| s2cid = 85895322 }}</ref><ref>{{cite journal | last1 = Ohmura | first1 = Y. | last2 = Kawachi | first2 = M. | last3 = Kasai | first3 = F. | last4 = Watanabe | first4 = M. | year = 2006| title = Genetic combinations of symbionts in a vegetatively reproducing lichen, ''Parmotrema tinctorum'', based on ITS rDNA sequences" (2006) | journal = Bryologist | volume = 109 | pages = 43–59 | doi=10.1639/0007-2745(2006)109[0043:gcosia]2.0.co;2| s2cid = 83701566 }}</ref> These multiple genotypes may better enable response to adaptation to environmental changes, and enable the lichen to inhabit a wider range of environments.<ref>{{cite journal | last1 = Piercey-Normore | year = 2006 | title = The lichen-forming asco-mycete ''Evernia mesomorpha'' associates with multiplegenotypes of ''Trebouxia jamesii'' | journal = New Phytologist | volume = 169 | issue = 2| pages = 331–344 | doi=10.1111/j.1469-8137.2005.01576.x | pmid=16411936| doi-access = free | bibcode = 2006NewPh.169..331P }}</ref>

===Controversy over classification method and species names===
There are about 20,000 known lichen [[species]].<ref name=UCLS>{{cite web|url=http://www.ucmp.berkeley.edu/fungi/lichens/lichensy.html|title=Lichens: Systematics, University of California Museum of Paleontology|access-date=10 October 2014|archive-url=https://web.archive.org/web/20150224172131/http://www.ucmp.berkeley.edu/fungi/lichens/lichensy.html|archive-date=24 February 2015|url-status=live}}</ref> But what is meant by "species" is different from what is meant by biological species in plants, animals, or fungi, where being the same species implies that there is a common [[ancestral lineage]].<ref name=UCLS/> Because lichens are combinations of members of two or even three different biological [[Kingdom (biology)|kingdom]]s, these components ''must'' have a ''different'' ancestral lineage from each other. By convention, lichens are still called "species" anyway, and are classified according to the species of their fungus, not the species of the algae or cyanobacteria. Lichens are given the same scientific name ([[binomial name]]) as the fungus in them, which may cause some confusion. The alga bears its own scientific name, which has no relationship to the name of the lichen or fungus.<ref name="Kirk pp.378-81"/>

Depending on context, "lichenized fungus" may refer to the entire lichen, or to the fungus when it is in the lichen, which can be grown in culture in isolation from the algae or cyanobacteria. Some algae and cyanobacteria are found naturally living outside of the lichen. The fungal, algal, or cyanobacterial component of a lichen can be grown by itself in culture. When growing by themselves, the fungus, algae, or cyanobacteria have very different properties than those of the lichen. Lichen properties such as growth form, physiology, and biochemistry, are very different from the combination of the properties of the fungus and the algae or cyanobacteria.

The same fungus growing in combination with different algae or cyanobacteria, can produce lichens that are very different in most properties, meeting non-DNA criteria for being different "species". Historically, these different combinations were classified as different species. When the fungus is identified as being the same using modern DNA methods, these apparently different species get reclassified as the ''same'' species under the current (2014) convention for classification by fungal component. This has led to debate about this classification convention. These apparently different "species" have their own independent evolutionary history.<ref name=WIL/><ref name="FSSD"/>

There is also debate as to the appropriateness of giving the same binomial name to the fungus, and to the lichen that combines that fungus with an alga or cyanobacterium ([[synecdoche]]). This is especially the case when combining the same fungus with different algae or cyanobacteria produces dramatically different lichen organisms, which would be considered different species by any measure other than the DNA of the fungal component. If the whole lichen produced by the same fungus growing in association with different algae or cyanobacteria, were to be classified as different "species", the number of "lichen species" would be greater.

===Diversity===
The largest number of lichenized fungi occur in the [[Ascomycota]], with about 40% of species forming such an association.<ref name="Kirk pp.378-81"/> Some of these lichenized fungi occur in orders with nonlichenized fungi that live as [[saprotroph]]s or [[parasite|plant parasites]] (for example, the [[Leotiales]], [[Dothideales]], and [[Pezizales]]). Other lichen fungi occur in only five [[order (biology)|orders]] in which all members are engaged in this habit (Orders [[Graphidales]], [[Gyalectales]], [[Peltigerales]], [[Pertusariales]], and [[Teloschistales]]).  Overall, about 98% of lichens have an ascomycetous mycobiont.<ref name="Lutzoni Pagel Reeb pp. 937–940">{{cite journal | last1=Lutzoni | first1=François | last2=Pagel | first2=Mark | last3=Reeb | first3=Valérie | title=Major fungal lineages are derived from lichen symbiotic ancestors | journal=Nature | volume=411 | issue=6840 | date=2001 | doi=10.1038/35082053 | pmid=11418855 | pages=937–940| bibcode=2001Natur.411..937L | s2cid=4414913 | url=https://www.semanticscholar.org/paper/f36413b98c70b197806bfb75444e49bf0777b1c8 }}</ref> Next to the Ascomycota, the largest number of lichenized fungi occur in the unassigned [[fungi imperfecti]], a catch-all category for fungi whose sexual form of reproduction has never been observed.{{citation needed|date=October 2014}} Comparatively few [[basidiomycetes]] are lichenized, but these include [[agarics]], such as species of ''[[Lichenomphalia]]'', [[clavarioid fungi]], such as species of ''[[Multiclavula]]'', and [[corticioid fungi]], such as species of ''[[Dictyonema]]''.

===Identification methods===
Lichen identification uses growth form, microscopy and reactions to chemical tests.

The outcome of the "Pd test" is called "Pd", which is also used as an abbreviation for the chemical used in the test, [[para-phenylenediamine]].<ref name=ASLGPZ/> If putting a drop on a lichen turns an area bright yellow to orange, this helps identify it as belonging to either the genus ''[[Cladonia]]'' or ''[[Lecanora]]''.<ref name=ASLGPZ/>

=== Evolution and paleontology ===
The fossil record for lichens is poor.<ref name=UCMPLFR>[http://www.ucmp.berkeley.edu/fungi/lichens/lichenfr.html "Lichens: Fossil Record"] {{Webarchive|url=https://web.archive.org/web/20100125101841/http://www.ucmp.berkeley.edu/fungi/lichens/lichenfr.html |date=25 January 2010 }}, University of California Museum of Paleontology.</ref> The extreme habitats that lichens dominate, such as tundra, mountains, and deserts, are not ordinarily conducive to producing fossils.<ref name=UCMPLFR/><ref name="urlFossil Record of Lichens"/> There are fossilized lichens embedded in amber. The fossilized ''Anzia'' is found in pieces of amber in northern Europe and dates back approximately 40 million years.<ref name=Poinar1992/> Lichen fragments are also found in fossil leaf beds, such as ''[[Lobaria]]''  from Trinity County in northern California, US, dating back to the early to middle [[Miocene]].<ref name=Peterson2000/>

The oldest fossil lichen in which both symbiotic partners have been recovered is ''[[Winfrenatia]]'', an early zygomycetous ([[Glomeromycota]]n) lichen symbiosis that may have involved controlled parasitism,{{citation needed|date=March 2016}} is permineralized in the [[Rhynie Chert]] of Scotland, dating from early [[Devonian|Early Devonian]], about 400 million years ago.<ref name=Taylor1995/> The slightly older fossil ''[[Spongiophyton]]'' has also been interpreted as a lichen on morphological<ref name=Taylor2004/> and isotopic<ref name=Jahren2003/> grounds, although the isotopic basis is decidedly shaky.<ref name=Fletcher2004/> It has been demonstrated that [[Silurian]]-[[Devonian]] fossils ''[[Nematothallus]]''<ref>{{cite journal |author1=Edwards D |author2=Axe L |year= 2012|title=Evidence for a fungal affinity for Nematasketum, a close ally of Prototaxites|journal=Botanical Journal of the Linnean Society|volume=168|pages=1–18|doi=10.1111/j.1095-8339.2011.01195.x|doi-access=free}}</ref> and ''[[Prototaxites]]''<ref>{{cite journal |author1=Retallack G.J. |author2=Landing, E.|year= 2014|title=Affinities and architecture of Devonian trunks of ''Prototaxites loganii''|journal=Mycologia|volume=106|issue=6|pages=1143–1156|doi=10.3852/13-390|pmid=24990121|s2cid=20013625}}</ref> were lichenized. Thus lichenized [[Ascomycota]] and [[Basidiomycota]] were a component of [[Llandovery epoch|Early Silurian]]-[[Devonian]] terrestrial ecosystems.<ref name=Karatygin2009/><ref name=Karatygin2007/> Newer research suggests that lichen evolved after the evolution of land plants.<ref>{{Cite web |url=https://scitechdaily.com/lichens-are-way-younger-than-scientists-thought-likely-evolved-millions-of-years-after-plants/ |title=Lichens Are Way Younger Than Scientists Thought – Likely Evolved Millions of Years After Plants |access-date=18 November 2019 |archive-url=https://web.archive.org/web/20191218121324/https://scitechdaily.com/lichens-are-way-younger-than-scientists-thought-likely-evolved-millions-of-years-after-plants/ |archive-date=18 December 2019 |url-status=live |date=15 November 2019 }}</ref>

The ancestral ecological state of both [[Ascomycota]] and [[Basidiomycota]] was probably [[saprobe|saprobism]], and independent lichenization events may have occurred multiple times.<ref>{{Cite journal|last1=Hansen|first1=Karen|last2=Perry|first2=Brian A.|last3=Dranginis|first3=Andrew W.|last4=Pfister|first4=Donald H.|date=May 2013|title=A phylogeny of the highly diverse cup-fungus family Pyronemataceae (Pezizomycetes, Ascomycota) clarifies relationships and evolution of selected life history traits|url=http://dx.doi.org/10.1016/j.ympev.2013.01.014|journal=Molecular Phylogenetics and Evolution|volume=67|issue=2|pages=311–335|doi=10.1016/j.ympev.2013.01.014|pmid=23403226|bibcode=2013MolPE..67..311H |issn=1055-7903}}</ref><ref name=Schoch2009/> In 1995, Gargas and colleagues proposed that there were at least five independent origins of lichenization; three in the basidiomycetes and at least two in the Ascomycetes.<ref name=Gargas1995/> Lutzoni et al. (2001) suggest lichenization probably evolved earlier and was followed by multiple independent losses. Some non-lichen-forming fungi may have secondarily lost the ability to form a lichen association. As a result, lichenization has been viewed as a highly successful nutritional strategy.<ref name=Honegger1998/><ref name=Wedin2004/>

Lichenized [[Glomeromycota]] may extend well back into the Precambrian. Lichen-like fossils consisting of coccoid cells ([[cyanobacteria]]?) and thin filaments (mucoromycotinan [[Glomeromycota]]?) are permineralized in marine [[phosphorite]] of the [[Doushantuo Formation]] in southern China. These fossils are thought to be 551 to 635 million years old or [[Ediacaran]].<ref name=Yuan2005/> Ediacaran [[acritarchs]] also have many similarities with [[Glomeromycota]]n vesicles and spores.<ref>{{cite journal |author=Retallack G.J. |year=2015 |title=Acritarch evidence of a late Precambrian adaptive radiation of Fungi. |journal=Botanica Pacifica |volume=4 |issue=2 |doi=10.17581/bp.2015.04203 |url=http://botsad.ru/media/aux/bp/BP_2015_4_2_retallack.pdf |pages=19–33 |access-date=22 December 2016 |archive-url=https://web.archive.org/web/20161222222005/http://botsad.ru/media/aux/bp/BP_2015_4_2_retallack.pdf |archive-date=22 December 2016 |url-status=live |doi-access=free }}</ref> It has also been claimed that [[Ediacaran biota|Ediacaran fossils]] including ''[[Dickinsonia]]'',<ref name=Retallack2007/> were lichens,<ref name=Retallack1994/> although this claim is controversial.<ref name=Switek2012/> Endosymbiotic [[Glomeromycota]] comparable with living ''[[Geosiphon]]'' may extend back into the [[Proterozoic]] in the form of 1500 million year old ''[[Horodyskia]]''<ref>{{cite journal |author1=Retallack, G.J. |author2=Dunn, K.L. |author3=Saxby, J. |year=2015 |title=Problematic Mesoproterozoic fossil Horodyskia from Glacier National Park, Montana, USA. |journal=Precambrian Research |volume=226 |pages=125–142 |doi=10.1016/j.precamres.2012.12.005|bibcode=2013PreR..226..125R }}</ref> and 2200 million year old ''[[Diskagma]]''.<ref name="Retallack-2013">{{cite journal |author1=Retallack, G.J. |author2=Krull, E.S. |author3=Thackray, G.D. |author4=Parkinson, D.|year= 2013|title=Problematic urn-shaped fossils from a Paleoproterozoic (2.2 Ga) paleosol in South Africa. |journal=Precambrian Research|volume= 235 |pages=71–87|doi=10.1016/j.precamres.2013.05.015|bibcode=2013PreR..235...71R}}</ref> Discovery of these fossils suggest that fungi developed symbiotic partnerships with photoautotrophs long before the evolution of vascular plants, though the Ediacaran lichen hypothesis is largely rejected due to an inappropriate definition of lichens based on taphonomy and substrate ecology.<ref>{{Citation|last1=Lücking|first1=Robert|title=Ediacarans, Protolichens, and Lichen-Derived Penicillium|date=2018|url=http://dx.doi.org/10.1016/b978-0-12-813012-4.00023-1|work=Transformative Paleobotany|pages=551–590|publisher=Elsevier|isbn=978-0-12-813012-4|access-date=2020-11-14|last2=Nelsen|first2=Matthew P.|doi=10.1016/b978-0-12-813012-4.00023-1}}</ref> However, a 2019 study by the same scientist who rejected the Ediacaran lichen hypothesis, Nelsen, used new time-calibrated phylogenies to conclude that there is no evidence of lichen before the existence of vascular plants.<ref>{{Cite journal |last1=Nelsen |first1=Matthew P. |last2=Lücking |first2=Robert |last3=Boyce |first3=C. Kevin |last4=Lumbsch |first4=H. Thorsten |last5=Ree |first5=Richard H. |date=January 2020 |title=No support for the emergence of lichens prior to the evolution of vascular plants |url=https://onlinelibrary.wiley.com/doi/10.1111/gbi.12369 |journal=Geobiology |language=en |volume=18 |issue=1 |pages=3–13 |doi=10.1111/gbi.12369 |pmid=31729136 |bibcode=2020Gbio...18....3N |s2cid=208034624 |issn=1472-4677}}</ref>

Lecanoromycetes, one of the most common classes of lichen-forming fungi, diverged from its ancestor, which may have also been lichen forming, around 258 million years ago, during the late Paleozoic period. However, the closely related clade Euritiomycetes appears to have become lichen-forming only 52 million years ago, during the early Cenozoic period.<ref>{{Cite journal |last1=Song |first1=Hyeunjeong |last2=Kim |first2=Ki-Tae |last3=Park |first3=Sook-Young |last4=Lee |first4=Gir-Won |last5=Choi |first5=Jaeyoung |last6=Jeon |first6=Jongbum |last7=Cheong |first7=Kyeongchae |last8=Choi |first8=Gobong |last9=Hur |first9=Jae-Seoun |last10=Lee |first10=Yong-Hwan |date=2022-06-24 |title=A comparative genomic analysis of lichen-forming fungi reveals new insights into fungal lifestyles |journal=Scientific Reports |language=en |volume=12 |issue=1 |pages=10724 |doi=10.1038/s41598-022-14340-5 |pmid=35750715 |pmc=9232553 |bibcode=2022NatSR..1210724S |issn=2045-2322}}</ref>