Editing Living fossil

{{Short description|Organism resembling a form long shown in the fossil records}}
{{Redirect|Living Fossil|the story by L. Sprague de Camp|Living Fossil (short story)}}
{{use dmy dates|cs1-dates=ly|date=November 2024}}{{EngvarB|date=January 2019}}
[[File:Coelacanth CAS 1.JPG|thumb|upright=1.35|The [[coelacanth]]s were thought to have gone extinct {{Ma|66}}, until a living specimen belonging to the [[order (biology)|order]] was discovered in 1938.]]

A ''' living fossil ''' is a [[Deprecation|deprecated]] term for an [[extant taxon]] that [[phenotypically]] resembles related [[species]] known only from the fossil record. To be considered a living fossil, the fossil species must be old relative to the time of origin of the extant [[clade]]. Living fossils commonly are of species-poor lineages, but they need not be. While the [[body plan]] of a living fossil remains superficially similar, it is never the same species as the remote relatives it resembles, because [[genetic drift]] would inevitably change its chromosomal structure.

Living fossils exhibit [[punctuated equilibrium|stasis]] (also called "bradytely") over geologically long time scales. Popular literature may wrongly claim that a "living fossil" has undergone no significant evolution since fossil times, with practically no [[molecular evolution]] or [[Morphology (biology)|morphological]] changes. Scientific investigations have repeatedly discredited such claims.<ref name="Casane">{{Cite journal|title = Why coelacanths are not 'living fossils'|journal = BioEssays|date = 2013-04-01|issn = 1521-1878|pages = 332–338|volume = 35|issue = 4|doi = 10.1002/bies.201200145|language = en|first1 = Didier|last1 = Casane|first2 = Patrick|last2 = Laurenti|pmid=23382020|s2cid = 2751255|doi-access = free}}</ref><ref>{{Cite journal|title = Multiple global radiations in tadpole shrimps challenge the concept of 'living fossils'|journal = PeerJ|pmc = 3628881|pmid = 23638400|volume = 1|doi = 10.7717/peerj.62|first1 = Thomas C.|last1 = Mathers|first2 = Robert L.|last2 = Hammond|first3 = Ronald A.|last3 = Jenner|first4 = Bernd|last4 = Hänfling|first5 = Africa|last5 = Gómez|pages=e62|year = 2013 | doi-access=free }}</ref><ref>{{Cite journal|title = Relict species: a relict concept?|journal = Trends in Ecology & Evolution|date = 2014-01-12|issn = 0169-5347|pmid = 25454211|pages = 655–663|volume = 29|issue = 12|doi = 10.1016/j.tree.2014.10.002|language = en|first1 = Philippe|last1 = Grandcolas|first2 = Romain|last2 = Nattier|first3 = Steve|last3 = Trewick| bibcode=2014TEcoE..29..655G }}</ref>

The minimal superficial changes to living fossils are mistakenly declared as an absence of evolution, but they are examples of [[stabilizing selection]], which is an [[evolutionary process]]—and perhaps the dominant process of [[morphological evolution]].<ref>{{cite journal|last1=Lynch|first1=M|title=The rate of evolution in mammals from the standpoint of the neutral expectation|journal=The American Naturalist|date=1990|volume=136|issue=6|pages=727–741|doi=10.1086/285128|s2cid=11055926}}</ref>

The term is currently deprecated among paleontologists and evolutionary biologists.

==Characteristics==
{{multiple image
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| caption1 = 170 million-year-old fossil ''Ginkgo'' leaves
| caption2 = Living ''[[Ginkgo biloba]]'' plant
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Living fossils have two main characteristics, although some have a third:
# Living organisms that are members of a [[taxon]] that has remained  recognizable in the fossil record over an unusually long time span.
# They show little morphological divergence, whether from early members of the lineage, or among [[Extant taxon|extant species]]. 
# They tend to have little taxonomic diversity.<ref>{{cite book |last1=Eldridge |first1=Niles |last2=Stanley |first2=Steven |title=Living Fossils |date=1984 |publisher=Springer-Verlag |location=New York}}</ref>
The first two are required for recognition as a living fossil; some authors also require the third, others merely note it as a frequent trait.

Such criteria are neither well-defined nor clearly quantifiable, but modern methods for analyzing evolutionary dynamics can document the distinctive tempo of stasis.<ref>{{cite journal |last1=Butler |first1=M. |last2=King |first2=A. |title=Phylogenetic comparative analysis: A modeling approach for adaptive evolution |journal=The American Naturalist |date=2004 |volume=164 |issue=6 |pages=683–695 |doi=10.1086/426002 |pmid=29641928|s2cid=4795316 }}</ref><ref>{{cite journal |last1=Hansen |first1=T. |last2=Martins |first2=E. |title=Translating between microevolutionary process and macroevolutionary patterns: The correlation structure of interspecific data |journal=Evolution |date=1996 |volume=50 |issue=4 |pages=1404–1417 |doi=10.2307/2410878 |pmid=28565714 |jstor=2410878}}</ref><ref>{{cite journal |last1=Harmon |first1=L. |last2=Losos |first2=J. |last3=Davies |first3=T. |last4=Gillespie |first4=R. |last5=Gittleman |first5=J. |last6=Jennings |first6=W. |last7=Kozak |first7=K. |last8=McPeek |first8=M. |last9=Moreno-Roark |first9=F. |last10=Near |first10=T. |last11=Purvis |first11=A. |last12=Ricklefs |first12=R. |last13=Schluter |first13=D. |last14=Schulte |first14=J. |last15=Seehausen |first15=O. |last16=Sidlauskas |first16=B. |last17=Torres-Carvajal |first17=O. |last18=Weir |first18=J. |last19=Mooers |first19=A |display-authors=6 |title=Early bursts of body size and shape evolution are rare in comparative data |journal=Evolution |year=2010 |volume=64 |issue=8 |pages=2385–2396 |doi=10.1111/j.1558-5646.2010.01025.x |pmid=20455932|s2cid=17544335 }}</ref> Lineages that exhibit stasis over very short time scales are not considered living fossils; what is poorly-defined is the time scale over which the morphology must persist for that lineage to be recognized as a living fossil.

The term ''living fossil'' is much misunderstood in popular media in particular, in which it often is used meaninglessly. In professional literature the expression seldom appears and must be used with far more caution, although it has been used inconsistently.<ref>{{cite journal |vauthors=Nagalingum NS, Marshall CR, Quental TB, Rai HS, Little DP, Mathews S |title=Recent synchronous radiation of a living fossil |journal=Science |date=2011-11-11 |volume=334 |issue=6057 |pages=796–799 |doi=10.1126/science.1209926 |pmid=22021670 |bibcode=2011Sci...334..796N |s2cid=206535984 |publication-date=2011-10-20 <!-- e-Pub -->}}</ref><ref>{{cite report |author1=Cavin, Lionel |author2=Guinot, Guillaume |title=Coelacanths as "almost living fossils" |series=Perspective Article |date=13 August 2014 |doi=10.3389/fevo.2014.00049 |publisher=Département de Géologie et Paléontologie |department=Muséum d'Histoire Naturelle |location=Genève, Switzerland|doi-access=free }}</ref>

One example of a concept that could be confused with "living fossil" is that of a "[[Lazarus taxon]]", but the two are not equivalent; a Lazarus taxon (whether a single species or a [[Systematics|group of related species]]) is one that suddenly reappears, either in the fossil record or in nature, as if the fossil had "come to life again".<ref name=":2">{{cite journal |vauthors=Dawson MR, Marivaux L, Li CK, Beard KC, Métais G |title=Laonastes and the "Lazarus effect" in recent mammals |journal=Science |date=2006-03-10 |volume=311 |issue=5766 |pages=1456–1458|doi=10.1126/science.1124187 |pmid=16527978 |bibcode=2006Sci...311.1456D |s2cid=25506765}}</ref> In contrast to "Lazarus taxa", a living fossil in most senses is a species or lineage that has undergone exceptionally little change throughout a long fossil record, giving the impression that the extant taxon had remained identical through the entire fossil and modern period. Because of the mathematical inevitability of [[genetic drift]], though, the DNA of the modern species is necessarily different from that of its distant, similar-looking ancestor. They almost certainly would not be able to cross-reproduce, and are not the same species.<ref>{{cite web |date=6 Jul 2016 |url=https://www.theguardian.com/science/2016/jul/06/why-its-time-to-make-living-fossils-extinct |author=Mark Carnall |title=Let's make living fossils extinct |work=The Guardian}}</ref>

The average [[Beta diversity#Absolute species turnover|species turnover]] time, meaning the time between when a species first is established and when it finally disappears, varies widely among [[phylum (biology)|phyla]], but averages about 2–3{{nbsp}}million years.<ref>{{Cite book |last=Lawton |first=John H. |url=https://archive.org/details/extinctionrates00lawt/mode/2up?q=million |title=Extinction rates |last2=May |first2=Robert M. (Robert McCredie) |date=1995 |publisher=Oxford; New York : Oxford University Press |others=Internet Archive |isbn=978-0-19-854829-4}}</ref> A living taxon that had long been thought to be extinct could be called a Lazarus taxon once it was discovered to be still extant. A dramatic example was the order [[Coelacanthiformes]], of which the genus ''[[Latimeria]]'' was found to be extant in 1938. About that there is little debate – however, whether ''Latimeria'' resembles early members of its lineage sufficiently closely to be considered a living fossil as well as a Lazarus taxon has been denied by some authors in recent years.<ref name="Casane"/>

Coelacanths disappeared from the fossil record some 80{{nbsp}}million years ago (in the upper [[Cretaceous]] period) and, to the extent that they exhibit low rates of morphological evolution, extant species qualify as living fossils. It must be emphasised that this criterion reflects fossil evidence, and is totally independent of whether the taxa had been subject to selection at all, which all living populations continuously are, whether they remain genetically unchanged or not.<ref name="Yadav2009"/>

This apparent stasis, in turn, gives rise to a great deal of confusion – for one thing, the fossil record seldom preserves much more than the general morphology of a specimen. To determine much about its physiology is seldom possible; not even the most dramatic examples of living fossils can be expected to be without changes, no matter how persistently constant their fossils and the extant specimens might seem. To determine much about [[noncoding DNA]] is hardly ever possible, but even if a species were hypothetically unchanged in its physiology, it is to be expected from the very nature of the reproductive processes, that its non-functional [[genomics|genomic]] changes would continue at more-or-less standard rates. Hence, a fossil lineage with apparently constant morphology need not imply equally constant physiology, and certainly neither implies any cessation of the basic evolutionary processes such as natural selection, nor reduction in the usual rate of change of the noncoding DNA.<ref name="Yadav2009">{{cite book |author=Yadav, P.R. |title=Understanding Palaeontology |date=1 January 2009 |publisher=Discovery Publishing House |isbn=978-81-8356-477-9 |pages=4 ff |url=https://books.google.com/books?id=zAWNWNWcQ08C&pg=PP4}}</ref>

Some living fossils are taxa that were known from palaeontological fossils before living representatives were discovered. The most famous examples of this are:
* [[Coelacanth]]iform fishes (2 species)
* ''Metasequoia'', the [[dawn redwood]] discovered in a remote Chinese valley (1 species)
* [[Glypheoidea|Glypheoid lobsters]] (2 species)
* [[Mymarommatidae|Mymarommatid wasps]] (10 species)
* [[Eomeropidae|Eomeropid scorpionflies]] (1 species)
* [[Jurodidae|Jurodid beetles]] (1 species)
* [[Echinothurioida|Soft sea urchins]] (59 species)

All the above include taxa that originally were described as fossils but now are known to include still-extant species.

Other examples of living fossils are single living species that have no close living relatives, but are survivors of large and widespread groups in the fossil record. For example:
* ''[[Ginkgo biloba]]''
* ''[[Syntexis libocedrii]]'', the cedar wood wasp
* [[Dinoflagellates]] (typified on coccoid [[dinocysts]]: occasionally calcareous cell remnants)

All of these were described from fossils before later being found alive.<ref name="Caloperosum">{{cite journal |last1=Montresor |first1=M. |last2=Janofske |first2=D. |last3=Willems |first3=H. |year=1997 |title=The cyst-theca relationship in ''Calciodinellum operosum'' emend. (Peridiniales, Dinophyceae) and a new approach for the study of calcareous cysts |journal=Journal of Phycology |volume=33 |issue=1 |pages=122–131 |doi=10.1111/j.0022-3646.1997.00122.x|bibcode=1997JPcgy..33..122M |s2cid=84169394 }}</ref><ref name="Postricari">{{cite journal |last1=Gu |first1=H. |last2=Kirsch |first2=M. |last3=Zinßmeister |first3=C. |last4=Söhner |first4=S. |last5=Meier |first5=K.J.S. |last6=Liu |first6=T. |last7=Gottschling |first7=M. |year=2013 |title=Waking the dead: Morphological and molecular characterization of extant †''Posoniella tricarinelloides'' (Thoracosphaeraceae, Dinophyceae) |journal=Protist |volume=164 |issue=5 |pages=583–597 |doi=10.1016/j.protis.2013.06.001 |pmid=23850812}}</ref><ref name="dx.doi.org">{{cite journal |last1=Mertens |first1=K.N. |last2=Takano |first2=Y. |last3=Head |first3=M.J. |last4=Matsuoka |first4=K. |year=2014 |title=Living fossils in the Indo-Pacific warm pool: A refuge for thermophilic dinoflagellates during glaciations |journal=Geology |volume=42 |issue=6 |pages=531–534 |doi=10.1130/G35456.1|bibcode=2014Geo....42..531M }}</ref>

The fact that a living fossil is a surviving representative of an archaic lineage does not imply that it must retain all the "primitive" features ([[Plesiomorphy|plesiomorphies]]) of its ancestral lineage. Although it is common to say that living fossils exhibit "morphological stasis", stasis, in the scientific literature, does not mean that any species is strictly identical to its ancestor, much less remote ancestors.

Some living fossils are relicts of formerly diverse and morphologically varied lineages, but not all survivors of ancient lineages necessarily are regarded as living fossils. See for example the uniquely and highly autapomorphic [[oxpecker]]s, which appear to be the only survivors of an ancient lineage related to [[starling]]s and [[mockingbird]]s.<ref name="Zuccon">{{cite journal |author1=Zuccon, Dario |author2=Cibois, Anne |author3=Pasquet, Eric |author4=Ericson, Per G.P. |year=2006 |title=Nuclear and mitochondrial sequence data reveal the major lineages of starlings, mynas and related taxa |journal=[[Molecular Phylogenetics and Evolution]] |volume=41 |issue=2 |pages=333–344 |doi=10.1016/j.ympev.2006.05.007 |pmid=16806992 |bibcode=2006MolPE..41..333Z |url=http://www.nrm.se/download/18.25ba04a21296cc434f980001039/Zuccon+et+al+2006+Starlings+MPEV.pdf |access-date=2011-02-20 |archive-date=2021-10-25 |archive-url=https://web.archive.org/web/20211025143403/https://www.nrm.se/download/18.25ba04a21296cc434f980001039/Zuccon+et+al+2006+Starlings+MPEV.pdf |url-status=dead }}</ref>

== Evolution and living fossils ==
The term ''living fossil'' is usually reserved for species or larger clades that are exceptional for their lack of morphological diversity and their exceptional conservatism, and several hypotheses could explain morphological stasis on a geologically long time-scale. Early analyses of evolutionary rates emphasized the persistence of a taxon rather than rates of evolutionary change.<ref>{{cite book |last1=Simpson |first1=George |title=The Major Features of Evolution |publisher=Columbia University Press |url=https://archive.org/details/majorfeaturesofe0000simp |url-access=registration |date=1953 |location=New York}}</ref> Contemporary studies instead analyze rates and modes of phenotypic evolution, but most have focused on clades that are thought to be adaptive radiations rather than on those thought to be living fossils. Thus, very little is presently known about the evolutionary mechanisms that produce living fossils or how common they might be. Some recent studies have documented exceptionally low rates of ecological and phenotypic evolution despite rapid speciation.<ref>{{cite journal |last1=Kozack |first1=K. |last2=Weisrock |first2=D. W. |last3=Larson |first3=A. |title=Rapid lineage accumulation in a non-adaptive radiation: phylogenetic analysis of diversification rates in eastern North American woodland salamanders (Plethodontidae: Plethodon) |journal=Proceedings of the Royal Society B: Biological Sciences |date=2006 |volume=273 |issue=1586 |pages=539–546 |doi=10.1098/rspb.2005.3326 |pmid=16537124 |pmc=1560065}}</ref> This has been termed a "non-adaptive radiation" referring to diversification not accompanied by adaptation into various significantly different niches.<ref>{{cite journal |last=Gittenberger |first=E. |title=What about non-adaptive radiation? |journal=Biological Journal of the Linnean Society |date=1991 |volume=43 |issue=4 |pages=263–272 |doi=10.1111/j.1095-8312.1991.tb00598.x}}</ref> Such radiations are explanation for groups that are morphologically conservative. Persistent adaptation within an adaptive zone is a common explanation for morphological stasis.<ref>{{cite journal |last1=Estes |first1=Suzanne |last2=Arnold |first2=Stevan |title=Resolving the paradox of stasis: Models with stabilizing selection explain evolutionary divergence on all timescales |journal=The American Naturalist |date=2007 |volume=169 |issue=2 |pages=227–244 |doi=10.1086/510633 |pmid=17211806 |s2cid=18734233}}</ref> The subject of very low evolutionary rates, however, has received much less attention in the recent literature than that of high rates.

Living fossils are not expected to exhibit exceptionally low rates of molecular evolution, and some studies have shown that they do not.<ref>{{cite web |title=Diversification in Ancient Tadpole Shrimps Challenges the Term 'Living Fossil' |work=Science Daily |url=https://www.sciencedaily.com/releases/2013/04/130402091641.htm |access-date=2 April 2013 |date=2 April 2013}}</ref><ref name=Falsity>{{Cite web |author=Ed Yong |date=Apr 2, 2013 |title=The Falsity of Living Fossils |url=http://www.the-scientist.com/?articles.view/articleNo/34927/title/The-Falsity-of-Living-Fossils/ |website=The Scientist |access-date=2015-12-03 |url-access=registration}}</ref> For example, on tadpole shrimp (''[[Triops]]''), one article notes, "Our work shows that organisms with conservative body plans are constantly radiating, and presumably, adapting to novel conditions... I would favor retiring the term 'living fossil' altogether, as it is generally misleading."<ref name=Falsity/> Some scientists instead prefer a new term stabilomorph, being defined as "an effect of a specific formula of adaptative strategy among organisms whose taxonomic status does not exceed genus-level. A high effectiveness of adaptation significantly reduces the need for differentiated phenotypic variants in response to environmental changes and provides for long-term evolutionary success."<ref>{{Cite journal |last1=Kin |first1=Adrian |last2=Błażejowski |first2=Błażej |date=2014-10-02 |title=The Horseshoe Crab of the Genus Limulus: Living Fossil or Stabilomorph? |journal=PLOS ONE |language=en |volume=9 |issue=10 |at=e108036 |doi=10.1371/journal.pone.0108036 |issn=1932-6203 |pmc=4183490 |pmid=25275563 |bibcode=2014PLoSO...9j8036K |doi-access=free}}</ref>

The question posed by several recent studies pointed out that the morphological conservatism of coelacanths is not supported by paleontological data.<ref>{{cite journal | vauthors = Friedman M, Coates MI, Anderson P | year = 2007 | title = First discovery of a primitive coelacanth fin fills a major gap in the evolution of lobed fins and limbs | journal = Evolution & Development | volume = 9 | issue = 4| pages = 329–37 | doi = 10.1111/j.1525-142X.2007.00169.x | pmid=17651357| s2cid = 23069133 }}</ref><ref>{{cite journal | vauthors = Friedman M, Coates MI | year = 2006 | title = A newly recognized fossil coelacanth highlights the early morphological diversification of the clade | journal = Proc. R. Soc. B | volume = 273 | issue = 1583| pages = 245–250 | doi = 10.1098/rspb.2005.3316 | pmid=16555794 | pmc=1560029}}</ref> In addition, it was shown recently that studies concluding that a slow rate of molecular evolution is linked to morphological conservatism in coelacanths are biased by the ''[[A priori and a posteriori|a priori]]'' hypothesis that these species are 'living fossils'.<ref name="Casane"/> Accordingly, the genome stasis hypothesis is challenged by the recent finding that the genome of the two extant coelacanth species ''L. chalumnae'' and ''L. menadoensis'' contain multiple species-specific insertions, indicating transposable element recent activity and contribution to post-speciation genome divergence.<ref name="Naville">{{cite journal | vauthors = Naville M, Chalopin D, Casane D, Laurenti P, Volff JN |date=July–August 2015 | title = The coelacanth: Can a "living fossil" have active transposable elements in its genome? | journal = Mobile Genetic Elements | volume = 5| issue = 4| pages = 55–9| doi = 10.1080/2159256X.2015.1052184 | pmid=26442185| pmc=4588170}}</ref> Such studies, however, challenge only a genome stasis hypothesis, not the hypothesis of exceptionally low rates of phenotypic evolution.

==History==
The term was coined by [[Charles Darwin]] in his ''[[On the Origin of Species]]'' from 1859, when discussing ''[[Ornithorhynchus]]'' (the platypus) and ''[[Lepidosiren]]'' (the South American lungfish):

{{Blockquote|All fresh-water basins, taken together, make a small area compared with that of the sea or of the land; and, consequently, the competition between fresh-water productions will have been less severe than elsewhere; new forms will have been more slowly formed, and old forms more slowly exterminated. And it is in fresh water that we find seven genera of Ganoid fishes, remnants of a once preponderant order: and in fresh water we find some of the most anomalous forms now known in the world, as the ''Ornithorhynchus'' and ''Lepidosiren'', which, like fossils, connect to a certain extent orders now widely separated in the natural scale. These anomalous forms may almost be called living fossils; they have endured to the present day, from having inhabited a confined area, and from having thus been exposed to less severe competition.|''[[On the Origin of Species]]'', 1859<ref>''[[On the Origin of Species]]'', 1859, [http://darwin-online.org.uk/content/frameset?pageseq=122&itemID=F373&viewtype=text p. 107].</ref>}}

===Other definitions===

==== Long-enduring ====
[[File:Rhynchocyon petersi from side.jpg|thumb|[[Elephant shrew]]s resemble the extinct ''[[Leptictidium]]'' of [[Eocene]] Europe.]]
A living taxon that lived through a large portion of [[geologic time]].

The [[Australian lungfish]] (''Neoceratodus fosteri''), also known as the Queensland lungfish, is an example of an organism that meets this criterion. Fossils identical to modern specimens have been dated at over 100{{nbsp}}million years old. Modern Queensland lungfish have existed as a species for almost 30{{nbsp}}million years.<ref>{{Cite web |title=Ann Kemps Lungfish - Queensland - Australia |url=http://annekempslungfish.com.au/lungfish_habitat.html |access-date=2025-01-18 |website=annekempslungfish.com.au}}</ref> The contemporary [[nurse shark]] has existed for more than 112{{nbsp}}million years, making this species one of the oldest, if not actually the oldest extant vertebrate species.

==== Resembles ancient species ====
A living taxon [[Morphology (biology)|morphologically]] and/or [[Physiology|physiologically]] resembling a fossil taxon through a large portion of geologic time (morphological stasis).<ref>{{cite web|url=http://www.uchospitals.edu/news/2006/20061026-lamprey.html |title=The University of Chicago Medical Center: ''Scientists find lamprey a 'living fossil' '' |publisher=Uchospitals.edu |date=2006-10-26 |access-date=2012-05-16}}</ref>

==== Retains many ancient traits ====
[[File:Liphistius malayanus 44930086.jpg|thumb|More primitive [[trapdoor spider]]s, such as this ''[[Liphistius]] malayanus'', have segmented plates on the dorsal surface of the abdomen and cephalothorax, a character shared with scorpions, making it probable that after the spiders diverged from the scorpions, the earliest unique ancestor of trapdoor species was the first to split off from the lineage that contains all other [[Extant taxon|extant]] spiders.]]

A living taxon with many characteristics believed to be primitive. This is a more neutral definition. However, it does not make it clear whether the taxon is truly old, or it simply has many plesiomorphies. Note that, as mentioned above, the converse may hold for true living fossil taxa; that is, they may possess a great many derived features ([[autapomorphy|autapomorphies]]), and not be particularly "primitive" in appearance.

==== Relict population ====
Any one of the above three definitions, but also with a [[relict distribution]] in [[Refugium (population biology)|refuges]].

Some paleontologists believe that living fossils with large distributions (such as ''[[Triops cancriformis]]'') are not real living fossils. In the case of ''Triops cancriformis'' (living from the [[Triassic]] until now), the Triassic specimens lost most of their appendages (mostly only [[carapace]]s remain), and they have not been thoroughly examined since 1938.

==== Low diversity ====
Any of the first three definitions, but the [[clade]] also has a low taxonomic diversity (low diversity lineages).

[[Oxpecker]]s are [[morphology (biology)|morphologically]] somewhat similar to [[starlings]] due to shared plesiomorphies, but are uniquely adapted to feed on parasites and blood of large land mammals, which has always obscured their relationships. This lineage forms part of a radiation that includes [[Sturnidae]] and [[Mimidae]], but appears to be the most ancient of these groups. [[Biogeography]] strongly suggests that oxpeckers originated in eastern [[Asia]] and only later arrived in Africa, where they now have a relict distribution.<ref name="Zuccon"/>

The two living species thus seem to represent an entirely extinct and (as [[Passerida]] go) rather ancient lineage, as certainly as this can be said in the absence of actual fossils. The latter is probably due to the fact that the oxpecker lineage never occurred in areas where conditions were good for fossilization of small bird bones, but of course, fossils of ancestral oxpeckers may one day turn up enabling this theory to be tested.

=== Operational definition ===
An operational definition was proposed in 2017, where a 'living fossil' lineage has a slow rate of evolution and occurs close to the middle of morphological variation (the centroid of morphospace) among related taxa (i.e. a species is morphologically conservative among relatives).<ref name="Herrera-Flores 2017">{{cite journal |doi=10.1111/pala.12284 |title=Macroevolutionary patterns in Rhynchocephalia: is the tuatara (Sphenodon punctatus) a living fossil? |journal=Palaeontology|volume=60 |issue=3 |pages=319–328 |year=2017 |last1=Herrera-Flores |first1=Jorge A. |last2=Stubbs |first2=Thomas L. |last3=Benton |first3=Michael J. |doi-access=free |bibcode=2017Palgy..60..319H }}</ref> The scientific accuracy of the [[morphometric]] analyses used to classify [[tuatara]] as a living fossil under this definition have been criticised however,<ref name="Vaux 2019">{{cite journal |doi=10.1111/pala.12402 |title=Tuatara and a new morphometric dataset for Rhynchocephalia: Comments on Herrera-Flores et al. |journal=Palaeontology|volume=62 |issue=2 |pages=321–334 |year=2019 |last1=Vaux |first1=Felix |last2=Morgan-Richards |first2=Mary |last3=Daly |first3=Elizabeth E. |last4=Trewick |first4=Steven A.|bibcode=2019Palgy..62..321V |s2cid=134902015 }}</ref> which prompted a rebuttal from the original authors.<ref name="Herrera-Flores 2019">{{cite journal |doi=10.1111/pala.12404 |title=Reply to comments on: Macroevolutionary patterns in Rhynchocephalia: is the tuatara (Sphenodon punctatus) a living fossil? |journal=Palaeontology|volume=62 |issue=2 |pages=335–338 |year=2019 |last1=Herrera-Flores |first1=Jorge A. |last2=Stubbs |first2=Thomas L. |last3=Benton |first3=Michael J. |bibcode=2019Palgy..62..335H |hdl=1983/846d212a-6eb6-494e-855f-e0684bede158 |s2cid=133726749 |url=https://research-information.bris.ac.uk/ws/files/198134843/Full_text_PDF_accepted_author_manuscript_.pdf }}</ref>

==Examples==
Some of these are informally known as "living fossils".
[[File:Ginkgo Stamm.jpg|thumb|upright|[[Ginkgo]]s not only have existed for a long time, but also have a long life span, with some having an age of over 2,500 years. Six specimens survived the [[atomic bombings of Hiroshima and Nagasaki|atomic bombing of Hiroshima]], 1 to 2 [[kilometer]]s from [[ground zero]]. They still live there today.]]
[[File:OsmundaRegalis.jpg|thumb|Ferns were the [[Dominance (ecology)|dominant]] plant group in the [[Jurassic]] period, with some species, such as ''[[Osmunda claytoniana]]'', maintaining [[Punctuated equilibrium|evolutionary stasis]] for at least 180 million years.<ref name="pmid24653037">{{cite journal |vauthors=Bomfleur B, McLoughlin S, Vajda V |title=Fossilized nuclei and chromosomes reveal 180 million years of genomic stasis in royal ferns |journal=Science |volume=343 |issue=6177 |pages=1376–7 |date=March 2014 |pmid=24653037 |doi=10.1126/science.1249884 |bibcode=2014Sci...343.1376B |s2cid=38248823 |url=http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-315}}</ref><ref name="Palaeos website">{{cite web|last=Kazlev|first=M. Alan|year=2002|url=http://www.palaeos.com/Mesozoic/Jurassic/Jurassic.htm|title=Palaeos website|archive-url=https://web.archive.org/web/20060105125654/http://www.palaeos.com/Mesozoic/Jurassic/Jurassic.htm |archive-date=2006-01-05|access-date=July 22, 2008}}</ref>]]

===Bacteria===
* [[Cyanobacteria]] – the oldest living fossils, emerging 3.5 billion years ago. They exist as single bacteria or in the form of [[stromatolite]]s, layered rocks produced by colonies of cyanobacteria.<ref>{{Cite web |url=http://ircamera.as.arizona.edu/NatSci102/NatSci/text/extfirstlife.htm|title=cyanobacteria|website=ircamera.as.arizona.edu|access-date=2019-04-27|archive-url=https://web.archive.org/web/20190503162408/http://ircamera.as.arizona.edu/NatSci102/NatSci/text/extfirstlife.htm|archive-date=2019-05-03}}</ref>

===Protists===
* The [[dinoflagellate]] †''Calciodinellum operosum''.<ref name="Caloperosum"/>
* The [[dinoflagellate]] †''Dapsilidinium pastielsii''.<ref name="dx.doi.org"/>
* The [[dinoflagellate]] †''Posoniella tricarinelloides''.<ref name="Postricari"/>
* The [[coccolithophore]] ''Tergestiella adriatica''.<ref name="Hagino et al. 2015">{{cite journal|last1=Hagino|first1=K. |last2=Young|first2=J. R.|last3=Bown|first3=P. R.|last4=Godrijan |first4=J.|last5=Kulhanek|first5=D.|last6=Kogane|first6=K. |last7=Horiguchi|first7=T.|title=Re-discovery of a "living fossil" coccolithophore from the coastal waters of Japan and Croatia |journal=Marine Micropaleontology |date=2015 |volume=116 |issue=1 |pages=28–37 |doi=10.1016/j.marmicro.2015.01.002 |bibcode=2015MarMP.116...28H |url=http://discovery.ucl.ac.uk/1462916/}}</ref>

===Plants===
* [[Moss]]
*'''[[Pteridophytes]]'''
** [[Horsetail]]s – ''Equisetum''
** [[Lycopodiophyta|Lycopods]] 
** [[Tree fern]]s and [[ferns]] 
*'''[[Gymnosperm]]s'''
** [[Conifers]]
***''[[Agathis]]'' – kauri in New Zealand, Australia and the Pacific and almasiga in the Philippines
*** ''[[Araucaria araucana]]'' – the monkey puzzle tree (as well as other extant ''[[Araucaria]]'' species)
*** ''[[Metasequoia]]'' – dawn redwood (Cupressaceae; related to ''[[Sequoia (genus)|Sequoia]]'' and ''[[Sequoiadendron]]'')
*** ''[[Sciadopitys]]'' – a unique conifer endemic to Japan known in the fossil record for about 230 million years.
*** ''[[Taiwania|Taiwania cryptomerioides]]'' – one of the largest tree species in Asia.
*** ''[[Wollemia]]'' tree (''[[Araucariaceae]]'' – a borderline example, related to ''[[Agathis]]'' and ''[[Araucaria]]'')<ref>{{cite journal | last1 = Chambers | first1 = T.C. | last2 = Drinnan | first2 = A.N. | last3 = McLoughlin | first3 = S. | year = 1998 | title = Some morphological features of Wollemi Pine (Wollemia nobilis: Araucariaceae) and their comparison to Cretaceous plant fossils | journal = International Journal of Plant Sciences | volume = 159 | pages = 160–171 | doi=10.1086/297534| s2cid = 84425685 }}</ref><ref>{{cite journal | author = McLoughlin S., Vajda V. | year = 2005 | title = Ancient wollemi pines resurgent | journal = American Scientist | volume = 93 | issue = 6| pages = 540–547 | doi=10.1511/2005.56.981| last2 = Vajda }}</ref>
** [[Cycad]]s – although this has been challenged by multiple lines of evidence<ref>{{Cite journal |last1=Nagalingum |first1=N. S. |last2=Marshall |first2=C. R. |last3=Quental |first3=T. B. |last4=Rai |first4=H. S. |last5=Little |first5=D. P. |last6=Mathews |first6=S. |date=2011-11-11 |title=Recent Synchronous Radiation of a Living Fossil |url=https://www.science.org/doi/10.1126/science.1209926 |journal=Science |language=en |volume=334 |issue=6057 |pages=796–799 |doi=10.1126/science.1209926 |pmid=22021670 |bibcode=2011Sci...334..796N |issn=0036-8075|url-access=subscription }}</ref><ref>{{Cite journal |last1=Coiro |first1=Mario |last2=Seyfullah |first2=Leyla Jean |date=2024-03-14 |title=Disparity of cycad leaves dispels the living fossil metaphor |journal=Communications Biology |language=en |volume=7 |issue=1 |page=328 |doi=10.1038/s42003-024-06024-9 |issn=2399-3642 |pmc=10940627 |pmid=38485767}}</ref> 
** ''[[Ginkgo biloba|Ginkgo]]'' tree (Ginkgoaceae)
** ''[[Welwitschia]]''
*'''[[Angiosperm]]s'''
** ''[[Amborella]]'' – a plant from New Caledonia, possibly closest to base of the [[angiosperms|flowering plants]]
** ''[[Magnolia]] –'' a genus whose form is little changed since the earliest days of flowering plant evolution in the Cretaceous and possibly earlier<ref>{{Cite web |last=Vallejo-Marin |first=Mario |date=2017-08-01 |title=Revealed: the first ever flower, 140m years ago, looked like a magnolia |url=http://theconversation.com/revealed-the-first-ever-flower-140m-years-ago-looked-like-a-magnolia-81861 |access-date=2023-05-17 |website=The Conversation |language=en}}</ref>[https://arboretum.harvard.edu/wp-content/uploads/2020/08/tree-of-the-month-magnolias.pdf]
** ''[[Trapa]]'' – water caltrops, seeds, and leaves of numerous extinct species are known all the way back to the Cretaceous.
** ''[[Nelumbo]]'' – several species of lotus flower are known exclusively from fossils dating back to the Cretaceous.
** ''[[Sassafras]]'' – many fossils of sassafras are known from the late cretaceous through the late pleistocene.
** ''[[Platanus]]'' Sycamore fossils are very abundant throughout the northern hemisphere with several extinct species. Sycamore leaves and fruits are quite common in plant fossils. Sycamores exhibit many primitive features as well such their exfoliating bark which is a result of a lack of elasticity. Platanus Occidentalis fossils are known from the pliocene and the pleistocene in North America.
** ''[[Tupelo (tree)|Nyssa]]'' Blackgum fossils go way back to the late cretaceous period. Many extinct species are recorded as well.
** ''[[Liriodendron]]'' Fossils from the cretaceous and the tertiary period are found with many extinct species. Tulip Trees at one point were present in Europe during the cretaceous and the early paleocene. Liriodendron Tulipifera fossils dating from the pliocene and pleistocene were discovered at the chowan formation in North Carolina.
** ''[[Liquidambar]]'' Sweetgums appeared during the mid-late cretaceous and several extinct species are found throughout Asia, Europe and North America. The genus was once widespread in Europe and Asia especially during the miocene. The American Sweetgum is a living fossil itself since fossil specimens dating from the miocene, pliocene and pleistocene were discovered in the eastern United States

===Fungi===
* ''[[Neolecta]]''

===Animals===
[[File:Wild shortbeak echidna.jpg|thumb|[[Echidna]]s are one of few mammals to lay eggs.]] 
;Vertebrates:
[[File:Hoatzins in Ecuador.jpg|thumb|[[Hoatzin]] hatch with two visible claws on their wings, but the claws fall out once the birds reach maturity.]] 
[[File:Crocodylus acutus jalisco mexico.jpg|thumb|[[Crocodilia]]ns survived the [[Cretaceous–Paleogene extinction event|K–Pg extinction event]] that killed off the non-avian dinosaurs.]]
[[File:Tuatara.jpg|thumb|[[Tuatara]] are reptiles, yet retain more primitive characteristics than lizards and snakes.]]
[[File:Mitsukurina owstoni, Pengo.jpg|thumb|The [[goblin shark]] is the only [[Extant taxon|extant]] representative of the [[family (biology)|family]] [[Mitsukurinidae]], a lineage some 125&nbsp;million years old ([[early Cretaceous]]).]]
* '''Mammals'''
** [[Aardvark]] (''Orycteropus afer'')
** [[Amami rabbit]] (''Pentalagus furnessi'')<ref>{{cite journal |author1=Robinson, T. |author2=Yang, F. |author3=Harrison, W. |year=2002 |title=Chromosome painting refines the history of genome evolution in hares and rabbits (order Lagomorpha) |pmid=12438803 |journal=Cytogenetic and Genome Research |pages=223–227 |volume=96 |issue=1–4 |doi=10.1159/000063034|s2cid=19327437 }}</ref>
**[[Nesolagus]] (''Asian striped rabbits'')
** [[Chevrotain]] (''Tragulidae'')<ref>{{cite book|editor-last1=Eldridge|editor-first1=Niles|editor-last2=Stanley|editor-first2=Steven M.|year=1984|title=Living Fossils|chapter=Tragulids as Living Fossils|series=Casebooks in Earth Sciences|pages=87–94|doi=10.1007/978-1-4613-8271-3_9|isbn=978-1-4613-8273-7 }}</ref>
** [[Chousingha]] (''Tetracerus quadricornis'')
** [[Elephant shrew]] (Macroscelidea)<ref>{{Cite journal |last=Tabuce |first=Rodolphe |date=2017-03-23 |title=3D model related to the publication: New remains of Chambius kasserinensis from the Eocene of Tunisia and evaluation of proposed affinities for Macroscelidea (Mammalia, Afrotheria) |url=http://morphomuseum.com/articles/view/37 |journal=MorphoMuseuM |volume=3 |issue=2 |pages=e1 |doi=10.18563/m3.3.2.e1|url-access=subscription }}</ref>
** [[Giant panda]] (''Ailuropoda melanoleuca'')
** [[Baiji]] (''Lipotes vexillifer) (One living species)''
** [[Ganges river dolphin]] (''Platanista gangetica)''<ref name=":1">{{Cite journal |last1=Braulik |first1=Gill |last2=Atkore |first2=Vidyadhar |last3=Shahnawaz Khan |first3=Mohammad |last4=Malla |first4=Sabita |date=July 2021 |title=Review of Scientific Knowledge of the Ganges river dolphin |url=https://riverdolphins.org/wp-content/uploads/2021/07/Ganges-River-dolphin-Scientific-Knowledge-Review-July2021.pdf |journal=World Wildlife Fund |pages=5}}</ref>
** [[Indus river dolphin]] (''Platanista minor''')'''''<ref name=":1" />
** [[Hawaiian monk seal]] (''Neomonachus schauinslandi'')
** [[Koala]] (''Phascolarctos cinereus'')
** [[Laotian rock rat]] (''Laonastes aenigmamus'')<ref name=":2" />
** [[Monito del monte]] (''Dromiciops gliroides'')
** [[Monotreme]]s (the [[platypus]] and [[echidna]])
** [[Mountain beaver]] (''Aplodontia rufa'')
** [[Okapi]] (''Okapia johnstoni'')
** [[Opossum]]s (''Didelphidae'')
** [[Clouded leopard]] (''Neofelis nebulousa'')
** [[Bush dog]] (''Speothos venaticus'')
** [[Maned wolf]] (''Chrysocyon brachyurus'')
** [[Red panda]] (''Ailurus fulgens'')<ref>{{cite news |url=https://nationalzoo.si.edu/animals/red-panda |title=Red panda |date=2016-04-22 |publisher=Smithsonian Institution |department=National Zoo |place=Washington, DC |access-date=2017-05-04 |language=en |quote=Red pandas are considered by many to be living fossils. They have no close living relatives, and their nearest fossil ancestors, [[Parailurus]], lived 3–4&nbsp;million years ago.}}</ref>
** [[Solenodon]] (''[[Cuban solenodon|Solenodon cubanus]]'' and ''[[Hispaniolan solenodon|Solenodon paradoxus]]'')
** [[Shrew opossum]] (''Caenolestidae'')
** [[Spectacled bear]] (''Tremarctos ornatus'')
** [[False killer whale]] (''Pseudorca crassidens'')
** [[Pygmy right whale]] (''Caperea marginata'')<ref name=Fordyce2013>{{cite journal |last1=Fordyce |first1=R.E. |last2=Marx |first2=F.G. |title=The pygmy right whale ''Caperea marginata'': The last of the cetotheres |journal=Proceedings of the Royal Society&nbsp;B: Biological Sciences |year=2013 |volume=280 |issue=1753 |pages=1–6 |doi=10.1098/rspb.2012.2645 |pmid=23256199 |pmc=3574355}}</ref><ref>{{cite news |url=http://www.csmonitor.com/Science/2012/1219/Extinct-whale-found-Odd-looking-pygmy-whale-traced-back-2-million-years |title='Extinct' whale found: Odd-looking pygmy whale traced back 2&nbsp;million years |newspaper=Christian Science Monitor |date=April 23, 2012 |access-date=December 19, 2012}}</ref>
**[[Pacarana]] (''Dinomys branickii'')
** [[Rhinoceroses]] (Rhinocerotidae)<ref>{{cite book|editor-last1=West|editor-first1=Gary|editor-last2=Heard|editor-first2=Darryl|editor-last3=Caulkett|editor-first3=Nigel|last1=Radcliffe|first1=Robin W.|last2=Morkel|first2=Peter vdB.|year=2014|title=Zoo Animal and Wildlife Immobilization and Anesthesia|chapter=Chapter 54: Rhinoceroses|edition=2nd|doi=10.1002/9781118792919.ch54}}</ref>
** [[Tapirs]] (Tapiridae)<ref>{{cite book|editor-last1=Eldridge|editor-first1=Niles|editor-last2=Stanley|editor-first2=Steven M.|last=Janis |first=Christine M.|year=1984|title=Living Fossils|chapter=Tapirs as Living Fossils|series=Casebooks in Earth Sciences|pages=80–86 |doi=10.1007/978-1-4613-8271-3_8|isbn=978-1-4613-8273-7}}</ref>
* '''Birds'''
** [[Pelican]]s (''Pelecanus'') – [[Morphology (biology)|form]] has been virtually unchanged since the [[Eocene]], and is noted to have been even more conserved across the Cenozoic than that of crocodiles.<ref>{{cite magazine |last=Switek |first=Brian |url=https://www.wired.com/wiredscience/2011/03/repost-the-pelicans-beak-success-and-evolutionary-stasis/ |title=The pelican's beak: Success and evolutionary stasis |type=repost |series=Wired Science |magazine=Wired |volume=152 |pages=15–20 |doi=10.1007/s10336-010-0537-5 |date=2011-03-21 |access-date=2013-06-10}}</ref>
** [[Acanthisittidae]] (New Zealand "wrens") – 2&nbsp;living species, a few more recently [[extinct]]. Distinct lineage of [[Passeriformes]].
** [[Broad-billed sapayoa]] (''Sapayoa aenigma'') – One living species. Distinct lineage of [[Tyranni]].
** [[Bearded reedling]] (''Panurus biarmicus'') – One living species. Distinct lineage of [[Passerida]] or [[Sylvioidea]].
**[[Picathartes]] (rockfowls)
** [[Coliiformes]] (mousebirds) – 6&nbsp;living species in 2&nbsp;genera. Distinct lineage of [[Neoaves]].
** [[Hoatzin]] (''Ophisthocomus hoazin'') – One living species. Distinct lineage of [[Neoaves]].
** [[Magpie goose]] (''Anseranas semipalmata'') – One living species. Distinct lineage of [[Anseriformes]].
** [[Sandhill crane]] (''Antigone canadensis'') – Oldest living species.
** [[Seriema]] (''Cariamidae'') – 2&nbsp;living species. Distinct lineage of [[Cariamae]].
** [[Tinamiformes]] (tinamous) 50&nbsp;living species. Distinct lineage of [[Palaeognathae]].
* '''Reptiles'''
** [[Crocodilia]] ([[crocodile]]s, [[gavial]]s, [[caiman]]s and [[alligator]]s)
** [[Pig-nosed turtle]] (''Carettochelys insculpta'')
**[[Hickatee]] (''Dermatemys mawii'')
** [[Snapping turtle]] (''[[Chelydridae]]'') family
** [[Tuatara]] (''[[Sphenodon punctatus]]'' and ''[[Sphenodon guntheri]]'')<ref name="Herrera-Flores 2017"/>
**[[Asian forest tortoise]] (''[[Manouria emys]]'')
**[[Impressed tortoise]] (''[[Manouria impressa]]'')
**[[Xenopeltis|Sunbeam snake]] (''[[Xenopeltis hainanensis]]'' and ''[[Xenopeltis unicolor]]'')
**[[Leatherback sea turtle]] (''Dermochelys coriacea'')
* '''Amphibians'''
** [[Giant salamanders]] (''[[Cryptobranchus]]'' and ''[[Andrias]]'')
** [[Hula painted frog]] (''[[Latonia nigriventer]]'')<ref>{{cite news |last=Morelle |first=Rebecca |author-link=Rebecca Morelle |title=Rediscovered hula painted frog 'is a living fossil' |url=https://www.bbc.co.uk/news/science-environment-22770959 |publisher=BBC News |access-date=4 June 2013 |date=2013-06-04}}</ref>
** [[Purple frog]] (''Nasikabatrachus sahyadrensis'')
[[File:Nautilus belauensis profile.jpg|thumb|[[Nautilus]] retain the external spiral shell that its other relatives have lost.]] 
[[File:Horseshoe Crab.jpg|thumb|upright|With little change over the last 450&nbsp;million years, the [[horseshoe crab]]s appear as living fossils.]]
* '''Jawless fish'''
** [[Hagfish]] (''Myxinidae'') family
** [[Lamprey]] (''Petromyzontiformes'')
* '''Bony fish'''
** [[Arowana]] and [[arapaima]] (''Osteoglossidae'')
** [[Bowfin]] (''Amia calva'')
** [[Coelacanth]] (the lobed-finned ''Latimeria menadoensis'' and ''Latimeria chalumnae'')
** [[Gar]] (''Lepisosteidae'')
** [[Queensland lungfish]] (''Neoceratodus fosteri'')
** [[African lungfish]] (''Protopterus sp.'')
** [[Sturgeons]] and [[paddlefish]] ([[Acipenseriformes]])
** [[Bichir]] (family ''Polypteridae'')
** ''[[Protanguilla palau]]''
** [[Mudskipper]] (''Oxudercinae'')
* '''Sharks'''
** [[Blind shark]] (''Brachaelurus waddi'')
** [[Bullhead shark]] (''Heterodontus sp.'')
** [[Cow shark]] (sixgill sharks and relatives) (''Hexanchidae'')
** [[Elephant shark]] (''Callorhinchus milii'')
** [[Frilled shark]] (''Chlamydoselachus sp.'')
** [[Goblin shark]] (''Mitsukurina owstoni'')
** [[Gulper shark]] (''Centrophorus sp.'')

;Invertebrates:
* '''Insects'''
** ''[[Cyatta abscondita ]]''(most recent common relative of ''[[Atta (ant)|Atta]]'' and ''[[Acromyrmex]]'' ant genera)
** [[Heloridae|Helorid wasps]] (1 living genus, 11 extinct genera)
** [[Mantophasmatidae|Mantophasmatodea]] (gladiators; a few living species)
** [[Meropeidae]] (3 living species, 4 extinct)
** ''[[Telephone-pole beetle|Micromalthus debilis]]'' (a [[beetle]])
** [[Mymarommatidae|Mymarommatid wasps]] (10 living species in genus ''Palaeomymar'')
** [[Nevrorthidae]] (3 species-poor genera)
** ''[[Nothomyrmecia]]'' (known as the 'dinosaur ant')
** ''[[Notiothauma reedi]]'' (a [[scorpionfly]] relative)
** [[Orussidae]] (parasitic wood wasps; about 70 living species in 16 genera)
** [[Peloridiidae]] (peloridiid bugs; fewer than 30 living species in 13 genera)
** [[Rhinorhipus|Rhinorhipid beetles]] (1 living species, Triassic origin)
** [[Rotoitidae|Rotoitid wasps]] (2 living species, 14 extinct)
** ''[[Sikhotealinia zhiltzovae]]'' (a [[Jurodidae|jurodid]] [[beetle]])
** ''[[Syntexis libocedrii]]'' (Anaxyelidae cedar wood wasp)
** ''[[Tricholepidion gertschi]]'' (silverfish with nearest relatives in Eocene)
* '''Crustaceans'''
** [[Glypheidea]] (2&nbsp;living species: ''Neoglyphea inopinata'' and ''Laurentaeglyphea neocaledonica'')
** [[Mantis shrimp|Stomatopods]] (mantis shrimp)
** [[Polychelida]] (deep sea blind lobster)
** ''[[Triopsidae]]'' (also known as tadpole shrimp; [[notostraca]]n [[crustacean]]s)
* '''Molluscs'''
** [[Nautilina]] (e.g., ''[[Nautilus pompilius]]'')
** ''[[Neopilina]]'' – [[Monoplacophora]]n
** [[Pleurotomariidae|Slit snail]] (e.g., ''[[Entemnotrochus rumphii]]'')
** ''[[Vampyroteuthis infernalis]]'' – the [[vampire squid]]
** [[Pleuroceridae|Pleurocerid]] snails<ref name=":0">{{Cite journal |last1=Dillon |first1=Robert T. |last2=Robinson |first2=John D. |date=2009 |title=The snails the dinosaurs saw: are the pleurocerid populations of the Older Appalachians a relict of the Paleozoic Era? |url=http://dx.doi.org/10.1899/08-034.1 |journal=Journal of the North American Benthological Society |volume=28 |issue=1 |pages=1–11 |doi=10.1899/08-034.1 |s2cid=85340338 |issn=0887-3593|url-access=subscription }}</ref>
* '''Other invertebrates'''
** [[Crinoid]]s
** [[Horseshoe crab]]s (only 4&nbsp;living species of the class [[Xiphosura]], family [[Limulidae]]<!--: ''[[Limulus polyphemus]]'', ''[[Tachypleus gigas]]'', ''[[Tachypleus tridentatus]]'', and ''[[Carcinoscorpius rotundicauda]]''-->)
** ''[[Lingula (genus)|Lingula anatina]]'' (an inarticulate [[brachiopod]])
** [[Liphistiidae]] (trapdoor spiders)
** [[Onychophora]]ns (velvet worms)
** ''[[Rhabdopleura]]'' (a [[hemichordate]])
** ''[[Valdiviathyris quenstedti]]'' (a [[craniforma]]n brachiopod)
** ''[[Paleodictyon nodosum]]'' (unknown)

==See also==
* [[Relict (biology)]]
* [[Breeding back]]
* [[Lazarus taxon]]

== Notes ==
Baiji is not officially classified as extinct, but instead critically endangered, possibly extinct and has the unofficial status of [[functional extinction]].<ref>{{Cite journal |last=Zhou |first=Kaiya |last2=Smith |first2=Brian |last3=Randall Reeves (IUCN SSC Cetacean Specialist Group) |last4=Braulik |first4=Gillian |last5=Barlow |first5=Jay |last6=Robert Pitman (Protected Resources Division, Southwest Fisheries Science Center/NOAA) |last7=Ding Wang (Institute of Hydrobiology, Chinese Academy of Sciences) |date=2017-08-20 |title=IUCN Red List of Threatened Species: Lipotes vexillifer |url=https://www.iucnredlist.org/species/12119/50362206 |journal=IUCN Red List of Threatened Species |archive-url=http://web.archive.org/web/20250105123957/https://www.iucnredlist.org/species/12119/50362206 |archive-date=2025-01-05}}</ref>

==References==
{{Reflist|30em}}

==External links==
{{Wiktionary}}
*[https://web.archive.org/web/20100805092533/http://mytriops.com/articles/triops_intro.stm MyTriops introduces ''Triops'' as living fossils]

{{Extinction}}
{{Authority control}}

[[Category:Evolutionary biology concepts]]
[[Category:Extinction]]
[[Category:Fossils]]