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{{redirect|Transitional forms|the hardcore punk music album|Sharptooth}}
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[[File:Archaeopteryx lithographica (Berlin specimen).jpg|thumb|upright|''[[Archaeopteryx]]'' is one of the most famous transitional fossils and gives evidence for the [[evolution]] of birds from [[Theropoda|theropod]] [[dinosaur]]s.]]▼
{{Paleontology|cTopic=Fossil record}}
A '''transitional fossil''' is any [[fossil]]ized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group.<ref name="Freeman">{{harvnb|Freeman|Herron|2004|p=816}}</ref> This is especially important where the descendant group is sharply differentiated by [[gross anatomy]] and mode of living from the ancestral group. These fossils serve as a reminder that taxonomic divisions are [[human]] constructs that have been imposed in hindsight on a continuum of variation. Because of the incompleteness of the fossil record, there is usually no way to know exactly how close a transitional fossil is to the point of divergence. Therefore, it cannot be assumed that transitional fossils are direct ancestors of more recent groups, though they are frequently used as models for such ancestors.<ref name=Prothero/>
In 1859, when [[Charles Darwin]]'s ''[[On the Origin of Species]]'' was first published, the fossil record was poorly known. Darwin described the perceived lack of transitional fossils as "the most obvious and gravest objection which can be urged against my theory," but he explained it by relating it to the extreme imperfection of the geological record.<ref>{{harvnb|Darwin|1859|pp=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F373&pageseq=297 279–280]}}</ref> He noted the limited collections available at the time but described the available information as showing patterns that followed from his theory of [[evolution|descent with modification]] through [[natural selection]].<ref>{{harvnb|Darwin|1859|pp=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F373&pageseq=359 341–343]}}</ref> Indeed, ''[[Archaeopteryx]]'' was discovered just two years later, in 1861, and represents a classic transitional form between earlier, non-avian [[dinosaur]]s and [[bird]]s. [[List of transitional fossils|Many more transitional fossils]] have been discovered since then, and there is now abundant evidence of how all [[class (biology)|classes]] of [[vertebrate]]s are related, including many transitional fossils.<ref name="NS2645">{{cite journal |last=Prothero |first=Donald R. |author-link=Donald Prothero |date=1 March 2008 |title=Evolution: What missing link? |url=https://www.newscientist.com/article/mg19726451.700-evolution-what-missing-link.html?full=true |journal=[[New Scientist]] |issue=2645 |pages=35–41 |issn=0262-4079 |doi=10.1016/s0262-4079(08)60548-5 |volume=197|url-access=subscription }}</ref> Specific examples of class-level transitions are: [[Evolution of tetrapods|tetrapod]]s and [[Evolution of fish|fish]], [[Origin of birds|birds and dinosaurs]], and [[Evolution of mammals|mammals and "mammal-like reptiles"]].
The term "missing link" has been used extensively in popular writings on [[human evolution]] to refer to a perceived gap in the [[Hominidae|hominid]] evolutionary record. It is most commonly used to refer to any new transitional fossil finds. Scientists, however, do not use the term, as it refers to a pre-evolutionary view of nature.
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===''Archaeopteryx''===
{{Main|Origin of birds}}
[[File:Archæopteryx, fig 1, Nordisk familjebok.png|thumb|upright|A historic 1904 reconstruction of ''[[Archaeopteryx|Archæopteryx]]'']]▼
▲[[File:Archaeopteryx lithographica (Berlin specimen).jpg|thumb|upright|''[[Archaeopteryx]]'' is one of the most famous transitional fossils and gives evidence for the [[evolution]] of birds from [[Theropoda|theropod]] [[dinosaur]]s.]]
''[[Archaeopteryx]]'' is a [[genus]] of [[Theropoda|theropod]] dinosaur closely related to the birds. Since the late 19th century, it has been accepted by palaeontologists, and celebrated in lay reference works, as being the oldest known bird, though a study in 2011 has cast doubt on this assessment, suggesting instead that it is a non-[[Avialae|avialan]] dinosaur closely related to the origin of birds.<ref name="Xiaotingia">{{cite journal |author1=Xing Xu |author-link1=Xu Xing (paleontologist) |author2=Hailu You |author3=Kai Du |author4=Fenglu Han |date=28 July 2011 |title=An ''Archaeopteryx''-like theropod from China and the origin of Avialae |journal=[[Nature (journal)|Nature]] |volume=475 |issue=7357 |pages=465–470 |doi=10.1038/nature10288 |issn=0028-0836 |pmid=21796204 |s2cid=205225790 }}</ref>
It lived in what is now southern Germany in the [[Late Jurassic]] [[Period (geology)|period]] around 150 million years ago, when Europe was an [[archipelago]] in a shallow warm tropical sea, much closer to the equator than it is now. Similar in shape to a [[European magpie]], with the largest individuals possibly attaining the size of a [[raven]],<ref name="Erickson etal 2009">{{cite journal |last1=Erickson |first1=Gregory M. |author-link1=Gregory M. Erickson |last2=Rauhut |first2=Oliver W. M. |author3=Zhonghe Zhou |author-link3=Zhou Zhonghe |last4=Turner |first4=Alan H. |last5=Inouye |first5=Brian D. |author6=Dongyu Hu |last7=Norell |first7=Mark A. |date=9 October 2009 |title=Was Dinosaurian Physiology Inherited by Birds? Reconciling Slow Growth in ''Archaeopteryx'' |journal=[[PLOS One]] |volume=4 |issue=10 |page=e7390 |bibcode=2009PLoSO...4.7390E |doi=10.1371/journal.pone.0007390 |issn=1545-7885 |pmc=2756958 |pmid=19816582 |display-authors=3 |doi-access=free }}</ref> ''Archaeopteryx'' could grow to about 0.5 metres (1.6 ft) in length. Despite its small size, broad wings, and inferred ability to fly or glide, ''Archaeopteryx'' has more in common with other small [[Mesozoic]] dinosaurs than it does with modern birds. In particular, it shares the following features with the [[
The first complete specimen was announced in 1861, and ten more ''Archaeopteryx'' fossils have been found since then. Most of the eleven known fossils include impressions of feathers—among the oldest direct evidence of such structures. Moreover, because these feathers take the advanced form of [[flight feather]]s, ''Archaeopteryx'' fossils are evidence that feathers began to evolve before the Late Jurassic.<ref>{{harvnb|Wellnhofer|2004|pp=282–300}}</ref>
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The [[cetacea]]ns (whales, dolphins and porpoises) are [[marine mammal]] descendants of land [[mammal]]s. The [[Pakicetidae|pakicetid]]s are an [[Extinction|extinct]] [[family (biology)|family]] of hoofed mammals that are the earliest whales, whose closest sister group is ''[[Indohyus]]'' from the family [[Raoellidae]].<ref name=science_news_2>{{cite news |author=Northeastern Ohio Universities Colleges of Medicine and Pharmacy |author-link=Northeast Ohio Medical University |date=21 December 2007 |title=Whales Descended From Tiny Deer-like Ancestors |url=https://www.sciencedaily.com/releases/2007/12/071220220241.htm |work=[[Science Daily]] |___location=Rockville, MD |publisher=ScienceDaily, LLC |access-date=2015-05-15}}</ref><ref>{{harvnb|Gingerich|Russell|1981}}</ref> They lived in the Early [[Eocene]], around 53 million years ago. Their fossils were first discovered in North Pakistan in 1979, at a river not far from the shores of the former [[Tethys Sea]].<ref>{{harvnb|Castro|Huber|2003}}</ref>{{page needed|date=March 2012}} Pakicetids could hear under water, using enhanced bone conduction, rather than depending on [[tympanic membrane]]s like most land mammals. This arrangement does not give directional hearing under water.<ref name=hearing>{{cite journal |last1=Nummela |first1=Sirpa |last2=Thewissen |first2=J. G. M. |last3=Bajpai |first3=Sunil |last4=Hussain |first4=S. Taseer |last5=Kumar |first5=Kishor |date=12 August 2004 |title=Eocene evolution of whale hearing |journal=Nature |volume=430 |issue=7001 |pages=776–778 |bibcode=2004Natur.430..776N |doi=10.1038/nature02720 |issn=0028-0836 |pmid=15306808 |s2cid=4372872 |display-authors=3 }}</ref>
''[[Ambulocetus natans]]'', which lived about 49 million years ago, was discovered in Pakistan in 1994. It was probably amphibious, and looked like a [[crocodile]].<ref name="skeleton">{{cite journal |last1=Thewissen |first1=J. G. M. |last2=Williams |first2=Ellen M. |last3=Roe |first3=Lois J. |last4=Hussain |first4=S. Taseer |date=20 September 2001 |title=Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls |journal=Nature |volume=413 |issue=6853 |pages=277–281 |doi=10.1038/35095005 |issn=0028-0836 |pmid=11565023 |display-authors=3 |bibcode=2001Natur.413..277T |s2cid=4416684 }}</ref> In the Eocene, [[Ambulocetidae|ambulocetid]]s inhabited the bays and estuaries of the Tethys Ocean in northern Pakistan.<ref name=radiations>{{cite journal |last1=Thewissen |first1=J. G. M. |last2=Williams |first2=Ellen M. |date=November 2002 |title=The Early Radiations of Cetacea (Mammalia): Evolutionary Pattern and Developmental Correlations |journal=[[Annual Review of Ecology, Evolution, and Systematics|Annual Review of Ecology and Systematics]] |volume=33 |issue=1 |pages=73–90 |doi=10.1146/annurev.ecolsys.33.020602.095426 |bibcode=2002AnRES..33...73T |issn=1545-2069 }}</ref> The fossils of ambulocetids are always found in near-shore shallow marine deposits associated with abundant marine plant fossils and [[Littoral zone|littoral]] [[Mollusca|mollusc]]s.<ref name=radiations/> Although they are found only in marine deposits, their oxygen isotope values indicate that they consumed water with a range of degrees of salinity, some specimens showing no evidence of sea water consumption and others none of fresh water consumption at the time when their teeth were fossilized. It is clear that ambulocetids tolerated a wide range of salt concentrations.<ref name="poster">{{cite journal |last1=Thewissen |first1=J. G. M. |last2=Bajpai |first2=Sunil |date=December 2001 |title=Whale Origins as a Poster Child for Macroevolution |journal=[[BioScience]] |volume=51 |issue=12 |pages=1037–1049 |doi=10.1641/0006-3568(2001)051[1037:WOAAPC]2.0.CO;2 |issn=0006-3568 |doi-access=free }}</ref> Their diet probably included land animals that approached water for drinking, or freshwater aquatic organisms that lived in the river.<ref name=radiations/> Hence, ambulocetids represent the transition phase of cetacean ancestors between freshwater and marine habitat.
===''Tiktaalik''===
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[[Pleuronectiformes]] (flatfish) are an [[order (biology)|order]] of [[Actinopterygii|ray-finned fish]]. The most obvious characteristic of the modern flatfish is their asymmetry, with both eyes on the same side of the head in the adult fish. In some families the eyes are always on the right side of the body (dextral or right-eyed flatfish) and in others they are always on the left (sinistral or left-eyed flatfish). The primitive [[spiny turbot]]s include equal numbers of right- and left-eyed individuals, and are generally less asymmetrical than the other families. Other distinguishing features of the order are the presence of protrusible eyes, another adaptation to living on the [[seabed]] ([[benthos]]), and the extension of the dorsal fin onto the head.<ref>{{harvnb|Chapleau|Amaoka|1998|pp=223–226}}</ref>
''[[Amphistium]]'' is a 50-million-year-old fossil fish identified as an early relative of the flatfish, and as a transitional fossil.<ref name="NaEvo">{{cite news |last=Minard |first=Anne |date=9 July 2008 |title=Odd Fish Find Contradicts Intelligent-Design Argument |url=http://news.nationalgeographic.com/news/2008/07/080709-evolution-fish.html |work=National Geographic News |___location=Washington, D.C. |publisher=[[National Geographic Society]] |archive-url=https://web.archive.org/web/20080804234422/http://news.nationalgeographic.com/news/2008/07/080709-evolution-fish.html|archive-date=4 August 2008|access-date=2008-07-17}}</ref> In ''Amphistium'', the transition from the typical symmetric head of a vertebrate is incomplete, with one eye placed near the top-center of the head.<ref name="FriedmanFlatfish">{{cite journal |last=Friedman |first=Matt |date=10 July 2008 |title=The evolutionary origin of flatfish asymmetry |journal=Nature |volume=454 |issue=7201 |pages=209–212 |bibcode=2008Natur.454..209F |doi=10.1038/nature07108 |issn=0028-0836 |pmid=18615083 |s2cid=4311712 }}</ref> Paleontologists concluded that "the change happened gradually, in a way consistent with evolution via [[natural selection]]—not suddenly, as researchers once had little choice but to believe."<ref name="NaEvo" />
''Amphistium'' is among the many fossil fish species known from the [[Monte Bolca]] ''[[Lagerstätte]]'' of [[Lutetian]] Italy. ''[[Heteronectes]]'' is a related, and very similar fossil from slightly earlier strata of France.<ref name="FriedmanFlatfish" />
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===Post-Darwin===
▲[[File:Archæopteryx, fig 1, Nordisk familjebok.png|thumb|upright|A historic 1904 reconstruction of ''[[Archaeopteryx|Archæopteryx]]'']]
[[File:Rhynia reconstruction.svg|thumb|upright|Reconstruction of ''[[Rhynia]]'']]
The idea that animal and plant species were not constant, but changed over time, was suggested as far back as the 18th century.<ref>{{cite journal |last1=Archibald |first1=J. David |date=August 2009 |title=Edward Hitchcock's Pre-Darwinian (1840) 'Tree of Life' |url=http://www.bio.sdsu.edu/faculty/archibald.html/Archibald09JHB42p561.pdf |journal=[[Journal of the History of Biology]] |volume=42 |issue=3 |pages=561–592 |doi=10.1007/s10739-008-9163-y |issn=0022-5010 |pmid=20027787 |citeseerx=10.1.1.688.7842 |s2cid=16634677 }}</ref> Darwin's ''On the Origin of Species'', published in 1859, gave it a firm scientific basis. A weakness of Darwin's work, however, was the lack of palaeontological evidence, as pointed out by Darwin himself. While it is easy to imagine [[natural selection]] producing the variation seen within [[genus|genera]] and families, the transmutation between the higher categories was harder to imagine. The dramatic find of the [[Archaeopteryx#History of discovery|London specimen]] of ''[[Archaeopteryx]]'' in 1861, only two years after the publication of Darwin's work, offered for the first time a link between the [[class (biology)|class]] of the highly derived birds, and that of the more basal [[reptile]]s.{{sfn|Darwin|1859|loc=[http://www.readprint.com/chapter-2217/Charles-Darwin Chapter 10]}} In a letter to Darwin, the palaeontologist [[Hugh Falconer]] wrote:
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==Missing links==
[[File:Pithecanthropus-erectus.jpg|thumb|left|"[[Java Man]]" or ''Pithecanthropus erectus'' (now ''[[Homo erectus]]''), the original "missing link" found in Java in 1891–92]]
[[File:Human
<!-- The term "missing link" refers back to the originally static pre-evolutionary concept of the [[great chain of being]], a [[Deism|deist]] idea that all existence is linked, from the lowest [[dirt]], through the living [[kingdom (biology)|kingdoms]] to angels and finally to God.{{sfn|Lovejoy|1936}}-->The idea of all living things being linked through some sort of transmutation process predates Darwin's theory of evolution. [[Jean-Baptiste Lamarck]] envisioned that life was generated constantly in the form of the simplest creatures, and strove [[Orthogenesis|towards complexity and perfection]] (i.e. humans) through a progressive series of lower forms.<ref>{{harvnb|Lamarck|1815–1822}}</ref> In his view, lower animals were simply newcomers on the evolutionary scene.<ref>{{cite journal |last=Appel |first=Toby A. |date=Fall 1980 |title=Henri De Blainville and the Animal Series: A Nineteenth-Century Chain of Being |journal=Journal of the History of Biology |volume=13 |issue=2 |pages=291–319 |doi=10.1007/BF00125745 |jstor=4330767 |s2cid=83708471 |issn=0022-5010 }}</ref>
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[[File:Punctuated Equilibrium.svg|thumb|upright=1.7|Sudden jumps with apparent gaps in the fossil record have been used as evidence for [[punctuated equilibrium]]. Such jumps can be explained either by [[macromutation]] or simply by relatively rapid episodes of gradual evolution by natural selection, since a period of say 10,000 years barely registers in the fossil record.]]
While "missing link" is still a popular term, well-recognized by the public and often used in the popular media,<ref name="zimmer">{{cite journal |last=Zimmer |first=Carl |author-link=Carl Zimmer |date=19 May 2009 |title=Darwinius: It delivers a pizza, and it lengthens, and it strengthens, and it finds that slipper that's been at large under the chaise lounge [sic] for several weeks... |url=http://blogs.discovermagazine.com/loom/2009/05/19/darwinius-it-delivers-a-pizza-and-it-lengthens-and-it-strengthens-and-it-finds-that-slipper-thats-been-at-large-under-the-chaise-lounge-for-several-weeks/ |journal=[[Discover (magazine)|The Loom]] |type=Blog |___location=Waukesha, WI |publisher=[[Kalmbach Publishing]] |access-date=2011-09-10 |archive-date=5 September 2011 |archive-url=https://web.archive.org/web/20110905220434/http://blogs.discovermagazine.com/loom/2009/05/19/darwinius-it-delivers-a-pizza-and-it-lengthens-and-it-strengthens-and-it-finds-that-slipper-thats-been-at-large-under-the-chaise-lounge-for-several-weeks/ |url-status=dead }}</ref> the term is avoided in scientific publications.<ref name="NS2645" /> Some bloggers have called it "inappropriate";<ref name="hm">{{cite web |url=http://www.biologytimes.com/why-the-term-missing-links-is-inappropriate/ |title=Why the term 'missing links' is inappropriate |last=Sambrani |first=Nagraj |date=10 June 2009 |website=Biology Times |type=Blog |access-date=2015-05-19}}</ref> both because the links are no longer "missing", and because human evolution is no longer believed to have occurred in terms of a single linear progression.<ref name="NS2645" /><ref name="nomorelink">{{cite news |author=<!--Staff writer(s); no by-line.--> |title=Newly found fossils could link to human ancestor |url=http://www.cbc.ca/news/technology/newly-found-fossils-could-link-to-human-ancestor-1.930070 |work=[[CBC News]] |___location=Ottawa, Ontario, Canada |publisher=[[Canadian Broadcasting Corporation]] |date=8 April 2010 |access-date=2015-05-19 |quote=It's tempting to call the new species a 'missing link' between earlier species and modern humans, but scientists say the concept no longer applies, given new knowledge of human evolution. [...] Researchers now say the evolution of humans consisted of a number of diverse species in many branches, not a single smooth line from ape-like species to humans.}}</ref>
==Punctuated equilibrium==
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* {{cite news |last=Lloyd |first=Robin |date=11 February 2009 |title=Fossils Reveal Truth About Darwin's Theory |url=http://www.livescience.com/3306-fossils-reveal-truth-darwin-theory.html |work=[[LiveScience]] |___location=Ogden UT |publisher=[[Purch]] |access-date=2015-05-19}}
* {{cite web |url=http://www.talkorigins.org/faqs/faq-transitional.html |title=Transitional Vertebrate Fossils FAQ |last=Hunt |first=Kathleen |date=17 March 1997 |website=[[TalkOrigins Archive]] |publisher=The TalkOrigins Foundation, Inc. |___location=Houston, TX |access-date=2015-05-19}}
* {{cite web |url=http://tiktaalik.uchicago.edu/ |title=''Tiktaalik roseae'' |publisher=[[University of Chicago]] |___location=Chicago, IL |access-date=2015-05-19 |archive-date=12 November 2011 |archive-url=https://web.archive.org/web/20111112134921/http://tiktaalik.uchicago.edu/ |url-status=dead }}
* {{cite web |url=http://collections.tepapa.govt.nz/exhibitions/whales/Segment.aspx?irn=161 |title=Whales Tohorā |publisher=[[Museum of New Zealand Te Papa Tongarewa]] |___location=Wellington, New Zealand |access-date=2015-05-19}}
* {{cite web |url=http://www.ucmp.berkeley.edu/diapsids/avians.html |title=Are Birds Really Dinosaurs? |last=Hutchinson |first=John R. |date=22 January 1998 |website=DinoBuzz |publisher=[[University of California Museum of Paleontology]] |___location=Berkeley, CA |access-date=2015-05-19}}
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