Editing Transitional fossil

{{Short description|Type of fossilized remains}}
{{redirect|Transitional forms|the hardcore punk music album|Sharptooth}}
{{Use dmy dates|date=September 2020}}
[[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.

==Evolutionary and phylogenetic taxonomy==
===Transitions in phylogenetic nomenclature===
{{Main|Phylogenetic nomenclature}}
{{See also|Evolutionary taxonomy}}
[[File:Spindle diagram.jpg|thumb|left|Traditional spindle diagram showing the [[vertebrate]]s classes "budding" off from each other. Transitional fossils typically represent animals from near the branching points.]]
In evolutionary taxonomy, the prevailing form of [[Taxonomy (biology)|taxonomy]] during much of the 20th century and still used in non-specialist textbooks, [[Taxon|taxa]] based on morphological similarity are often drawn as "bubbles" or "spindles" branching off from each other, forming evolutionary trees.<ref>For example, see {{harvnb|Benton|1997}}</ref> Transitional forms are seen as falling between the various groups in terms of anatomy, having a mixture of characteristics from inside and outside the newly branched [[clade]].{{sfn|Prothero|2007|p=[https://archive.org/details/evolutionwhatfos00prot_0/page/84 84]}}

With the establishment of [[cladistics]] in the 1990s, relationships commonly came to be expressed in [[cladogram]]s that illustrate the branching of the evolutionary lineages in stick-like figures. The different so-called "natural" or "[[Monophyly|monophyletic]]" groups form nested units, and only these are given [[phylogenetic nomenclature|phylogenetic names]]. While in traditional classification tetrapods and fish are seen as two different groups, phylogenetically tetrapods are considered a branch of fish. Thus, with cladistics there is no longer a transition between established groups, and the term "transitional fossils" is a [[misnomer]]. Differentiation occurs within groups, represented as branches in the cladogram.<ref name="Palaeos">{{cite web |url=http://palaeos.com/vertebrates/tetrapoda/amphibians.html |title=Amphibians, Systematics, and Cladistics |last=Kazlev |first=M. Alan |website=[[Palaeos]] |access-date=2012-05-09}}</ref>

In a cladistic context, transitional organisms can be seen as representing early examples of a branch, where not all of the traits typical of the previously known descendants on that branch have yet evolved.{{sfn|Prothero|2007|p=[https://archive.org/details/evolutionwhatfos00prot_0/page/127 <!-- quote=fossil "transitional form". --> 127]}} Such early representatives of a group are usually termed "[[Basal (phylogenetics)|basal taxa]]" or "[[Sister group|sister taxa]],"{{sfn|Prothero|2007|p=[https://archive.org/details/evolutionwhatfos00prot_0/page/263 <!-- quote=fossil "transitional form". --> 263]}} depending on whether the fossil organism belongs to the daughter clade or not.<ref name="Palaeos" />

===Transitional versus ancestral===
A source of confusion is the notion that a transitional form between two different taxonomic groups must be a direct ancestor of one or both groups. The difficulty is exacerbated by the fact that one of the goals of evolutionary taxonomy is to identify taxa that were ancestors of other taxa. However, because evolution is a branching process that produces a complex bush pattern of related [[species]] rather than a linear process producing a ladder-like progression, and because of the incompleteness of the fossil record, it is unlikely that any particular form represented in the fossil record is a direct ancestor of any other. Cladistics deemphasizes the concept of one taxonomic group being an ancestor of another, and instead emphasizes the identification of sister taxa that share a more recent common ancestor with one another than they do with other groups. There are a few exceptional cases, such as some marine [[plankton]] [[Micropaleontology|microfossil]]s, where the fossil record is complete enough to suggest with confidence that certain fossils represent a population that was actually ancestral to a later population of a different species.<ref>{{cite journal |last1=Prothero |first1=Donald R. |last2=Lazarus |first2=David B. |date=June 1980 |title=Planktonic Microfossils and the Recognition of Ancestors |journal=[[Systematic Biology]] |volume=29 |issue=2 |pages=119–129 |doi=10.1093/sysbio/29.2.119 |issn=1063-5157 }}</ref> But, in general, transitional fossils are considered to have features that illustrate the transitional anatomical features of actual common ancestors of different taxa, rather than to ''be'' actual ancestors.<ref name=Prothero>{{harvnb|Prothero|2007|pp=133–135}}</ref>

==Prominent examples==

===''Archaeopteryx''===
{{Main|Origin of birds}}
[[File:Archæopteryx, fig 1, Nordisk familjebok.png|thumb|upright|A historic 1904 reconstruction of ''[[Archaeopteryx|Archæopteryx]]'']]
''[[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&nbsp;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&nbsp;metres (1.6&nbsp;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 [[Deinonychosauria|deinonychosaurs]] ([[Dromaeosauridae|dromaeosaur]]s and [[Troodontidae|troodontid]]s): jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes ("killing claw"), [[feather]]s (which suggest [[homeothermy]]), and various skeletal features.<ref name="Yalden 1">{{cite journal |last=Yalden |first=Derek W. |author-link=Derek Yalden |date=September 1984 |title=What size was ''Archaeopteryx''? |journal=[[Zoological Journal of the Linnean Society]] |volume=82 |issue=1–2 |pages=177–188 |doi=10.1111/j.1096-3642.1984.tb00541.x |issn=0024-4082 }}</ref> These features make ''Archaeopteryx'' a clear candidate for a transitional fossil between dinosaurs and birds,<ref name="UCal MoP">{{cite web |url=http://www.ucmp.berkeley.edu/diapsids/birds/archaeopteryx.html |title=''Archaeopteryx'': An Early Bird |author=<!--Staff writer(s); no by-line.--> |website=[[University of California Museum of Paleontology]] |publisher=[[University of California, Berkeley]] |location=Berkeley, CA |access-date=2006-10-18}}</ref> making it important in the study both of dinosaurs and of the origin of birds.

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>

===''Australopithecus afarensis''===
{{main|Australopithecus afarensis|Human evolution}}
{{see also|List of human evolution fossils}}
[[File:Lucy Skeleton cropped.jpg|thumb|75px|''[[Australopithecus afarensis|A. afarensis]]'' - walking posture]]
The hominid ''Australopithecus afarensis'' represents an evolutionary transition between modern bipedal humans and their quadrupedal [[ape]] ancestors. A number of traits of the ''A. afarensis'' skeleton strongly reflect bipedalism, to the extent that some researchers have suggested that bipedality evolved long before ''A. afarensis''.<ref name="Lovejoy1988">{{cite journal |last=Lovejoy |first=C. Owen |author-link=Owen Lovejoy (anthropologist) |date=November 1988 |title=Evolution of Human walking |url=http://users.clas.ufl.edu/krigbaum/proseminar/Lovejoy_1988_SA.pdf |journal=[[Scientific American]] |volume=259 |issue=5 |pages=82–89 |doi=10.1038/scientificamerican1188-118 |issn=0036-8733 |pmid=3212438 |bibcode=1988SciAm.259e.118L }}</ref> In overall anatomy, the pelvis is far more human-like than ape-like. The [[Ilium (bone)|iliac blades]] are short and wide, the sacrum is wide and positioned directly behind the hip joint, and there is clear evidence of a strong attachment for the [[Rectus femoris muscle|knee extensors]], implying an upright posture.<ref name="Lovejoy1988" />{{rp|122}}

While the [[pelvis]] is not entirely like that of a human (being markedly wide, or flared, with laterally orientated iliac blades), these features point to a structure radically remodelled to accommodate a significant degree of [[bipedalism]]. The [[femur]] angles in toward the knee from the [[hip]]. This trait allows the foot to fall closer to the midline of the body, and strongly indicates habitual bipedal locomotion. Present-day humans, [[orangutan]]s and [[spider monkey]]s possess this same feature. The feet feature [[adducted]] big toes, making it difficult if not impossible to grasp branches with the [[hindlimb]]s. Besides locomotion, ''A. afarensis'' also had a slightly larger brain than a modern [[Common chimpanzee|chimpanzee]]<ref name="SmithsonianHuman">{{cite web |url=http://humanorigins.si.edu/evidence/human-fossils/species/australopithecus-afarensis |title=''Australopithecus afarensis'' |website=Human Evolution |publisher=[[Smithsonian Institution]]'s Human Origins Program |location=Washington, D.C. |access-date=2015-05-15}}</ref> (the closest living relative of humans) and had teeth that were more human than ape-like.<ref>{{cite journal |last1=White |first1=Tim D. |author-link1=Tim D. White |last2=Suwa |first2=Gen |last3=Simpson |first3=Scott |last4=Asfaw |first4=Berhane |author-link4=Berhane Asfaw |date=January 2000 |title=Jaws and teeth of ''Australopithecus afarensis'' from Maka, Middle Awash, Ethiopia |journal=[[American Journal of Physical Anthropology]] |volume=111 |issue=1 |pages=45–68 |doi=10.1002/(SICI)1096-8644(200001)111:1<45::AID-AJPA4>3.0.CO;2-I |issn=0002-9483 |pmid=10618588}}</ref>

===Pakicetids, ''Ambulocetus''===
{{Main|Evolution of cetaceans}}
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| caption1 =Reconstruction of ''[[Pakicetus]]''
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| caption2 =Skeleton of ''[[Ambulocetus natans]]''
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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&nbsp;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&nbsp;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 |pages=73–90 |doi=10.1146/annurev.ecolsys.33.020602.095426 |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''===
{{Main|Tiktaalik|Evolution of tetrapods}}
[[File:Tiktaalik Chicago.JPG|thumb|left|''[[Tiktaalik|Tiktaalik roseae]]'' had [[Spiracle (vertebrates)|spiracle]]s (air holes) above the eyes.]]
[[File:Tiktaalik BW.jpg|thumb|250px|Life restoration of ''Tiktaalik roseae'']]
''Tiktaalik'' is a genus of extinct [[Sarcopterygii|sarcopterygian]] (lobe-finned fish) from the Late [[Devonian]] period, with many features akin to those of tetrapods (four-legged animals).<ref name="Nature">{{cite journal |last1=Daeschler |first1=Edward B. |author-link1=Ted Daeschler |last2=Shubin |first2=Neil H. |author-link2=Neil Shubin |last3=Jenkins | first3=Farish A. Jr. |author-link3=Farish Jenkins |date=6 April 2006 |title=A Devonian tetrapod-like fish and the evolution of the tetrapod body plan |journal=Nature |volume=440 |issue=7085 |pages=757–763 |bibcode=2006Natur.440..757D |doi=10.1038/nature04639 |issn=0028-0836 |pmid=16598249 |doi-access=free }}</ref> It is one of several lines of ancient sarcopterygians to develop adaptations to the oxygen-poor shallow water habitats of its time—adaptations that led to the evolution of tetrapods.<ref name="scientificamerican">{{cite journal |last=Clack |first=Jennifer A. |author-link=Jenny Clack |date=December 2005 |title=Getting a Leg Up on Land |journal=Scientific American |volume=293 |pages=100–107 |issue=6 |bibcode=2005SciAm.293f.100C |doi=10.1038/scientificamerican1205-100 |issn=0036-8733 |pmid=16323697 }}</ref> Well-preserved fossils were found in 2004 on [[Ellesmere Island]] in [[Nunavut]], Canada.<ref>{{cite journal |last=Easton |first=John |date=23 October 2008 |title=''Tiktaalik's'' internal anatomy explains evolutionary shift from water to land |url=http://chronicle.uchicago.edu/081023/tiktaalik.shtml |journal=University of Chicago Chronicle |issue=3 |volume=28 |issn=1095-1237 |access-date=2012-04-19 }}</ref>

''Tiktaalik'' lived approximately 375&nbsp;million years ago. [[Paleontology|Paleontologist]]s suggest that it is representative of the transition between non-tetrapod vertebrates such as ''[[Panderichthys]]'', known from fossils 380&nbsp;million years old, and early tetrapods such as ''[[Acanthostega]]'' and ''[[Ichthyostega]]'', known from fossils about 365&nbsp;million years old. Its mixture of primitive fish and derived tetrapod characteristics led one of its discoverers, [[Neil Shubin]], to characterize ''Tiktaalik'' as a "[[Tetrapodomorpha|fishapod]]."<ref>{{cite news |last=Wilford |first=John Noble |author-link=John Noble Wilford |date=5 April 2006 |title=Scientists Call Fish Fossil the 'Missing Link' |url=https://www.nytimes.com/2006/04/05/science/05cnd-fossil.html |newspaper=[[The New York Times]] |access-date=2015-05-17}}</ref><ref>{{harvnb|Shubin|2008}}</ref> Unlike many previous, more fish-like transitional fossils, the "fins" of ''Tiktaalik'' have basic wrist bones and simple rays reminiscent of fingers. They may have been [[weight-bearing]]. Like all modern tetrapods, it had rib bones, a mobile neck with a separate pectoral girdle, and lungs, though it had the gills, scales, and fins of a fish.<ref name="Nature" />  However in a 2008 paper by Boisvert at al. it is noted that ''Panderichthys'', due to its more derived distal portion, might be closer to tetrapods than ''Tiktaalik'', which might have independently developed similarities to tetrapods by convergent evolution.<ref>{{cite web|url=http://uu.diva-portal.org/smash/get/diva2:221262/FULLTEXT01 |title=Pectoral fin info |publisher=uu.diva-portal.org |access-date=2021-05-09}}</ref>

Tetrapod footprints found in Poland and reported in ''[[Nature (journal)|Nature]]'' in January 2010 were "securely dated" at 10&nbsp;million years older than the oldest known [[Elpistostegalia|elpistostegids]]<ref>{{cite journal |last1=Niedźwiedzki |first1=Grzegorz |last2=Szrek |first2=Piotr |last3=Narkiewicz |first3=Katarzyna |last4=Narkiewicz |first4=Marek |last5=Ahlberg |first5=Per E. |author-link5=Per E. Ahlberg |date=7 January 2010 |title=Tetrapod trackways from the early Middle Devonian period of Poland |journal=Nature |volume=463 |issue=7227 |pages=43–48 |doi=10.1038/nature08623 |issn=0028-0836 |pmid=20054388 |display-authors=3 |bibcode=2010Natur.463...43N |s2cid=4428903 }}</ref> (of which ''Tiktaalik'' is an example), implying that animals like ''Tiktaalik'', possessing features that evolved around 400&nbsp;million years ago, were "late-surviving relics rather than direct transitional forms, and they highlight just how little we know of the earliest history of land vertebrates."<ref>{{cite journal |author=<!--Staff writer(s); no by-line.--> |date=7 January 2010 |title=Four feet in the past: trackways pre-date earliest body fossils |url=http://www.nature.com/nature/journal/v463/n7277/edsumm/e100107-01.html |type=Editor's summary |journal=Nature |volume=463 |issue=7227 |issn=0028-0836}}</ref>

===''Amphistium''===
{{Main|Flatfish#Evolution}}
[[File:Pseudopleuronectes americanus.jpg|thumb|left|Modern [[flatfish]] are asymmetrical, with both eyes on the same side of the head.]]
[[File:Amphistium.JPG|thumb|Fossil of ''[[Amphistium]]'' with one eye at the top-center of the head]]

[[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" />

===''Runcaria''===
[[File:Runcaria megasporangium and cupule drawing.jpg|thumb|80px|The [[Devonian]] fossil plant ''[[Runcaria]]'' resembles a seed but lacks a solid seed coat and means to guide pollen.]]
{{Main|Runcaria|Evolution of plants#Seeds}}

A Middle Devonian precursor to [[Spermatophyte|seed plant]]s has been identified from Belgium, predating the earliest seed plants by about 20&nbsp;million years. ''Runcaria'', small and radially symmetrical, is an integumented [[Sporangium|megasporangium]] surrounded by a [[Calybium and cupule|cupule]]. The megasporangium bears an unopened [[distal]] extension protruding above the multilobed [[Integument#Botanical usage|integument]]. It is suspected that the extension was involved in [[Anemophily|anemophilous pollination]]. ''Runcaria'' sheds new light on the sequence of character acquisition leading to the seed, having all the qualities of seed plants except for a solid [[seed coat]] and a system to guide the pollen to the seed.<ref>{{cite journal |last1=Gerrienne |first1=Philippe |last2=Meyer-Berthaud |first2=Brigitte |last3=Fairon-Demaret |first3=Muriel |last4=Streel |first4=Maurice |last5=Steemans |first5=Philippe |date=29 October 2004 |title=Runcaria, a Middle Devonian Seed Plant Precursor |journal=[[Science (journal)|Science]] |volume=306 |issue=5697 |pages=856–858 |bibcode=2004Sci...306..856G |doi=10.1126/science.1102491 |issn=0036-8075 |pmid=15514154 |s2cid=34269432 |display-authors=3 }}</ref>

==Fossil record==
{{See also|Taphonomy}}

Not every transitional form appears in the [[fossil record]], because the fossil record is not complete. Organisms are only rarely preserved as fossils in the best of circumstances, and only a fraction of such fossils have been discovered. Paleontologist [[Donald Prothero]] noted that this is illustrated by the fact that the number of species known through the fossil record was less than 5% of the number of known living species, suggesting that the number of species known through fossils must be far less than 1% of all the species that have ever lived.<ref name=Prothero2007pp5053>{{harvnb|Prothero|2007|pp=50–53}}</ref>

Because of the specialized and rare circumstances required for a biological structure to fossilize, logic dictates that known fossils represent only a small percentage of all life-forms that ever existed—and that each discovery represents only a snapshot of evolution. The transition itself can only be illustrated and corroborated by transitional fossils, which never demonstrate an exact half-way point between clearly divergent forms.<ref name="CC200">{{cite web |url=http://www.talkorigins.org/indexcc/CC/CC200.html |title=Claim CC200: Transitional fossils |date=5 November 2006 |editor-last=Isaak |editor-first=Mark |website=[[TalkOrigins Archive]] |publisher=The TalkOrigins Foundation, Inc. |location=Houston, TX |access-date=2009-04-30}}</ref>

The fossil record is very uneven and, with few exceptions, is heavily slanted toward organisms with hard parts, leaving most groups of [[soft-bodied organism]]s with little to no fossil record.<ref name=Prothero2007pp5053/> The groups considered to have a good fossil record, including a number of transitional fossils between traditional groups, are the vertebrates, the [[echinoderm]]s, the [[brachiopod]]s and some groups of [[arthropod]]s.<ref>{{harvnb|Donovan|Paul|1998}}</ref>

==History==
{{see also|History of paleontology|Timeline of paleontology}}

===Post-Darwin===
[[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:

<blockquote>Had the Solnhofen quarries been commissioned—by august command—to turn out a strange being à la Darwin—it could not have executed the behest more handsomely—than in the ''Archaeopteryx''.<ref>{{cite journal |last=Williams |first=David B. |date=September 2011 |title=Benchmarks: September 30, 1861: Archaeopteryx is discovered and described |url=http://www.earthmagazine.org/article/benchmarks-september-30-1861-archaeopteryx-discovered-and-described |journal=EARTH |issn=1943-345X |access-date=2012-02-24 |archive-url=https://web.archive.org/web/20120507072544/http://www.earthmagazine.org/article/benchmarks-september-30-1861-archaeopteryx-discovered-and-described |archive-date=7 May 2012  }}</ref></blockquote>

Thus, transitional fossils like ''Archaeopteryx'' came to be seen as not only corroborating Darwin's theory, but as icons of evolution in their own right.<ref>{{harvnb|Wellnhofer|2009}}</ref> For example, the Swedish [[encyclopedic dictionary]] ''Nordisk familjebok'' of 1904 showed an inaccurate ''Archaeopteryx'' reconstruction (see illustration) of the fossil, "ett af de betydelsefullaste paleontologiska fynd, som någonsin gjorts" ("one of the most significant paleontological discoveries ever made").<ref>{{harvnb|Leche|1904|pp=[https://runeberg.org/nfba/0758.html 1379–1380]}}</ref>

===The rise of plants===
Transitional fossils are not only those of animals. With the increasing mapping of the [[Phylum#Land plant phyla (divisions)|divisions]] of plants at the beginning of the 20th century, the search began for the ancestor of the [[vascular plant]]s. In 1917, [[Robert Kidston]] and [[William Henry Lang]] found the remains of an extremely primitive plant in the [[Rhynie chert]] in [[Aberdeenshire]], Scotland, and named it ''[[Rhynia]]''.<ref name="Kidston1917">{{cite journal |last1=Kidston |first1=Robert |author-link1=Robert Kidston |last2=Lang |first2=William Henry |author-link2=William Henry Lang |date=27 February 1917 |title=XXIV.—On Old Red Sandstone Plants showing Structure, from the Rhynie Chert Bed, Aberdeenshire. Part I. Rhynia Gwynne-Vaughanii, Kidston and Lang |url=https://www.biodiversitylibrary.org/item/130747#page/961/mode/1up |journal=Transactions of the Royal Society of Edinburgh |volume=51 |issue=3 |pages=761–784 |doi=10.1017/S0263593300006805 |s2cid=251580286 |issn=0080-4568 |oclc=704166643 |access-date=2015-05-18 }}</ref>

The ''Rhynia'' plant was small and stick-like, with simple [[Dichotomy|dichotomously]] branching stems without leaves, each tipped by a [[sporangium]]. The simple form echoes that of the [[sporophyte]] of [[mosses]], and it has been shown that ''Rhynia'' had an [[alternation of generations]], with a corresponding [[gametophyte]] in the form of crowded tufts of diminutive stems only a few millimetres in height.<ref name="Kerpetal2004">{{cite journal |last1=Kerp |first1=Hans |last2=Trewin |first2=Nigel H. |last3=Hass |first3=Hagen |year=2003 |title=New gametophytes from the Early Devonian Rhynie chert |journal=Transactions of the Royal Society of Edinburgh: Earth Sciences |volume=94 |issue=4 |pages=411–428 |doi=10.1017/S026359330000078X |s2cid=128629425 |issn=0080-4568 }}</ref> ''Rhynia'' thus falls midway between mosses and early vascular plants like [[fern]]s and [[Lycopodiopsida|clubmoss]]es. From a carpet of moss-like gametophytes, the larger ''Rhynia'' sporophytes grew much like simple clubmosses, spreading by means of horizontal growing stems growing [[rhizoid]]s that anchored the plant to the substrate. The unusual mix of moss-like and vascular traits and the extreme structural simplicity of the plant had huge implications for botanical understanding.<ref>{{harvnb|Andrews|1967|p=32}}</ref>

==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 pedigree.jpg|thumb|right|The human pedigree back to [[amoeba]] shown as a reinterpreted [[Great chain of being|chain of being]] with living and fossil animals. From G. Avery's critique of [[Ernst Haeckel]], 1873.]]

<!-- 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>

After ''On the Origin of Species'', the idea of "lower animals" representing earlier stages in evolution lingered, as demonstrated in [[Ernst Haeckel]]'s figure of the human pedigree.{{sfn|Haeckel|2011|p=216}} While the vertebrates were then seen as forming a sort of evolutionary sequence, the various [[class (biology)|classes]] were distinct, the undiscovered intermediate forms being called "missing links."

The term was first used in a scientific context by [[Charles Lyell]] in the third edition (1851) of his book ''Elements of Geology'' in relation to missing parts of the [[Geologic time scale|geological column]], but it was popularized in its present meaning by its appearance on page xi of his book ''[[Geological Evidences of the Antiquity of Man]]'' of 1863. By that time, it was generally thought that the end of the [[last glacial period]] marked the first appearance of humanity; Lyell drew on new findings in his ''Antiquity of Man'' to put the origin of human beings much further back. Lyell wrote that it remained a profound mystery how the huge gulf between man and beast could be bridged.<ref>{{cite journal |last=Bynum |first=William F. |date=Summer 1984 |title=Charles Lyell's ''Antiquity of Man'' and its critics |journal=Journal of the History of Biology |volume=17 |issue=2 |pages=153–187 |doi=10.1007/BF00143731 |jstor=4330890 |s2cid=84588890 |issn=0022-5010 }}</ref> Lyell's vivid writing fired the public imagination, inspiring [[Jules Verne]]'s ''[[Journey to the Center of the Earth]]'' (1864) and [[Louis Figuier]]'s 1867 second edition of ''La Terre avant le déluge'' ("Earth before the Flood"), which included dramatic illustrations of savage men and women wearing animal skins and wielding stone axes, in place of the [[Garden of Eden]] shown in the 1863 edition.{{sfn|Browne|2003|pp=130, 218, 515}}

The search for a fossil showing transitional traits between apes and humans, however, was fruitless until the young Dutch geologist [[Eugène Dubois]] found a skullcap, a molar and a [[femur]] on the banks of [[Solo River]], [[Java]] in 1891. The find combined a low, ape-like [[skull roof]] with a brain estimated at around 1000&nbsp;cc, midway between that of a chimpanzee and an adult human. The single molar was larger than any modern human tooth, but the femur was long and straight, with a knee angle showing that "[[Java Man]]" had walked upright.<ref>{{harvnb|Swisher|Curtis|Lewin|2001}}</ref> Given the name ''[[Homo erectus|Pithecanthropus erectus]]'' ("erect ape-man"), it became the first in what is now a long [[list of human evolution fossils]]. At the time it was hailed by many as the "missing link," helping set the term as primarily used for human fossils, though it is sometimes used for other intermediates, like the dinosaur-bird intermediary ''[[Archaeopteryx]]''.<ref>{{harvnb|Reader|2011}}</ref><ref>{{cite web |last=Benton |first=Michael J. |author-link=Michael Benton |date=March 2001 |title=Evidence of Evolutionary Transitions |url=http://www.actionbioscience.org/evolution/benton2.html |website=[[actionbioscience]] |publisher=[[American Institute of Biological Sciences]] |location=Washington, D.C. |access-date=2012-03-29 |archive-date=26 April 2012 |archive-url=https://web.archive.org/web/20120426203938/http://www.actionbioscience.org/evolution/benton2.html  }}</ref> 
[[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==
{{main|Punctuated equilibrium}}

The theory of punctuated equilibrium developed by [[Stephen Jay Gould]] and [[Niles Eldredge]] and first presented in 1972<ref>{{harvnb|Eldredge|Gould|1972|pp=82–115}}</ref> is often mistakenly drawn into the discussion of transitional fossils.<ref>{{cite journal |last=Bates |first=Gary |date=December 2006 |title=That quote!—about the missing transitional fossils |url=http://creation.com/that-quote-about-the-missing-transitional-fossils |journal=Creation |volume=29 |issue=1 |pages=12–15 |issn=0819-1530 |access-date=2014-07-06}}
*{{cite web |url=http://www.talkorigins.org/faqs/patterson.html |title=Patterson Misquoted: A Tale of Two 'Cites' |last=Theunissen |first=Lionel |date=24 June 1997 |website=[[TalkOrigins Archive]] |publisher=The TalkOrigins Foundation, Inc. |location=Houston, TX |access-date=2015-05-19}}</ref> This theory, however, pertains only to well-documented transitions within taxa or between closely related taxa over a geologically short period of time. These transitions, usually traceable in the same geological outcrop, often show small jumps in morphology between extended periods of morphological stability. To explain these jumps, Gould and Eldredge envisaged comparatively long periods of genetic stability separated by periods of rapid evolution. Gould made the following observation concerning creationist misuse of his work to deny the existence of transitional fossils:

{{blockquote|Since we proposed punctuated equilibria to explain trends, it is infuriating to be quoted again and again by [[Creationism|creationists]]—whether through design or stupidity, I do not know—as admitting that the fossil record includes no transitional forms. The punctuations occur at the level of species; directional trends (on the staircase model) are rife at the higher level of transitions within major groups. | [[Stephen Jay Gould]] |''[[The Panda's Thumb (book)|The Panda's Thumb]]''{{sfn|Gould|1980|p=189}}}}

==See also==
* [[Crocoduck]]
* [[Evidence of common descent]]
* [[Missing link (human evolution)|Missing link]]
* [[Speciation]]

==References==
{{Reflist}}

==Sources==
{{Refbegin}}
* {{cite book |last=Andrews | first=Henry N. Jr. |title=Studies in Paleobotany |year=1967 |orig-date=Originally published 1961 |edition=Reprint |others=Chapter on palynology by Charles J. Felix |location=New York |publisher=[[John Wiley & Sons]] |lccn=61006768 |oclc=12877482 }}
* {{cite book |last=Benton |first=Michael J. |author-link=Michael Benton |year=1997 |edition=2nd |title=Vertebrate Palaeontology |location=London |publisher=[[Chapman & Hall]] |isbn=978-0-412-73810-4 |oclc=37378512 |title-link=Vertebrate Palaeontology (Benton) }}
* {{cite book |last=Browne |first=Janet |author-link=Janet Browne |year=2003 |orig-date=Originally published 2002 |title=Charles Darwin: The Power of Place |volume=2 |location=London |publisher=[[Jonathan Cape]] |isbn=978-0-7126-6837-8 |oclc=806284755 }}
* {{cite book |last1=Castro |first1=Peter |last2=Huber |first2=Michael E. |year=2003 |title=Marine Biology |others=Original art work by William Ober and Claire Garrison |edition=4th |location=New York |publisher=[[McGraw Hill Education|McGraw-Hill]] |isbn=978-0-07-029421-9 |lccn=2002190248 |oclc=49259996 |url=https://archive.org/details/marinebiology00pete }}
* {{cite book |last1=Chapleau |first1=François |last2=Amaoka |first2=Kunio |year=1998 |chapter=Flatfishes |editor1-last=Paxton |editor1-first=John R. |editor2-last=Eschmeyer |editor2-first=William M. |title=Encyclopedia of Fishes |others=Illustrations by David Kirshner |edition=2nd |location=San Diego, CA |publisher=[[Academic Press]] |isbn=978-0-12-547665-2 |lccn=98088228 |oclc=39641701 }}
* {{cite book |last=Darwin |first=Charles |author-link=Charles Darwin |year=1859 |title=On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life |edition=1st |location=London |publisher=[[John Murray (publishing house)|John Murray]] |lccn=06017473 |oclc=741260650 |title-link=On the Origin of Species }} The book is available from [http://darwin-online.org.uk/content/frameset?pageseq=1&itemID=F373&viewtype=side The Complete Work of Charles Darwin Online]. Retrieved 2015-05-13.
* {{cite book |editor1-last=Donovan |editor1-first=Stephen K. |editor1-link=Stephen Donovan |editor2-last=Paul |editor2-first=Christopher R. C. |year=1998 |title=The Adequacy of the Fossil Record |location=Chichester; New York |publisher=John Wiley & Sons |isbn=978-0-471-96988-4 |lccn=98010110 |oclc=38281286 }}
* {{cite book |last1=Eldredge |first1=Niles |author-link1=Niles Eldredge |last2=Gould |first2=Stephen Jay |author-link2=Stephen Jay Gould |year=1972 |chapter=Punctuated equilibria: an alternative to phyletic gradualism |editor1-last=Schopf |editor1-first=Thomas J. M. |title=Models in Paleobiology |location=San Francisco, CA |publisher=Freeman, Cooper |isbn=978-0-87735-325-6 |lccn=72078387 |oclc=572084 }}
* {{cite book |last1=Freeman |first1=Scott |last2=Herron |first2=Jon C. |year=2004 |title=Evolutionary Analysis |edition=3rd |location=Upper Saddle River, NJ |publisher=[[Pearson Education]] |isbn=978-0-13-101859-4 |lccn=2003054833 |oclc=52386174 |url=https://archive.org/details/prenticehallguid00meli }}
* {{cite book |last1=Gingerich |first1=Philip D. |author-link1=Philip D. Gingerich |last2=Russell |first2=Donald E. |year=1981 |title=Pakicetus inachus, a New Archaeocete (Mammalia, Cetacea) From the Early-Middle Eocene Kuldana Formation of Kohat (Pakistan) |url=http://deepblue.lib.umich.edu/bitstream/handle/2027.42/48501/ID352.pdf?sequence=2&isAllowed=y|type=Research report |series=Contributions from the Museum of Paleontology |volume=25 |issue=11 |location=Ann Arbor, MI |publisher=[[University of Michigan Museum of Natural History|Museum of Paleontology, University of Michigan]] |pages=235–246 |issn=0097-3556 |lccn=82621252 |oclc=8263404 }}
* {{cite book |last=Gould |first=Stephen Jay |author-link=Stephen Jay Gould |year=1980 |title=The Panda's Thumb: More Reflections in Natural History |edition=1st |location=New York |publisher=[[W. W. Norton & Company]] |isbn=978-0-393-01380-1 |lccn=80015952 |oclc=6331415 |title-link=The Panda's Thumb (book) }}
* {{cite book |last=Haeckel |first=Ernst |author-link=Ernst Haeckel |title=The Evolution of Man |year=2011 |orig-date=Originally published 1912; London: [[Charles Watts (secularist)|Watts & Co.]] |volume=1 |others=Translated from the German by [[Joseph McCabe]] |edition=5th enlarged |location=Hamburg, Germany |publisher=Tredition Classics |isbn=978-3-8424-6302-8 |oclc=830523724 }}
* {{cite book |last=Lamarck |first=Jean-Baptiste |author-link=Jean-Baptiste Lamarck |year=1815–1822 |title=Histoire naturelle des animaux sans vertèbres |language=fr |location=Paris |publisher=Verdière |lccn=07018340 |oclc=5269931 }}
* {{cite book |last=Lovejoy |first=Arthur O. |author-link=Arthur Oncken Lovejoy |year=1936 |title=The Great Chain of Being: A Study of the History of an Idea |url=https://archive.org/details/greatchainofbein0000love |url-access=registration |series=William James Lectures, 1933 |location=Cambridge, MA |publisher=[[Harvard University Press]] |lccn=36014264 |oclc=192226 }}
* {{cite encyclopedia |last=Leche |first=V. |editor-last=Meijer |editor-first=Bernhard |encyclopedia=[[Nordisk familjebok]] |title=Archæopteryx |language=sv |edition=New, revised and richly illustrated |year=1904 |publisher=Nordisk familjeboks förlags aktiebolag |location=Stockholm |lccn=15023737 |oclc=23562281 }}
* {{cite book |last=Prothero |first=Donald R. |author-link=Donald Prothero |year=2007 |title=Evolution: What the Fossils Say and Why it Matters |others=Original illustrations by Carl Buell |location=New York |publisher=[[Columbia University Press]] |isbn=978-0-231-13962-5 |lccn=2007028804 |oclc=154711166 |url=https://archive.org/details/evolutionwhatfos00prot_0 }}
* {{cite book |last=Reader |first=John |year=2011 |title=Missing Links: In Search of Human Origins |others=Foreword by Andrew Hill |edition=Enlarged and updated |location=Oxford; New York |publisher=[[Oxford University Press]] |isbn=978-0-19-927685-1 |lccn=2011934689 |oclc=707267298 |url-access=registration |url=https://archive.org/details/missinglinksinse0000read }}
* {{cite book |last=Shubin |first=Neil |author-link=Neil Shubin |year=2008 |title=Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body |location=New York |publisher=[[Pantheon Books]] |isbn=978-0-375-42447-2 |lccn=2007024699 |oclc=144598195 |url=https://archive.org/details/yourinnerfishjou00shub_0 }}
* {{cite book |last1=Swisher |first1=Carl C. III |last2=Curtis |first2=Garniss H. |author-link2=Garniss Curtis |last3=Lewin |first3=Roger |author-link3=Roger Lewin |year=2001 |orig-date=Originally published 2000 |title=Java Man: How Two Geologists Changed Our Understanding of Human Evolution |location=Chicago, IL |publisher=[[University of Chicago Press]] |isbn=978-0-226-78734-3 |lccn=2001037337 |oclc=48066180 }}
* {{cite book |last=Wellnhofer |first=Peter |author-link=Peter Wellnhofer |year=2004 |chapter=The Plumage of ''Archaeopteryx'': Feathers of a Dinosaur? |editor1-last=Currie |editor1-first=Philip J. |editor1-link=Philip J. Currie |editor2-last=Koppelhus |editor2-first=Eva B. |editor3-last=Shugar |editor3-first=Martin A. |display-editors = 3 |editor4-last=Wright |editor4-first=Joanna L. |title=Feathered Dragons: Studies on the Transition from Dinosaurs to Birds |series=Life of the Past |location=Bloomington, IN |publisher=[[Indiana University Press]] |isbn=978-0-253-34373-4 |lccn=2003019035 |oclc=52942941 }}
* {{cite book |last=Wellnhofer |first=Peter |title=Archaeopteryx: The Icon of Evolution |year=2009 |others=Translated by Frank Haase; foreword by [[Luis M. Chiappe]] |edition=Revised English edition of the 1st German |location=München |publisher=Verlag Dr. Friedrich Pfeil |isbn=978-3-89937-108-6 |oclc=501736379 }}
{{Refend}}

==External links==
* {{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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{{Good article}}

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