Editing Transitional fossil (section)

==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}}
{{multiple image
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| image1 =Pakicetus BW.jpg
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| caption1 =Reconstruction of ''[[Pakicetus]]''
| image2 =Ambulocetus natans.jpg
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| caption2 =Skeleton of ''[[Ambulocetus natans]]''
}}

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>