Transitional fossil: Difference between revisions

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{{Main|Origin of birds}}

[[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 |authorlink1=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=harv}}</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. |authorlink1=Gregory M. Erickson |last2=Rauhut |first2=Oliver W. M. |author3=Zhonghe Zhou |authorlink3=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 |ref=harv}}</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 [[Dromaeosauridae#Deinonychosauria|deinonychosaur]]s ([[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. |authorlink=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=harv}}</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 |accessdate=2006-10-18}}</ref> making it important in the study both of dinosaurs and of the origin of birds.

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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 |authorlink=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 |accessdate=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=harv}}</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 |ref=harv|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=harv}}</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 |url=http://bioscience.oxfordjournals.org/content/51/12/1037.full.pdf |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 |accessdate=2015-05-16 |ref=harv}}</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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''Tiktaalik'' lived approximately 375 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 million years old, and early tetrapods such as ''[[Acanthostega]]'' and ''[[Ichthyostega]]'', known from fossils about 365 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 |authorlink=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]] |accessdate=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" />

Tetrapod footprints found in Poland and reported in ''[[Nature (journal)|Nature]]'' in January 2010 were "securely dated" at 10 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. |authorlink5=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 |ref=harv|bibcode=2010Natur.463...43N |s2cid=4428903 }}</ref> (of which ''Tiktaalik'' is an example), implying that animals like ''Tiktaalik'', possessing features that evolved around 400 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''===

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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]] |archiveurl=https://web.archive.org/web/20080804234422/http://news.nationalgeographic.com/news/2008/07/080709-evolution-fish.html|archivedate=4 August 2008|accessdate=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=harv}}</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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{{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 million years. ''Runcaria'', small and radially symmetrical, is an integumented [[Sporangium|megasporangium]] surrounded by a [[Calybium and cupule|cupule]]. The megasporangium bears an unopened [[Anatomical terms of ___location#Proximal and distal|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=harv}}</ref>

==Fossil record==

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[[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 |ref=harv|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 primitive [[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 |accessdate=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 |url-status=dead }}</ref></blockquote>

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[[File:Pithecanthropus-erectus.jpg|thumb|left|"[[Java Man]]" or ''Pithecanthropus erectus'' (now ''[[Homo erectus]]''), the original "missing link" found in Java in 1891–92.]]

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 is generated in the form of the simplest creatures constantly, and then strive [[Orthogenesis|towards complexity and perfection]] (i.e. humans) through a 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=harv}}</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, but Lyell drew on new findings in his ''Antiquity of Man'' to put the origin of human beings much further back in the deep geological past. 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=harv}}</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 idea of a "missing link" between humans and so-called "lower" animals remains lodged in the public imagination.<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 |accessdate=2015-05-19}}</ref> 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. |authorlink=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. |accessdate=2012-03-29}}</ref>