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Adding Origin of birds and Archaeopteryx, which is one of the most celebrated '''transitional fossils'''
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==Examples==
==Examples==
[[File:Archaeopteryx lithographica (Berlin specimen).jpg|thumb|right|The Berlin specimen]]
{{Main|List of transitional fossils}}
{{Main|List of transitional fossils}}

===Evolution of birds===
{{Main|Origin of birds}}

''[[Archaeopteryx]]'' is a [[genus]] of [[theropod]] [[dinosaur]] that is closely related to [[bird]]. Since the late 19th century, it has been generally accepted by palaeontologists, and celebrated in lay reference works, as being the oldest known [[bird]], though some more recent studies have cast doubt on this assessment, finding that it is instead a non-avialan dinosaur closely related to the origin of birds.<ref name=Xiaotingia>{{cite journal |title=An ''Archaeopteryx''-like theropod from China and the origin of Avialae |url=http://www.nature.com/nature/journal/v475/n7357/full/nature10288.html |date=28 July 2011 |journal=Nature |volume=475 |pages=465–470 |doi=10.1038/nature10288 |issue=7357 |author=Xing Xu, Hailu You, Kai Du and Fenglu Han |pmid=21796204}}</ref>

''Archaeopteryx'' lived in the [[Late Jurassic]] [[Period (geology)|Period]] around 150&nbsp;million years ago, in what is now southern [[Germany]] during a time when [[Europe]] was an archipelago of islands 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|last=Erickson|first=Gregory M.|coauthors=Rauhut, Oliver W. M., Zhou, Zhonghe, Turner, Alan H, Inouye, Brian D. Hu, Dongyu, Norell, Mark A.|year=2009|title=Was Dinosaurian Physiology Inherited by Birds? Reconciling Slow Growth in ''Archaeopteryx''|journal=PLoS ONE|volume=4|issue=10|page= e7390|url=http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0007390;jsessionid=F7E462DE00439EEA45BCC1AF96012EE0|accessdate=2009-10-25|doi=10.1371/journal.pone.0007390|pmid=19816582|bibcode = 2009PLoSO...4.7390E|editor1-last=Desalle|editor1-first=Robert|pmc=2756958 }}</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 bird]]s. In particular, it shares the following features with the [[deinonychosaur]]s ([[dromaeosaur]]s and [[troodontid]]s): jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes ("killing claw"), feathers (which also suggest [[homeothermy]]), and various skeletal features.<ref name="Yalden 1">{{cite journal |author=Yalden D.W. |title=What size was ''Archaeopteryx''? |journal=Zoological Journal of the Linnean Society|volume=82 |issue=1–2|pages=177–188 |year=1984 |url=http://www3.interscience.wiley.com/journal/120801944/abstract |doi=10.1111/j.1096-3642.1984.tb00541.x}}</ref>

The features above make ''Archaeopteryx'' a clear candidate for a transitional fossil between dinosaurs and birds.<ref name="UCal MoP">[http://www.ucmp.berkeley.edu/diapsids/birds/archaeopteryx.html ''Archaeopteryx'': An Early Bird] - [[University of California, Berkeley]] Museum of Paleontology. Retrieved 2006-OCT-18</ref><ref name="UCalg Long">[http://www.ucalgary.ca/~longrich/archaeopteryx.html ''Archaeopteryx lithographica''] - Nick Longrich, [[University of Calgary]]. Discusses how many wings an ''Archaeopteryx'' had and other questions.</ref> Thus, ''Archaeopteryx'' plays an important role not only in the study of the [[origin of birds]] but in the study of dinosaurs.

It was named from a [[feather]] in 1861. That same year, the first complete specimen of ''Archaeopteryx'' was announced. Over the years, ten more fossils of ''Archaeopteryx'' have surfaced. Most of these eleven fossils include impressions of feathers—among the oldest direct evidence of such structures. Moreover, because these feathers are of an advanced form ([[flight feather]]s), these fossils are evidence that the evolution of feathers began before the Late Jurassic.<ref name="weln 04">{{cite book|title=Feathered Dragons|year=2004|chapter=The Plumage of ''Archaeopteryx''|editor=Currie PJ, Koppelhus EB, Shugar MA, Wright JL|author=Wellnhofer P|pages=282–300|publisher=Indiana University Press|isbn=0-253-34373-9}}</ref>


===Evolution of cetaceans===
===Evolution of cetaceans===

Revision as of 23:11, 16 February 2012

The London specimen of Archaeopteryx, discovered only two years after the publication of On the Origin of Species
"Missing link" redirects here. For other uses, see Missing Link.

A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages (clades) diverge. It will have characteristics typical of organisms on both sides of the split, but because of the incompleteness of the fossil record, there is usually no way to know exactly how close it is to the actual point of divergence.

Transitional fossils serve as a reminder that taxonomic divisions are human constructs that have been imposed in hindsight on a continuum of variation. Numerous examples exist, including those between humans and other primates, tetrapods and fish, and birds and dinosaurs. The phrase "missing link" has been used extensively in popular writings on human evolution to refer to a perceived gap in the 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 is misleading and inaccurate.

History

Darwin

In 1859, when Charles Darwin's On the Origin of Species was first published, the fossil record was poorly known, and Darwin described the lack of transitional fossils as "the most obvious and gravest objection which can be urged against my theory", but explained it by relating it to the extreme imperfection of the geological record.[1] He noted the limited collections available at that time, but described the available information as showing patterns which followed from his theory of descent with modification through natural selection.[2] Indeed, Archaeopteryx was discovered just two years later, in 1861, and represents a classic transitional form between dinosaurs and birds. Many more transitional fossils have been discovered since then and it is now considered that there is abundant evidence of how all the classes of vertebrates are related, much of it in the form of transitional fossils.[3]

A popular term used to designate transitional forms is "missing links". The term "missing links" was first used by Charles Lyell in in his Elements of Geology of 1851, but was popularized in its present meaning by its appearance in Lyell's Geological Evidences of the Antiquity of Man of 1863, p. xi. By that time geologists had abandoned a literal Biblical account and it was generally thought that the end of the last glacial period marked the first appearance of humanity, a view Lyell's Elements presented. His Antiquity of Man drew on new findings to put the origin of human beings much further back in the deep geological past. Lyell's vivid writing fired the public imagination, inspiring Jules Verne's Journey to the Center of the Earth, and Louis Figuier's 1867 second edition of La Terre avant le déluge 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.[4]

The idea of a "missing link" between humans and so-called "lower" animals remains lodged in the public imagination.[5] The concept has been fueled by the successive discoveries of Australopithecus africanus (Taung Child), Australopithecus sediba,[6][7] Homo erectus (Peking Man, Java Man, Turkana boy), and other Hominina fossils.[8][9]The term tends to be used in the popular media, but is avoided in the scientific press as it relates to the links in the great chain of being, a static pre-evolutionary concept now abandoned. In reality, the discovery of more and more transitional fossils continues to add to knowledge of evolutionary transitions,[3][10].

Evolutionary taxonomy and cladistics

Main article: Cladistics

In evolutionary taxonomy, the prevailing form of taxonomy during much of the 20th century and still used in basal textbooks, taxa based on morphological similarity are often drawn as "bubbles" branching off from each other, forming evolutionary trees.[11] Transitional forms, are seen as falling between the various groups in term of anatomy, and are placed at the borders of these.

With the establishment of cladistic methods, relationships are now strictly expressed in so-called cladograms, illustrating the branching of the evolutionary lineages. The different so-called 'natural' or 'monophyletic' groups form nested units that do not overlap. Within cladistics there is thus no longer a transition between established groups, but a differentiation that occurs within groups, represented as a branching in the cladogram. In this context, transitional organisms can be conceptualized as representing early examples on the different branches of a cladogram, lying between a particular branching point and the "crown-group", i.e. the most-derived group, which is placed at the end of a lineage.

Transitional vs ancestral

A source of confusion is the concept that a transitional form between two different taxonomic group must be directly ancestral to one or both groups. This was exacerbated by the fact that one of the goals of evolutionary taxonomy was the attempt to identify taxa that were ancestral to other taxa. However, it is almost impossible to be sure that any form represented in the record is actually a direct ancestor of any other. In fact because evolution is a branching process that produces a complex bush pattern of related species rather than a linear process that produces a ladder like progression, and because of the incompleteness the fossil record, it is unlikely that any particular form represented in the fossil record is a direct ancestor of any other. Cladistics deemphasized the concept of one taxonmic group being an ancestor of another, and instead emphasizes the concept of identifying sister taxa that share a common ancestor with one another more recently than they do with other groups. There are a few exceptional cases, such as some marine plankton micro-fossils, 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, 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.[12]

Comparison to 'intermediate' forms

The terms 'transitional' and 'intermediate' are for the most part used as synonyms; however, a distinction between the two can be made:

  • "Transitional" can be used for those forms that do not have a significant number of unique derived traits that the derived relative does not possess as well. In other words, a transitional organism is morphologically close to the actual common ancestor it shares with its more derived relative.
  • "Intermediate" can be used for those forms that do have a large number of uniquely derived traits not connected to its derived relative.

According to this definition, Archaeopteryx, which does not show any derived traits that more derived birds do not possess as well, is transitional. In contrast, the platypus is intermediate because it retains certain reptilian traits no longer found in modern mammals and also possesses derived traits of a highly specialized aquatic animal.

Following this definition, all living organisms are in fact to be regarded as intermediate forms when they are compared to some other related life-form. Indeed there are many species alive today that can be considered to be intermediate between two or more groups.

Examples

The Berlin specimen
Main article: List of transitional fossils

Evolution of birds

Main article: Origin of birds

Archaeopteryx is a genus of theropod dinosaur that is closely related to bird. Since the late 19th century, it has been generally accepted by palaeontologists, and celebrated in lay reference works, as being the oldest known bird, though some more recent studies have cast doubt on this assessment, finding that it is instead a non-avialan dinosaur closely related to the origin of birds.[13]

Archaeopteryx lived in the Late Jurassic Period around 150 million years ago, in what is now southern Germany during a time when Europe was an archipelago of islands 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,[14] Archaeopteryx could grow to about 0.5 metres (1.6 ft) in length. Despite its small size, broad wings, and inferred ability to fly or glide, Archaeopteryx has more in common with other small Mesozoic dinosaurs than it does with modern birds. In particular, it shares the following features with the deinonychosaurs (dromaeosaurs and troodontids): jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes ("killing claw"), feathers (which also suggest homeothermy), and various skeletal features.[15]

The features above make Archaeopteryx a clear candidate for a transitional fossil between dinosaurs and birds.[16][17] Thus, Archaeopteryx plays an important role not only in the study of the origin of birds but in the study of dinosaurs.

It was named from a feather in 1861. That same year, the first complete specimen of Archaeopteryx was announced. Over the years, ten more fossils of Archaeopteryx have surfaced. Most of these eleven fossils include impressions of feathers—among the oldest direct evidence of such structures. Moreover, because these feathers are of an advanced form (flight feathers), these fossils are evidence that the evolution of feathers began before the Late Jurassic.[18]

Evolution of cetaceans

Main article: Evolution of cetaceans
Reconstruction of Pakicetus
Reconstruction of Ambulocetus natans

The cetaceans (whales, dolphins and porpoises) are marine mammal descendants of land mammals. The pakicetids are hoofed mammals that are the earliest whales, with Indohyus from family Raoellidae being the closest sister group.[19] [20] They lived in the early Eocene, around 53 million years ago. Their fossils were first discovered in North Pakistan in 1979, located at a river not far from the shores of former Tethys Sea.[21]

In 1994, Ambulocetus natans, which lived about 49 million years ago, was discovered in Pakistan. It was probably amphibious, and resembled the crocodile in its physical appearance.[22] However, pakicetids were able to listen underwater, by using enhanced bone conduction, rather than depending on tympanic membrane like general land mammals. This method of hearing does not give directional hearing underwater.[23] In the Eocene, ambulocetids inhabited the bays and estuaries of the Tethys Ocean in northern Pakistan.[24] ]] The fossils of ambulocetids are always found in near-shore shallow marine deposits associated with abundant marine plant fossils and littoral molluscs.[24] Although they are found only in marine deposits, their oxygen isotope values indicate that they consumed a range of water with different degree of salinity, with some specimens having no evidence of sea water consumption and others did not ingest fresh water at the time when their teeth are fossilized. It is clear that ambulocetids tolerated a wide range of salt concentrations.[25] Their diet probably included land animals that approached water for drinking or some freshwater aquatic organisms that lived in the river.[24] Hence, ambulocetids represent the transition phase of cetacean ancestors between fresh water and marine habitat.

Evolution of tetrapods

Main article: Tetrapods § Evolution
Life restoration of Tiktaalik roseae made for the National Science Foundation

Tiktaalik is a genus of extinct sarcopterygian (lobe-finned "fish") from the late Devonian period, with many features akin to those of tetrapods (four-legged animals).[26] It is an example from several lines of ancient sarcopterygian "fish" developing adaptations to the oxygen-poor shallow-water habitats of its time, which led to the evolution of tetrapods.[27] Well-preserved fossils were found in 2004 on Ellesmere Island in Nunavut, Canada.

Tiktaalik lived approximately 375 million years ago. Paleontologists suggest that it is representative of the transition between non-tetrapod vertebrates ("fish") 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 "fishapod".[28][29][30]

Evolution of the horse

Main article: Evolution of the horse

The reconstruction of the evolution of the horse and its relatives assembled by Othniel Charles Marsh from surviving fossils that form a single, consistently developing lineage with many "transitional" types, is often cited as a family tree. However, modern cladistics gives a different, multi-stemmed shrublike picture, with multiple innovations and many dead ends.

Evolution of seeding plants

Main article: Evolutionary history of plants § Seeds

A middle Devonian precursor to seed plants from Belgium has been identified predating the earliest seed plants by about 20 million years. Runcaria, small and radially symmetrical, is an integumented megasporangium surrounded by a cupule. The megasporangium bears an unopened distal extension protruding above the mutlilobed integument. It is suspected that the extension was involved in anemophilous pollination. Runcaria sheds new light on the sequence of character acquisition leading to the seed. Runcaria has all of the qualities of seed plants except for a solid seed coat and a system to guide the pollen to the seed.[31]

Limitations of the fossil record

Not every transitional form appears in the fossil record because the fossil record is nowhere near complete. Organisms are only rarely preserved as fossils in the best of circumstances and only a fraction of such fossils have ever been discovered. The paleontologist Donald Prothero noted that this is illustrated by the fact that the total number of species of all kinds known through the fossil record was less than 5% of the number of known living species, which suggests that the number of species known through fossils must be less than 1% of all the species that have ever lived.[32]

Due to the specialized and rare circumstances required for a biological structure to fossilize, only a very small percentage of all life-forms that ever have existed can be expected to be represented in discoveries and each represents only a snapshot of the process of evolution. The transition itself can only be illustrated and corroborated by transitional fossils, but it will never demonstrate an exact half-way point between clearly divergent forms.[33]

The fossil record is very uneven, and with few exceptions are heavily slanted toward organisms with hard parts, leaving most groups of soft-bodied organisms with little to no fossil record.[32] The groups considered to have a good fossil record, including a number of transitional fossils between traditional groups, are the vertebrates, the echinoderms, brachiopods and some groups of arthropods.[34]

Punctuated equilibrium

Main article: Punctuated equilibrium

The theory of punctuated equilibrium developed by Stephen Jay Gould and Niles Eldredge and first presented in 1972[35] is often mistakenly drawn into the discussion of transitional fossils. 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 of creationist misuse of his work to deny the existence of transitional fossils:

"Since we proposed punctuated equilibria to explain trends, it is infuriating to be quoted again and again by 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[36]

See also

Footnotes

  1. ^ Darwin 1859, pp. 279–280
  2. ^ Darwin 1859, pp. 341–343
  3. ^ a b Prothero, D (27 February 2008). "Evolution: What missing link?" (2645). New Scientist: 35–40. {{cite journal}}: Cite journal requires |journal= (help); Invalid |ref=harv (help)
  4. ^ Browne 2002, pp. 130, 218, 515
  5. ^ "Why the term "missing links" is inappropriate". Hoxful Monsters. 10 June 2009. Retrieved 10 September 2011.
  6. ^ "The 'missing link' - scientists discover our 'earliest' ancestors". The Telegraph. 10 September 2011. Retrieved 10 September 2011.
  7. ^ Carl Zimmer (8 September 2011). "The Verge of Human". Discover Magazine. Retrieved 10 September 2011.
  8. ^ "It's not a missing link". CBC News. 9 April 2010. Retrieved 10 September 2011.
  9. ^ Carl Zimmer (19 March 2009). "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 for several weeks…". Discover Magazine. Retrieved 10 September 2011.
  10. ^ E.g. Bentons Vertebrate Palaeontology, 2nd edition, 1997
  11. ^ Prothero 2007, pp. 133–135
  12. ^ Xing Xu, Hailu You, Kai Du and Fenglu Han (28 July 2011). "An Archaeopteryx-like theropod from China and the origin of Avialae". Nature. 475 (7357): 465–470. doi:10.1038/nature10288. PMID 21796204.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  13. ^ Erickson, Gregory M. (2009). Desalle, Robert (ed.). "Was Dinosaurian Physiology Inherited by Birds? Reconciling Slow Growth in Archaeopteryx". PLoS ONE. 4 (10): e7390. Bibcode:2009PLoSO...4.7390E. doi:10.1371/journal.pone.0007390. PMC 2756958. PMID 19816582. Retrieved 25 October 2009. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: unflagged free DOI (link)
  14. ^ Yalden D.W. (1984). "What size was Archaeopteryx?". Zoological Journal of the Linnean Society. 82 (1–2): 177–188. doi:10.1111/j.1096-3642.1984.tb00541.x.
  15. ^ Archaeopteryx: An Early Bird - University of California, Berkeley Museum of Paleontology. Retrieved 2006-OCT-18
  16. ^ Archaeopteryx lithographica - Nick Longrich, University of Calgary. Discusses how many wings an Archaeopteryx had and other questions.
  17. ^ Wellnhofer P (2004). "The Plumage of Archaeopteryx". In Currie PJ, Koppelhus EB, Shugar MA, Wright JL (ed.). Feathered Dragons. Indiana University Press. pp. 282–300. ISBN 0-253-34373-9.{{cite book}}: CS1 maint: multiple names: editors list (link)
  18. ^ Northeastern Ohio Universities Colleges of Medicine and Pharmacy (2007, December 21). "Whales Descended From Tiny Deer-like Ancestors". ScienceDaily. Retrieved 21 December 2007.{{cite web}}: CS1 maint: numeric names: authors list (link)
  19. ^ Philip D. Gingerich, D. E. Russell (1981). "Pakicetus inachus, a new archaeocete (Mammalia, Cetacea) from the early-middle Eocene Kuldana Formation of Kohat (Pakistan)". Univ. Mich. Contr. Mus. Paleont. 25: 235–246.
  20. ^ Castro, E. Huber, Peter, Michael (2003). Marine Biology (4 ed). McGraw-Hill.{{cite book}}: CS1 maint: multiple names: authors list (link)
  21. ^ J. G. M. Thewissen, E. M. Williams, L. J. Roe and S. T. Hussain (2001). "Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls". Nature. 413 (6853): 277–281. doi:10.1038/35095005. PMID 11565023.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  22. ^ Nummela, Sirpa (11 August 2004). "Eocene evolution of whale hearing". Nature. 430 (7001): 776–778. Bibcode:2004Natur.430..776N. doi:10.1038/nature02720. PMID 15306808. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  23. ^ a b c Thewissen, J. G. M. (1 November 2002). "THE EARLY RADIATIONS OF CETACEA (MAMMALIA): Evolutionary Pattern and Developmental Correlations". Annual Review of Ecology and Systematics. 33 (1): 73–90. doi:10.1146/annurev.ecolsys.33.020602.095426. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  24. ^ THEWISSEN, J. G. M. (1 January 2001). "Whale Origins as a Poster Child for Macroevolution". BioScience. 51 (12): 1037. doi:10.1641/0006-3568(2001)051[1037:WOAAPC]2.0.CO;2. ISSN 0006-3568. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  25. ^ Edward B. Daeschler, Neil H. Shubin and Farish A. Jenkins, Jr (6 April 2006). "A Devonian tetrapod-like fish and the evolution of the tetrapod body plan". Nature. 440 (7085): 757–763. doi:10.1038/nature04639. PMID 16598249.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  26. ^ Jennifer A. Clack, Scientific American, Getting a Leg Up on Land Nov. 21, 2005.
  27. ^ John Noble Wilford, The New York Times, Scientists Call Fish Fossil the Missing Link, Apr. 5, 2006.
  28. ^ Shubin, Neil (2008). Your Inner Fish. Pantheon. ISBN 9780375424472.
  29. ^ Shubin, Neil (2008). Your Inner Fish. Pantheon. ISBN 9780375424472.
  30. ^ "Science Magazine". Runcaria, a Middle Devonian Seed Plant Precursor. American Association for the Advancement of Science. 2011. Retrieved 22 March 2011.
  31. ^ a b Prothero 2007, pp. 50–53
  32. ^ Isaak, M (5 November 2006). "Claim CC200: There are no transitional fossils". TalkOrigins Archive. Retrieved 30 April 2009.
  33. ^ Donovan, S. K. and Paul, C. R. C. (eds) 1998: The adequacy of the fossil record, Wiley, New York, 312 pp.
  34. ^ Eldredge N & Gould SJ (1972). "Punctuated equilibria: an alternative to phyletic gradualism". In Schopf TJM (ed.). Models in paleobiology. San Francisco: W. H. Freeman. pp. 82–115. ISBN 0-87735-325-5.
  35. ^ Gould, Stephen (1980). The Panda's Thumb. New York: Norton. p. 189. ISBN 0393013804.

References

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