Transitional fossil: Difference between revisions

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A '''transitional fossil''' is any [[fossil]]ized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group.<ref name="Freeman">{{harvnb|Freeman|Herron|2004|p=816}}</ref> This is especially important where the descendant group is sharply differentiated by [[gross anatomy]] and mode of living from the ancestral group. These fossils serve as a reminder that taxonomic divisions are [[human]] constructs that have been imposed in hindsight on a continuum of variation. Because of the incompleteness of the fossil record, there is usually no way to know exactly how close a transitional fossil is to the point of divergence. Therefore, it cannot be assumed that transitional fossils are direct ancestors of more recent groups, though they are frequently used as models for such ancestors.<ref name=Prothero/>

In 1859, when [[Charles Darwin]]'s ''[[On the Origin of Species]]'' was first published, the fossil record was poorly known. Darwin described the perceived lack of transitional fossils as "the most obvious and gravest objection which can be urged against my theory," but he explained it by relating it to the extreme imperfection of the geological record.<ref>{{harvnb|Darwin|1859|pp=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F373&pageseq=297 279–280]}}</ref> He noted the limited collections available at the time but described the available information as showing patterns that followed from his theory of [[evolution|descent with modification]] through [[natural selection]].<ref>{{harvnb|Darwin|1859|pp=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F373&pageseq=359 341–343]}}</ref> Indeed, ''[[Archaeopteryx]]'' was discovered just two years later, in 1861, and represents a classic transitional form between earlier, non-avian [[dinosaur]]s and [[bird]]s. [[List of transitional fossils|Many more transitional fossils]] have been discovered since then, and there is now abundant evidence of how all [[class (biology)|classes]] of [[vertebrate]]s are related, including many transitional fossils.<ref name="NS2645">{{cite journal |last=Prothero |first=Donald R. |author-link=Donald Prothero |date=1 March 2008 |title=Evolution: What missing link? |url=https://www.newscientist.com/article/mg19726451.700-evolution-what-missing-link.html?full=true |journal=[[New Scientist]] |issue=2645 |pages=35–41 |issn=0262-4079 |doi=10.1016/s0262-4079(08)60548-5 |volume=197|url-access=subscription }}</ref> Specific examples of class-level transitions are: [[Evolution of tetrapods|tetrapod]]s and [[Evolution of fish|fish]], [[Origin of birds|birds and dinosaurs]], and [[Evolution of mammals|mammals and "mammal-like reptiles"]].

The term "missing link" has been used extensively in popular writings on [[human evolution]] to refer to a perceived gap in the [[Hominidae|hominid]] evolutionary record. It is most commonly used to refer to any new transitional fossil finds. Scientists, however, do not use the term, as it refers to a pre-evolutionary view of nature.

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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 |issue=1 |pages=73–90 |doi=10.1146/annurev.ecolsys.33.020602.095426 |bibcode=2002AnRES..33...73T |issn=1545-2069 }}</ref> The fossils of ambulocetids are always found in near-shore shallow marine deposits associated with abundant marine plant fossils and [[Littoral zone|littoral]] [[Mollusca|mollusc]]s.<ref name=radiations/> Although they are found only in marine deposits, their oxygen isotope values indicate that they consumed water with a range of degrees of salinity, some specimens showing no evidence of sea water consumption and others none of fresh water consumption at the time when their teeth were fossilized. It is clear that ambulocetids tolerated a wide range of salt concentrations.<ref name="poster">{{cite journal |last1=Thewissen |first1=J. G. M. |last2=Bajpai |first2=Sunil |date=December 2001 |title=Whale Origins as a Poster Child for Macroevolution |journal=[[BioScience]] |volume=51 |issue=12 |pages=1037–1049 |doi=10.1641/0006-3568(2001)051[1037:WOAAPC]2.0.CO;2 |issn=0006-3568 |doi-access=free }}</ref> Their diet probably included land animals that approached water for drinking, or freshwater aquatic organisms that lived in the river.<ref name=radiations/> Hence, ambulocetids represent the transition phase of cetacean ancestors between freshwater and marine habitat.

===''Tiktaalik''===