Transitional fossil: Difference between revisions

Revision as of 02:07, 11 December 2020

Revision as of 20:41, 18 December 2020

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{{redirect|Transitional forms|the hardcore punk music album|Sharptooth}}

{{short description|Fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group}}

{{redirect|Transitional forms|the hardcore punk music album|Sharptooth}}

{{Use dmy dates|date=September 2020}}

{{Paleontology|cTopic=Fossil record}}

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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 &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=harv}}</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 |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'' 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. |authorlink1=Ted Daeschler |last2=Shubin |first2=Neil H. |authorlink2=Neil Shubin |last3=Jenkins |first3=Farish A., Jr. |authorlink3=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 |ref=harv|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. |authorlink=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=harv}}</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 |accessdate=2012-04-19 |ref=harv}}</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 |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 &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. |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 &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''===

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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 &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 [[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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==See also==

* [[Crocoduck]]

{{Div col}}

* [[Evidence of common descent]]

* [[List of transitional fossils]]

* [[Speciation]]

* [[Crocoduck]]

{{div col end}}

==References==

{{Reflist|30em}}

==Sources==