Polydactyly in stem-tetrapods: Difference between revisions - Wikipedia

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{{shortShort description|The stateState of having more than five digits in ancient fish and tetrapods}}

[[File:FishapodsFins tetrapodsto hands.JPGpng|thumb|right|500px400x400px|Limb evolution: A.in ''[[Eusthenopteron]]'',Devonian Btetrapodomorphs from pectoral fins to forelimbs. From left to right: ''[[GogonasusEusthenopteron]]'', C. ''[[Panderichthys]]'', D. ''[[Tiktaalik]]'', E. ''[[AcanthostegaElpistostege]]'', F. ''[[IchthyostegaAcanthostega]]'' ( hindleg ), and G. ''[[Tulerpeton]]''. ]]

'''Polydactyly in stem-tetrapods''' should here be understood as having more than five digits to the finger or foot, a condition that was the natural state of affairs in the very firstearliest [[Stegocephalia|stegocephalians]] during the [[Vertebrate land invasion|evolution of terrestriality]]. The polydactyly in these largely [[aquatic animal]]s is not to be confused with [[polydactyly]] in the medical sense, i.e. it was not an anomaly in the sense it was not a [[congenital disorder|congenital condition]] of having more than the typical number of [[digit (anatomy)|digit]]s for a given [[taxon]].<ref name=hall>{{cite book |last1=Hall |first1=Brian K. |title=Evolutionary Developmental Biology |date=1998 |publisher=Springer Science & Business Media |isbn=978-0-412-78580-1 |page=262 }}</ref> Rather, it appears to be a result of the early [[evolution]] from a limb with a [[fin]] rather than digits.

"Living tetrapods, such as the frogs, turtles, birds and mammals, are a subgroup of the tetrapod lineage. The lineage also includes finned and limbed tetrapods that are more closely related to living tetrapods than to living lungfishes."<ref>{{Cite journal|last1=Lu|first1=Jing|last2=Zhu|first2=Min|last3=Long|first3=John A.|last4=Zhao|first4=Wenjin|last5=Senden|first5=Tim J.|last6=Jia|first6=Liantao|last7=Qiao|first7=Tuo|date=2012-10-23|title=The earliest known stem-tetrapod from the Lower Devonian of China|journal=Nature Communications|language=en|volume=3|issue=1|pages=1160|doi=10.1038/ncomms2170|pmid=23093197|bibcode=2012NatCo...3.1160L|issn=2041-1723|doi-access=free|hdl=1885/69314|hdl-access=free}}</ref> [[Tetrapod]]s evolved from animals with fins such as found in [[lobe-finned fish]]es. From this condition a new pattern of limb formation evolved, where the development axis of the limb rotated to sprout secondary axes along the lower margin, giving rise to a variable number of very stout skeletal supports for a paddle-like foot.<ref>{{cite journal |last1=Coates |first1=M. I. |last2=Clack |first2=J. A. |title=Polydactyly in the earliest known tetrapod limbs |journal=Nature |date=September 1990 |volume=347 |issue=6288 |pages=66–69 |doi=10.1038/347066a0 |bibcode=1990Natur.347...66C |s2cid=4319165 }}</ref> The condition is thought to have arisen from the loss of the [[Fish anatomy#Spines and rays|fin ray]]-forming proteins [[actinodin 1]] and [[actinodin 2]] or modification of the expression of [[HOXD13]].<ref name="zhang2010">{{cite journal |last1=Zhang |first1=Jing |last2=Wagh |first2=Purva |last3=Guay |first3=Danielle |last4=Sanchez-Pulido |first4=Luis |last5=Padhi |first5=Bhaja K. |last6=Korzh |first6=Vladimir |last7=Andrade-Navarro |first7=Miguel A. |last8=Akimenko |first8=Marie-Andrée |title=Loss of fish actinotrichia proteins and the fin-to-limb transition |journal=Nature |date=July 2010 |volume=466 |issue=7303 |pages=234–237 |doi=10.1038/nature09137 |pmid=20574421 |bibcode=2010Natur.466..234Z |s2cid=205221027 }}</ref><ref>{{cite journal |last1=Schneider |first1=Igor |last2=Shubin |first2=Neil H. |title=Making Limbs from Fins |journal=Developmental Cell |date=December 2012 |volume=23 |issue=6 |pages=1121–1122 |doi=10.1016/j.devcel.2012.11.011 |pmid=23237946 |doi-access=free }}</ref> It is still unknown why exactly this happens. "[[Sonic hedgehog|SHH]] is produced by the mesenchymal cells of the zone of polarizing activity (ZPA) found at the posterior margin of the limbs of all vertebrates with paired appendages, including the most primitive chondrichthyian fishes. Its expression is driven by a well-conserved limb-specific enhancer called the ZRS (zone of polarizing region activity regulatory sequence) that is located approximately 1 [[Base pair#Length measurements|Mb]] upstream of the coding sequence of ''Shh''."<ref>{{Cite journal|last1=Saxena|first1=Aditya|last2=Towers|first2=Matthew|last3=Cooper|first3=Kimberly L.|date=2017-02-05|title=The origins, scaling and loss of tetrapod digits|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|volume=372|issue=1713|doi=10.1098/rstb.2015.0482|issn=0962-8436|pmc=5182414|pmid=27994123}}</ref>

[[Devonian]] taxa were polydactylous. ''[[Acanthostega]]'' had eight digits on both the hindlimbs and forelimbs. ''[[Ichthyostega]]'', which was both more derived and more specialized, had seven digits on the hindlimb, though the hand is unknown. The yet-more-derived ''[[Tulerpeton]]'' had six toes on both the hindlimbs and forelimbs.<ref name=hall/>

Early groups like ''[[Acanthostega]]'' had eight digits, while the more derived ''[[Ichthyostega]]'' had seven digits, the yet-more derived ''[[Tulerpeton]]'' had six toes.<ref name=hall/> ''[[Crassigyrinus]]'' from the fossil-poor [[Romer's gap]] in early [[Carboniferous]] is usually thought to have had five digits to each foot. The [[Anthracosauria|Anthracosaurs]], which may be stem-tetrapods<ref>{{cite journal |last1=Laurin |first1=Michel |title=The importance of global parsimony and historical bias in understanding tetrapod evolution. Part I. Systematics, middle ear evolution and jaw suspension |journal=Annales des Sciences Naturelles - Zoologie et Biologie Animale |date=January 1998 |volume=19 |issue=1 |pages=1–42 |doi=10.1016/S0003-4339(98)80132-9 }}</ref><ref>{{cite journal |last1=Marjanović |first1=David |last2=Laurin |first2=Michel |title=The Origin(s) of Modern Amphibians: A Commentary |journal=Evolutionary Biology |date=September 2009 |volume=36 |issue=3 |pages=336–338 |doi=10.1007/s11692-009-9065-8 }}</ref> or reptiliomorphs,<ref>{{cite book |last1=Gauthier |first1=J. |first2=A. G. |last2=Kluge |first3=T. |last3=Rowe |year=1988 |chapter=The early evolution of the Amniota |pages=103-155 |editor1-last=Benton |editor1-first=Michael J. |title=The Phylogeny and Classification of the Tetrapods: Amphibians, reptiles, birds. Vol. 1 |publisher=Systematics Association |isbn=978-0-19-857705-8 }}</ref> retained the five-toe pattern still found in [[Amniotes]], while further reduction had taken place on other [[Labyrinthodont]] lines, leaving the forefoot with four toes and the hind foot with five, a pattern still found in [[Lissamphibia|modern amphibians]].<ref>[[Michael Benton|Benton, M.]] (2005): [[Vertebrate Palaeontology (Benton)|Vertebrate Palaeontology]] 3rd edition. Blackwell Publishing{{pn}}</ref> The increasing knowledge of Labyrinthodonts from Romer's gap has led to the challenging of the hypothesis that [[pentadactyly]], as displayed by most modern tetrapods, is [[plesiomorphic]]. The number of digits was once thought to have been reduced in [[amphibians]] and [[reptiles]] independently,<ref name=hall/><ref>{{cite journal |doi=10.1007/978-1-4615-3310-8_43 |s2cid=86020906 }}</ref> but more recent studies suggest that a single reduction occurred, along the tetrapod stem, in the Late Devonian or Early Carboniferous.<ref>{{cite journal |last1=Laurin |first1=Michel |title=A Reevaluation of the Origin of Pentadactyly |journal=Evolution |date=1998 |volume=52 |issue=5 |pages=1476–1482 |jstor=2411316 }}</ref><ref>{{cite journal |last1=Ruta |first1=Marcello |last2=Coates |first2=Michael I. |title=Dates, nodes and character conflict: Addressing the Lissamphibian origin problem |journal=Journal of Systematic Palaeontology |date=January 2007 |volume=5 |issue=1 |pages=69–122 |doi=10.1017/S1477201906002008 }}</ref> However, even the early [[Ichthyostegalia]]ns like ''Acanthostega'' and ''Ichthyostega'' appear to have had the forward ossified bony toes combined in a single stout digit, making them effectively five-toed.▼

▲EarlyIt groupsis likeunclear ''[[Acanthostega]]''whether hadpolydactylous eighttetrapods digits,survived whileto the more derived ''[[IchthyostegaCarboniferous]]'' had seven digits, the yet-more derived ''[[Tulerpeton]]'' had six toes.<ref name=hall/> ''[[Crassigyrinus]]'', from the fossil-poor [[Romer's gap]] in the early [[Carboniferous]], is usually thought to have had five digits to each foot. The [[Anthracosauria|Anthracosaursanthracosaurs]], which may be stem-tetrapods<ref>{{cite journal |last1=Laurin |first1=Michel |title=The importance of global parsimony and historical bias in understanding tetrapod evolution. Part I. Systematics, middle ear evolution and jaw suspension |journal=Annales des Sciences Naturelles - Zoologie et Biologie Animale |date=January 1998 |volume=19 |issue=1 |pages=1–42 |doi=10.1016/S0003-4339(98)80132-9 }}</ref><ref>{{cite journal |last1=Marjanović |first1=David |last2=Laurin |first2=Michel |title=The Origin(s) of Modern Amphibians: A Commentary |journal=Evolutionary Biology |date=September 2009 |volume=36 |issue=3 |pages=336–338 |doi=10.1007/s11692-009-9065-8 |bibcode=2009EvBio..36..336M |s2cid=12023942 |url=https://hal.archives-ouvertes.fr/hal-00549002/file/MARJANOVIC_David.pdf |access-date=2021-11-22 |archive-date=2020-11-06 |archive-url=https://web.archive.org/web/20201106144430/https://hal.archives-ouvertes.fr/hal-00549002/file/MARJANOVIC_David.pdf |url-status=live }}</ref> or reptiliomorphs,<ref>{{cite book |last1=Gauthier |first1=J. |first2=A. G. |last2=Kluge |first3=T. |last3=Rowe |year=1988 |chapter=The early evolution of the Amniota |pages=103-155103–155 |editor1-last=Benton |editor1-first=Michael J. |title=The Phylogeny and Classification of the Tetrapods: Amphibians, reptiles, birds. Vol. 1 |publisher=Systematics Association |isbn=978-0-19-857705-8 }}</ref> retained the five-toe pattern still found in [[Amniotesamniote]], whiles. furtherFurther reduction had taken place onin otherthe [[LabyrinthodontTemnospondyli|temnospondyls]] lines, leaving the forefoot with four toes and the hind foot with five, a pattern still found in [[Lissamphibia|modern amphibians]].<ref>[[Michael Benton|Benton, M.]] (2005): [[Vertebrate Palaeontology (Benton)|Vertebrate Palaeontology]] 3rd edition. Blackwell Publishing{{pn|date=October 2020}}</ref> The increasing knowledge of Labyrinthodontslabyrinthodonts from Romer's gap has led to the challenging of the hypothesis that [[pentadactyly]], as displayed by most modern tetrapods, is [[plesiomorphic]]. The number of digits was once thought to have been reduced in [[amphibians]] and [[reptiles]] independently,<ref name="hall" /><ref>{{cite journalbook |doi=10.1007/978-1-4615-3310-8_43 |s2cid=86020906 |chapter=New Palaeontological Contributions to Limb Ontogeny and Phylogeny |title=Developmental Patterning of the Vertebrate Limb |year=1991 |last1=Coates |first1=Michael |pages=325–337 |isbn=978-1-4613-6457-3 }}</ref> but more recent studies suggest that a single reduction occurred, along the tetrapod stem, in the Late Devonian or Early Carboniferous.<ref>{{cite journal |last1=Laurin |first1=Michel |title=A Reevaluation of the Origin of Pentadactyly |journal=Evolution |date=1998 |volume=52 |issue=5 |pages=1476–1482 |doi=10.1111/j.1558-5646.1998.tb02028.x |jstor=2411316 |pmid=28565380 |s2cid=33367097 |doi-access=free }}</ref><ref>{{cite journal |last1=Ruta |first1=Marcello |last2=Coates |first2=Michael I. |title=Dates, nodes and character conflict: Addressing the Lissamphibian origin problem |journal=Journal of Systematic Palaeontology |date=January 2007 |volume=5 |issue=1 |pages=69–122 |doi=10.1017/S1477201906002008 |bibcode=2007JSPal...5...69R |s2cid=86479890 }}</ref> However, evenEven the early [[Ichthyostegaliaichthyostegalia]]ns like ''Acanthostega'' and ''Ichthyostega'' appear to have had the forward ossified bony toes combined in a single stout digit, making them effectively five-toed.

== See also ==

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{{reflist}}

{{Commons category|Polydactyly_in_early_tetrapodsPolydactyly in early tetrapods}}

{{Tetrapodomorpha|R.}}

[[Category:Comparative anatomy]]