Editing Platypus (section)

== Evolution ==
{{cladogram
|caption=Evolutionary relationships between the platypus and other mammals<ref name="LecointreGuyader2006">{{cite book|last1=Lecointre|first1=Guillaume|last2=Le Guyader|first2=Hervé|title=The Tree of Life: A Phylogenetic Classification|url=https://archive.org/details/treeoflifephylog0000leco|url-access=registration|access-date=28 March 2015|year=2006|publisher=Harvard University Press|isbn=978-0-674-02183-9}}</ref>
|cladogram=
{{clade
|1={{clade
   |1={{clade
      |1='''Platypus'''
      |2=[[Echidna]]s
      }}
   |label2=''{{spaces}}live{{spaces}}birth{{spaces}}''
   |2={{clade
      |1=[[Marsupial]]s
      |label2=''{{spaces}}true{{spaces}}placenta{{spaces}}''
      |2=[[Placental]]s
      }}
   }}
}}
}}

The platypus and other monotremes were very poorly understood, and some of the 19th century myths that grew up around them{{snd}}for example, that the monotremes were "inferior" or [[Reptile|quasireptilian]]{{snd}}still endure.<ref name="Rodent">{{cite journal|journal=Philosophical Transactions: Biological Sciences|title=The platypus is not a rodent: DNA hybridization, amniote phylogeny and the palimpsest theory|author=Kirsch, John A. W. |author2=Mayer, Gregory C. |volume=353|issue=1372|pages=1221–37|date=29 July 1998|pmid=9720117|pmc=1692306|doi=10.1098/rstb.1998.0278}}</ref> In 1947, [[William King Gregory]] theorised that placental mammals and marsupials may have diverged earlier, and a subsequent branching divided the monotremes and marsupials, but later research and fossil discoveries have suggested this is incorrect.<ref name="Rodent" /><ref name="JM">{{cite journal |journal=Nature |volume=416 |issue= 6877|pages=165–8 |year=2002 |doi=10.1038/416165a |title=The first Jurassic mammal from South America |author=Rauhut, O.W.M.|author2= Martin, T.|author3= Ortiz-Jaureguizar, E.|author4= Puerta, P. |pmid=11894091|bibcode=2002Natur.416..165R|hdl=11336/99461 |s2cid=4346804 |hdl-access=free }}</ref> In fact, modern monotremes are the survivors of an early branching of the mammal tree, and a later branching is thought to have led to the [[marsupial]] and placental groups.<ref name="Rodent" /><ref name="JME">{{cite journal|journal=Journal of Mammalian Evolution|title=Evolution of the Monotremes: Phylogenetic Relationship to Marsupials and Eutherians, and Estimation of Divergence Dates Based on α-Lactalbumin Amino Acid Sequences|author=Messer, M.|author2= Weiss, A.S.|author3= Shaw, D.C.|author4= Westerman, M.|volume=5|issue=1|pages=95–105|date=March 1998 | doi = 10.1023/A:1020523120739|s2cid=39638466}}</ref> [[Molecular clock]] and fossil dating suggest platypuses split from [[echidna]]s around 19–48{{spaces}}million years ago.<ref name="Phillips">{{cite journal | author = Phillips MJ|author2= Bennett TH|author3= Lee MS | year = 2009 | title = Molecules, morphology, and ecology indicate a recent, amphibious ancestry for echidnas | journal =Proc. Natl. Acad. Sci. U.S.A. | volume = 106 | issue = 40| pages = 17089–94 | doi = 10.1073/pnas.0904649106 | pmid = 19805098 | pmc = 2761324 | bibcode = 2009PNAS..10617089P |doi-access= free}}</ref>
[[File:Steropodon BW.jpg|thumb|right|Reconstruction of ancient platypus relative ''Steropodon'']]

The oldest discovered fossil of the modern platypus dates back to about 100,000 years ago during the [[Quaternary]] period, though a limb bone of ''Ornithorhynchus'' is known from [[Pliocene]]-aged strata.<ref>{{cite book|author=Musser, A.M.|editor1=Merrick, J.R.|editor2=Archer, M.|editor3=Hickey, G.M.|editor4=Lee, M.S.Y.|year=2006|chapter=26. Furry Egg-layers: Monotreme Relationships and Radiations|title=Evolution and Biogeography of Australasian Vertebrates|publisher=Auscipub|pages=523–550|isbn=978-0975779002}}</ref> The extinct monotremes ''[[Teinolophos]]'' and ''[[Steropodon]]'' from the [[Cretaceous]] were once thought to be closely related to the modern platypus,<ref name="JM" /> but are now considered more basal taxa.<ref name="Thomas H. Rich 2016">{{Cite journal |last1=Rich |first1=Thomas H. |last2=Hopson |first2=James A. |last3=Gill |first3=Pamela G. |last4=Trusler |first4=Peter |last5=Rogers-Davidson |first5=Sally |last6=Morton |first6=Steve |last7=Cifelli |first7=Richard L. |last8=Pickering |first8=David |last9=Kool |first9=Lesley |year=2016 |title=The mandible and dentition of the Early Cretaceous monotreme Teinolophos trusleri |journal=Alcheringa: An Australasian Journal of Palaeontology |volume=40 |issue=4 |pages=475–501 |doi=10.1080/03115518.2016.1180034 |bibcode=2016Alch...40..475R |s2cid=89034974 |hdl=1885/112071 |hdl-access=free }}</ref> The fossilised ''Steropodon'' was discovered in [[New South Wales]] and is composed of an opalised lower jawbone with three molar teeth (whereas the adult contemporary platypus is toothless). The molar teeth were initially thought to be [[Tribosphenic molar|tribosphenic]], which would have supported a variation of Gregory's theory, but later research has suggested, while they have three cusps, they evolved under a separate process.<ref name="MS">{{cite journal|journal=Acta Palaeontologica Polonica|url=http://www.app.pan.pl/archive/published/app47/app47-487.pdf|title=New data on the Paleocene monotreme ''Monotrematum sudamericanum'', and the convergent evolution of triangulate molars|author=Pascual, R.|author2=Goin, F.J.|author3=Balarino, L.|author4=Udrizar Sauthier, D.E.|volume=47|issue=3|pages=487–492|year=2002|access-date=18 March 2009|archive-date=9 August 2017|archive-url=https://web.archive.org/web/20170809023141/http://www.app.pan.pl/archive/published/app47/app47-487.pdf|url-status=live}}</ref> The fossil jaw of ''Teinolophos'' is thought to be about 110{{spaces}}million years old, making it the oldest mammal fossil found in Australia. Unlike the modern platypus (and [[echidna]]s), ''Teinolophos'' lacked a beak.<ref name="Thomas H. Rich 2016" />

In 2024, [[Late Cretaceous]] ([[Cenomanian]])-aged fossil specimens of actual early platypus relatives were recovered from the same rocks as ''Steropodon'', including the [[Basal (phylogenetics)|basal]] ''[[Opalios]]'' and the more derived ''[[Dharragarra]]'', the latter of which may be the oldest member of the platypus [[Stem group|stem]]-lineage, as it retains the same [[Dentition|dental formula]] found in Cenozoic platypus relatives.<ref name=":0">{{cite journal |last1=Flannery |first1=Timothy F. |last2=McCurry |first2=Matthew R. |last3=Rich |first3=Thomas H. |last4=Vickers-Rich |first4=Patricia |last5=Smith |first5=Elizabeth T. |last6=Helgen |first6=Kristofer M. |title=A diverse assemblage of monotremes (Monotremata) from the Cenomanian Lightning Ridge fauna of New South Wales, Australia |journal=Alcheringa: An Australasian Journal of Palaeontology |date=2 April 2024 |volume=48 |issue=2 |pages=319–337 |doi=10.1080/03115518.2024.2348753 |doi-access=free |bibcode=2024Alch...48..319F }}</ref> ''[[Monotrematum]]'' and ''[[Patagorhynchus]]'', two other fossil relatives of the platypus, are known from the latest Cretaceous ([[Maastrichtian]]) and the mid-[[Paleocene]] of [[Argentina]], indicating that some monotremes managed to colonize [[South America]] from Australia when the two continents were connected via [[Antarctica]]. These are also considered potential members of the platypus stem-lineage.<ref name="MS" /><ref name="patagonia">{{Cite journal|title = A platypus in Patagonia (Ancient life – 1992)|last = Folger|first = Tim| date = 1993 |journal = Discover|volume = 14|issue = 1|page = 66}}</ref><ref name=":0" />  The closest fossil relative of the platypus was ''[[Obdurodon]]'', known from the late [[Oligocene]] to the [[Miocene]] of Australia. It closely resembled the modern platypus, aside from the presence of molar teeth.<ref name=":0" /> A fossilised tooth of the giant platypus ''[[Obdurodon tharalkooschild]]'' was dated 5–15{{spaces}}million years ago. Judging by the tooth, the animal measured 1.3 metres long, making it the largest platypus on record.<ref>{{cite news|url = http://www.zmescience.com/science/geology/platypus-fossil-paleontology-05112013/|title = 'Platypus-zilla' fossil unearthed in Australia|newspaper = ZME Science|last = Mihai|first = Andrei|year = 2013|access-date = 5 November 2013|archive-date = 21 July 2021|archive-url = https://web.archive.org/web/20210721004004/https://www.zmescience.com/science/geology/platypus-fossil-paleontology-05112013/|url-status = live}}</ref>

The loss of teeth in the modern platypus has long been enigmatic, as a distinctive lower molar tooth row was previously present in its lineage for over 95 million years. Even its closest relative, ''Obdurodon'', which otherwise closely resembles the platypus, retained this tooth row. More recent studies indicate that this tooth loss was a geologically very recent event, occurring only around the [[Pliocene|Plio]]-[[Pleistocene]] (around 2.5 million years ago), when the [[rakali]], a large semiaquatic [[rodent]], colonized Australia from [[New Guinea]]. The platypus, which previously fed on a wide array of hard and soft-bodied prey, was outcompeted by the rakali for hard-bodied prey such as crayfish and mussels. This competition may have selected for the loss of teeth in the platypus and their replacement by horny pads, as a way of specializing for softer-bodied prey, over which the rakali did not compete with it.<ref name=":0" />

=== Genome ===
[[File:Platypus skeleton Pengo.jpg|thumb|Platypus skeleton]]

Because of the early divergence from the [[Theria|therian mammals]] and the low numbers of extant monotreme species, the platypus is a frequent subject of research in evolutionary biology. In 2004, [[research]]ers at the [[Australian National University]] discovered the platypus has ten [[sex chromosome]]s, compared with two (XY) in most other mammals. These ten chromosomes form five unique pairs of XY in males and XX in females, i.e. males are X{{sub|1}}Y{{sub|1}}X{{sub|2}}Y{{sub|2}}X{{sub|3}}Y{{sub|3}}X{{sub|4}}Y{{sub|4}}X{{sub|5}}Y{{sub|5}}.<ref name="discover">{{cite web|url=http://discovermagazine.com/2005/apr/sex-ys-platypuses0425/|title=Sex, Ys, and Platypuses|publisher=Discover|first=Jocelyn|last=Selim|date=25 April 2005|access-date=7 May 2008|archive-date=16 May 2008|archive-url=https://web.archive.org/web/20080516233225/http://discovermagazine.com/2005/apr/sex-ys-platypuses0425|url-status=live}}</ref> One of the X chromosomes of the platypus has great homology to the bird Z chromosome.<ref>{{Cite journal|author1=Frank Grützner|author2=Willem Rens|author3=Enkhjargal Tsend-Ayush|author4=Nisrine El-Mogharbel|author5=Patricia C. M. O'Brien|author6=Russell C. Jones|author7=Malcolm A. Ferguson-Smith|author8=Jennifer A. Marshall Graves|title=In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes|journal=Nature|volume=432|issue=7019|pages=913–917|date=16 December 2004|doi=10.1038/nature03021|pmid=15502814|bibcode=2004Natur.432..913G|s2cid=4379897}}
</ref> The platypus genome also has both reptilian and mammalian genes associated with egg fertilisation.<ref name="draft_genome">{{cite journal| journal = Nature| volume = 453| issue = 7192| pages = 175–183| title = Genome analysis of the platypus reveals unique signatures of evolution| date = 8 May 2008| doi = 10.1038/nature06936| author = Warren, Wesley C.| pmid = 18464734| pmc = 2803040 |display-authors=etal|bibcode=2008Natur.453..175W}}</ref><ref>{{cite journal |title=Beyond the Platypus Genome – 2008 Boden Research Conference |journal=Reprod Fertil Dev |volume=21 |issue=8 |pages=i–ix, 935–1027 |year=2009 |url=http://www.publish.csiro.au/nid/44/issue/4849.htm |access-date=3 March 2012 |archive-date=21 November 2015 |archive-url=https://web.archive.org/web/20151121130525/http://www.publish.csiro.au/nid/44/issue/4849.htm |url-status=live }}</ref> Though the platypus lacks the mammalian sex-determining gene [[SRY]], a study found that the mechanism of sex determination is the [[Anti-Müllerian hormone|AMH gene]] on the oldest [[Y chromosome]].<ref>{{cite journal|last1=Cortez|first1=Diego|last2=Marin|first2=Ray|last3=Toledo-Flores|first3=Deborah|last4=Froidevaux|first4=Laure|last5=Liechti|first5=Angélica|last6=Waters|first6=Paul D.|last7=Grützner|first7=Frank|last8=Kaessmann|first8=Henrik|year=2014|title=Origins and functional evolution of Y chromosomes across mammals|journal=Nature|volume=508|issue=7497|pages=488–493|doi=10.1038/nature13151|pmid=24759410|bibcode=2014Natur.508..488C|s2cid=4462870}}</ref><ref>{{cite news|last1=Salleh|first1=Anna|title=Platypus Sex 'Master Switch' Identified|url=http://www.abc.net.au/science/articles/2014/05/05/3994897.htm|publisher=Australian Broadcasting Corporation|date=5 May 2014|access-date=5 June 2014|archive-date=6 July 2016|archive-url=https://web.archive.org/web/20160706211627/http://www.abc.net.au/science/articles/2014/05/05/3994897.htm|url-status=live}}</ref> A draft version of the platypus genome sequence was published in ''[[Nature (journal)|Nature]]'' on 8{{spaces}}May 2008, revealing both reptilian and mammalian elements, as well as two genes found previously only in birds, amphibians, and fish. More than 80% of the platypus's genes are common to the other mammals whose genomes have been sequenced.<ref name="draft_genome" /> An updated genome, the most complete on record, was published in 2021, together with the genome of the [[short-beaked echidna]].<ref>{{cite journal |last1=Zhou |first1=Yang |last2=Shearwin-Whyatt |first2=Linda |last3=Li |first3=Jing |last4=Song |first4=Zhenzhen |last5=Hayakawa |first5=Takashi |last6=Stevens |first6=David |last7=Fenelon |first7=Jane C. |last8=Peel |first8=Emma |last9=Cheng |first9=Yuanyuan |last10=Pajpach |first10=Filip |last11=Bradley |first11=Natasha |last12=Suzuki |first12=Hikoyu |last13=Nikaido |first13=Masato |last14=Damas |first14=Joana |last15=Daish |first15=Tasman |last16=Perry |first16=Tahlia |last17=Zhu |first17=Zexian |last18=Geng |first18=Yuncong |last19=Rhie |first19=Arang |last20=Sims |first20=Ying |last21=Wood |first21=Jonathan |last22=Haase |first22=Bettina |last23=Mountcastle |first23=Jacquelyn |last24=Fedrigo |first24=Olivier |last25=Li |first25=Qiye |last26=Yang |first26=Huanming |last27=Wang |first27=Jian |last28=Johnston |first28=Stephen D. |last29=Phillippy |first29=Adam M. |last30=Howe |first30=Kerstin |last31=Jarvis |first31=Erich D. |last32=Ryder |first32=Oliver A. |last33=Kaessmann |first33=Henrik |last34=Donnelly |first34=Peter |last35=Korlach |first35=Jonas |last36=Lewin |first36=Harris A. |last37=Graves |first37=Jennifer |last38=Belov |first38=Katherine |last39=Renfree |first39=Marilyn B. |last40=Grutzner |first40=Frank |last41=Zhou |first41=Qi |last42=Zhang |first42=Guojie |title=Platypus and echidna genomes reveal mammalian biology and evolution |journal=Nature |date=29 April 2021 |volume=592 |issue=7856 |pages=756–762 |doi=10.1038/s41586-020-03039-0 |pmid=33408411 |pmc=8081666 |bibcode=2021Natur.592..756Z |doi-access=free }}</ref>