Yinotheria

Yinotheria Temporal range:(possibleLate Triassicrecord)
Ambondro mahabojaw fragment
Shuotherium dongijawbone
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Subclass:	Yinotheria Chow and Rich, 1982
Subgroups
†Shuotheriidae Australosphenida Monotremata?

Yinotheriais a proposed basal subclass clade of crownmammalsuniting theShuotheriidae,an extinct group of mammals from the Jurassic of Eurasia, withAustralosphenida,a group of mammals known from the Jurassic to Cretaceous ofGondwana,which possibly include livingmonotremes.Today, there are only five survivingspeciesof monotremes which live inAustraliaandNew Guinea,consisting of theplatypusand four species ofechidna.Fossils of yinotheres have been found inBritain,China,Russia,MadagascarandArgentina.Contrary to other known crown mammals, they retained postdentary bones as shown by the presence of a postdentary trough. The extant members (monotremes) developed the mammalianmiddle earindependently.

Other studies have rejected Yinotheria, finding Shuotheriidae to be unrelated to Australosphenida and outside crown Mammalia.

Evolutionary history

According to genetic studies, Yinotheria diverged from othermammalsaround 220 to 210 million years ago, at some point in theTriassicorEarly Jurassic.The oldest-known fossils are a bit younger, dating around 168 to 163 million years in theMiddle Jurassic.These fossils are the generaPseudotribosof China,Shuotheriumof both China and the United Kingdom,ItatodonofSiberiaandParitatodonofKyrgyzstanand the UK.These, which belong to the familyShuotheriidae,are the only known northern hemisphere group of yinotherians.

The infraclassAustralosphenidaappeared around the same time as Shuotheriidae. The familyHenosferidae,comprising the generaHenosferus,Ambondro,andAsfaltomylos,has been found in the southern hemisphere at locations in Argentina and Madagascar. This suggests that this family could have been more widespread and diverse inGondwanaduring that time; however, due to their fragile state, some fossils might have been destroyed by geological events.

The familyAusktribosphenidaeand the firstmonotremesappeared in theEarly Cretaceous,in the region that is now known asAustralasia.Despite being found in the same region of the world and in the same time period, recent work has found that the olderHenosferidaeis the sister taxon toMonotremata,withAusktribosphenidaebeing the next sister taxa in Australosphenida.Ausktribosphenidae includes the generaBishopsandAusktribosphenos.

Some 110-million-year-old monotreme fossil jaw fragments were found atLightning Ridge,New South Wales.These fragments, from the speciesSteropodon galmani,are the oldest known monotreme fossils. Fossils from the generaKollikodon,Teinolophos,andObdurodonhave also been discovered. In 1991, a fossil tooth of a 61-million-year-old platypus was found in southernArgentina(since namedMonotrematum,though it is now considered to be anObdurodonspecies). (See fossil monotremes below.)Molecular clockand fossil dating give a wide range of dates for the split between echidnas and platypuses, with one survey putting the split at 19 to 48 million years ago,but another putting it at 17 to 89 million years ago.All these dates are more recent than the oldest known platypus fossils, suggesting that both the short-beaked and long-beaked echidna species are derived from a platypus-like ancestor.

Systematics

History of classification

Prototheria

See also:Prototheria

Originally, monotremes were classified as a subclass of mammals known asPrototheria.The names Prototheria,MetatheriaandEutheria(meaning "first beasts", "changed beasts", and "true beasts", respectively) refer to the three mammalian groupings that have living representatives. Each of the three may be defined as a totalcladecontaining a livingcrown-group(respectively, the Monotremata,MarsupialiaandPlacentalia) plus any fossil species that are more closely related to that crown-group than to any other living animals.

The threefold division of living mammals into monotremes, marsupials and placentals was already well established whenThomas Huxleyproposed the names Metatheria and Eutheria to incorporate the two latter groups in 1880. Initially treated as subclasses, Metatheria and Eutheria are by convention now grouped asinfraclassesof the subclassTheria,and in more recent proposals have been demoted further (tocohortsor evenmagnorders), ascladisticreappraisals of the relationships between living and fossil mammals have suggested that the Theria itself should be reduced in rank.

Prototheria, on the other hand, was generally recognised as a subclass until quite recently, on the basis of a hypothesis that defined the group by two supposedsynapomorphies:(1) formation of the side wall of the braincase from a bone called the anterior lamina, contrasting with thealisphenoidin therians; and (2) a linear alignment of molar cusps, contrasting with a triangular arrangement in therians. These characters appeared to unite monotremes with a range ofMesozoicfossil orders (Morganucodonta,Triconodonta,DocodontaandMultituberculata) in a broader clade for which the name Prototheria was retained, and of which monotremes were thought to be only the last surviving branch (Benton 2005: 300, 306).

Australosphenida hypothesis and Yinotheria

The evidence that was held to support Prototheria is now universally discounted. In the first place, the examination of embryos has revealed that the development of the braincase wall is essentially identical in therians and in 'prototherians': the anterior lamina simply fuses with the alisphenoid in therians, and therefore the 'prototherian' condition of the braincase wall is primitive for all mammals, while the therian condition can be derived from it. Additionally, the linear alignment of molar cusps is also primitive for all mammals. Therefore, neither of these states can supply a uniquely shared derived character that would support a 'prototherian' grouping of orders in contradistinction to Theria (Kemp 1983).

In a further reappraisal, the molars of embryonic and fossil monotremes (living monotreme adults are toothless) appear to demonstrate an ancestral pattern of cusps that is similar to the triangular arrangement observed in therians. Some peculiarities of this dentition support an alternative grouping of monotremes with certain recently discovered fossil forms into a proposed new clade known as theAustralosphenida,and also suggest that the triangular array of cusps may have evolved independently in australosphenidans and therians (Luoet al.2001, 2002). Australosphenida is characterized by the shared presence of a cingulum on the outer front corner of the lower molars, a short and broad talonid, a relatively low trigonid, and a triangulated last lower premolar.

The Australosphenida hypothesis remains controversial; for example, lingual cingula seem to be a presence in various non-australosphenidan mammalsand some work has shown the possibility of Eutheria being the sister group to Australosphenida, without monotremes.As a result, some taxonomists (e.g. McKenna & Bell 1997) prefer to maintain the name Prototheria as a fitting contrast to the other group of living mammals, the Theria. In theory, the Prototheria is taxonomically redundant, since Monotremata is currently the only order that can still be confidently included, but its retention might be justified if new fossil evidence, or a re-examination of known fossils, enables extinct relatives of the monotremes to be identified and placed within a wider grouping.

When systematic work was performed, it was also found that Australosphenida is the sister taxon toShuotheriidae,an obscure group of Mesozoic mammals that were found in what is nowChina.Kielan-Jaworowska, Cifelli & Luo 2002had this to say regarding the Shuotheriidae, particularlyShuotherium:

In our view, the most compelling evidence as to the affinities ofShuotheriumlies in the structure of the last premolar, which shares striking similarities to that of Australosphenida

Lower molar structure ofShuotheriumand Australosphenida is obviously quite different, and for this reason we do not placeShuotheriumwithin this Gondwanan clade. Based on the limited evidence available, however, we suggest thatShuotheriumis a viable sister-taxon to Australosphenida.

Yinotheria is named for this grouping.

Other scholars have rejected Yinotheria, finding instead that Shuotheriidae is closely related toDocodontaoutside crown Mammalia.

Taxonomy

See also:Monotremes § Fossil monotremes

In comparison toMetatheriaandEutheria,where there seems to be a better understanding on the relationships among taxa with substantial fossil evidence, Yinotheria has few fossils; mostly consisting of (with few exceptions) the jawbones and teeth. In addition, the group seems not to have been as diverse in their evolutionary history, in comparison to members of both Metatheria and Eutheria.Future analysis and better fossil remains could affect the membership of Yinotheria as well as rearranging and revising the relationships of stem-monotremes and crowned monotremes.

• Subclass YinotheriaChow & Rich 1982 sensu Kielan-Jaworowska, Cifelli & Luo 2004[PrototheriaGill 1872]
  • Order†ShuotheridaChow & Rich 1982[Shuotheridia; Shuotheria]
    • Family†ShuotheriidaeChow & Rich 1982
      • Genus†PseudotribosLuo, Ji & Yuan 2007
        • †Pseudotribos robustusLuo, Ji & Yuan 2007
      • Genus†ShuotheriumChow & Rich 1982
        • †Shuotherium dongiChow & Rich 1982
        • †Shuotherium shilongiWang et al. 1998
  • InfraclassAustralosphenidaLuo, Cifelli & Kielan-Jaworowska 2001 sensu Kielan-Jaworowska, Cifelli & Luo 2004
    • Order†AusktribosphenidaLuo, Cifelli & Kielan-Jaworowska 2001
      • Family†AusktribosphenidaeRich et al. 1997
        • Genus†BishopsRich et al. 2001
          • †Bishops whitmoreiRich et al. 2001
        • Genus†AusktribosphenosRich et al. 1997
          • †Ausktribosphenos nyktosRich et al. 1997
    • Order†HenosferidaAverianov & Lopatin 2011
      • Family†HenosferidaeRougier et al. 2007
        • Genus†Ambondro
          • †Ambondro mahaboFlynn et al. 1999
        • Genus†AsfaltomylosRauhut et al. 2002
          • †Asfaltomylos patagonicusRauhut et al. 2002
        • Genus†HenosferusRougier et al. 2007
          • †Henosferus molusRougier et al. 2007
    • OrderMonotremataBonaparte 1837 sensu Luo, Cifelli & Kielan-Jaworowska 2001
      • FamilyIncertae sedis
        • Genus†KryoryctesPridmore et al. 2005(either a stem-monotremeor junior synonym ofSteropodon)
          • †Kryoryctes cadburyiPridmore et al. 2005
      • FamilyTachyglossidaeGill 1872[EchidnidaeBurnett 1830] (echidnas, spiny anteaters)
        • Genus†MegalibgwiliaGriffiths, Wells & Barrie 1991(subjective synonym or subgenus ofZaglossus)
          • †Megalibgwilia robusta(Dun 1895)(subjective synonym ofZaglossus robustus)
          • †Megalibgwilia ramsayi(Owen 1884)(senior synonym ofMegalibgwilia owenii)
        • GenusZaglossusGill 1877[ProechidnaGervais 1877;AcanthoglossusGervais 1877;BruyniaDubois 1882;BruijiaThomas 1883;ProzaglossusKerbert 1913] (Long-beaked Echidnas)
          • †MurrayglossusGlauert 1914(Hackett's Giant Echidna) (might belong to a new genus)
          • Zaglossus bruijni(Peters & Doria 1876)(Western Long-beaked/Bruinj's/Brown False Echidna)
          • Zaglossus attenboroughiiFlannery & Groves 1998(Attenborough's/Sir David's/Cyclops Long-beaked Echidna)
          • Zaglossus bartoniThomas 1907(Eastern/Barton's Long-beaked/Grey False Echidna)
        • GenusTachyglossusIlliger 1811[AcanthonotusGoldfuss 1809;EchidnaCuvier 1798;EchinopusFischer de Waldheim 1814;SyphomiaRafinesque 1815]
          • Tachyglossus aculeatus(Shaw 1792) Illiger 1811(short-beaked Echidna)
      • Family†KollikodontidaeFlannery et al. 1995
        • Genus†KollikodonFlannery et al. 1995(might be amammaliformof uncertain placement)
          • †Kollikodon ritchieiFlannery et al. 1995
      • Family†SteropodontidaeArcher et al. 1985
        • Genus†TeinolophosRich et al. 1999(might be a basal monotreme or a stem-monotreme)
          • †Teinolophos trusleriRich et al. 1999
        • Genus†SteropodonArcher et al. 1985
          • †Steropodon galmaniArcher et al. 1985
      • FamilyOrnithorhynchidaeGray 1825
        • Genus†MonotrematumPascual et al. 1992(here considered to be a basal ornithorhynchid;others subjective synonym ofObdurodon)
          • †Monotrematum sudamericanumPascual et al. 1992
        • Genus†ObdurodonWoodburne & Tedford 1975
          • †Obdurodon insignisWoodburne & Tedford 1975
          • †Obdurodon tharalkooschildPian, Archer & Hand 2013
          • †Obdurodon dicksoniArcher et al. 1992(Riversleigh platypus)
        • GenusOrnithorhynchusBlumenbach 1800[DermipusWiedermann 1800;PlatypusShaw 1799 non Herbst 1793]
          • Ornithorhynchus anatinus(Shaw 1799) Blumenbach 1800(platypus)

Phylogeny

Below is a simplified tree on Averianovet al.,2014after Woodburn, 2003and Ashwell, 2013

Yinotheria	†Shuotheriidae †Pseudotribos †Shuotherium Australosphenida †Ausktribosphenidae †Bishops †Ausktribosphenos †Henosferidae †Henosferus †Ambondro †Asfaltomylos Monotremata

References

Bibliography

• Benton, Michael J.(2005).Vertebrate Palaeontology(3rd ed.). Oxford: Blackwell Publishing.ISBN0-632-05637-1.
• Dykes, Trevor."Mesozoic Mammals: Shuotheriidae & Australosphenida, an internet directory".Archived fromthe originalon July 16, 2012.Retrieved12 May2013.
• Kemp, T.S. (1983). "The relationships of mammals".Zoological Journal of the Linnean Society.77(4): 353–84.doi:10.1111/j.1096-3642.1983.tb00859.x.
• Kielan-Jaworowska, Z.; Cifelli, R.L.; Luo, Z. (2002). "Dentition and relationships of the Jurassic mammal Shuotherium".Acta Palaeontologica Polonica.47(3): 479–486.
• Luo, Z.-X.; Cifelli, R.L.; Kielan-Jaworowska, Z. (2001). "Dual origin of tribosphenic mammals".Nature.409(6816): 53–7.Bibcode:2001Natur.409...53L.doi:10.1038/35051023.PMID11343108.S2CID4342585.
• Luo, Z.-X.; Cifelli, R.L.; Kielan-Jaworowska, Z. (2002). "In quest for a phylogeny of Mesozoic mammals".Acta Palaeontologica Polonica.47:1–78.
• McKenna, Malcolm C., and Susan K. Bell. 1997.Classification of Mammals Above the Species Level.New York: Columbia University Press.ISBN0-231-11013-8
• Kielan-Jaworowska, Zofia; Cifelli, Richard L.; Luo, Zhe-Xi (2004).Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure.New York: Columbia University Press. pp. 214–215.

External links

• Media related toYinotheriaat Wikimedia Commons
• Data related toYinotheriaat Wikispecies