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The Australosphenida are a proposed infraclass of mammals within subclass
Yinotheria Yinotheria is a proposed basal subclass clade of crown mammals that contains a few fossils of the Mesozoic and the extant monotremes. Today, there are only five surviving species, which live in Australia and New Guinea, but fossils have been found i ...
. Today, there are only five surviving species, which live in
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, smal ...

Australia
and
New Guinea New Guinea (; Hiri Motu: ''Niu Gini''; id, Papua, historically ) is the List of islands by area, world's second-largest island, and with an area of , the largest island in the Southern Hemisphere. Located in Melanesia in the southwestern Pac ...

New Guinea
, but fossils have been found in
Madagascar Madagascar (; mg, Madagasikara), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), and previously known as the Malagasy Republic, is an island country in the Indian Ocean, ...
and
Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country located mostly in the southern half of South America. Sharing the bulk of the Southern Cone with Chile to the west, the country is also bordered b ...
. The surviving species consist of the
platypus The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the cl ...

platypus
and four species of echidna. 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 mammalian middle ear independently.


Taxonomy

This grouping includes the following taxa: * Monotremata, divided into the families
Ornithorhynchidae The Ornithorhynchidae are one of the two extant families in the order Monotreme, Monotremata, and contain the platypus and its extinct relatives. The other family is the Tachyglossidae, or echidnas. Within the Ornithorhynchidae are the genus, ge ...
(platypus), Steropodontidae and Tachyglossidae (echidnas) and the genus '' Kryoryctes'' *†Henosferidae, including the genus, genera ''Ambondro (genus), Ambondro'', ''Asfaltomylos'', and ''Henosferus'' from the Jurassic of Argentina and Madagascar. *†Ausktribosphenidae, including the genera ''Ausktribosphenos'', ''Bishops (genus), Bishops'' from the Middle Jurassic to Lower Cretaceous of Australia. *†Kollikodontidae, formerly included in true monotremes but now thought to form an outgroup. *†''Vincelestes'', sometimes recovered as an australosphenidan (when not inversely considered a cladotherian). *†''Tendagurutherium'', also recently recovered as an australosphenidan. The clade Australosphenida was proposed by Luo ''et al.'' (2001, 2002) and was initially left unranked, as the authors do not apply the Linnaean taxonomy, Linnaean hierarchy. In Benton (2005), it is ranked as a 'Linnaean taxonomy#Taxonomic ranks, superdivision', i.e. one or two levels below the infraclass.


Australosphenid teeth

The grouping embodies a hypothesis about the evolution of molar (tooth), molar teeth in mammals. Living monotremes are toothless as adults, but the juvenile platypus, fossil monotremes and Ausktribosphenida all share a pattern of three molar cusp (dentistry), cusps arranged in a triangle or V shape, which is known as the Tribosphenic molar, tribosphenic type of molar. Tribosphenic molars have long been held to characterize the subclass Theria (marsupials, placentals and their extinct relatives), while monotremes were thought to be related to fossil groups with a linear alignment of cusps: morganucodontids, docodonts, triconodonts and multituberculates, all of which were united with the monotremes into the 'subclass Prototheria'. Defined in this way, the 'Prototheria' is no longer recognised as a valid clade, since the linear cusp pattern is a primitive condition within Mammalia and cannot supply the shared derived character, which is required to establish a subgroup. Instead, the available evidence suggests that the monotremes descend from a Mesozoic radiation of tribosphenic mammals in the southern continents (hence the name Australosphenida, meaning 'southern wedges'), but this interpretation is highly controversial. According to Luo ''et al.'', tribosphenic molars were evolved by the Australosphenida independently of the true Tribosphenida, or Boreosphenida (that is, the therians and their relatives) in the northern continents. Others contend that the Ausktribosphenida in fact belong to the placentals and were therefore true tribosphenids, but unrelated to the ancestry of the monotremes. If this were confirmed, it would entail abandoning the clade Australosphenida. Most recent phylogenetics, phylogenetic studies validate this clade, lumping henosferids and aukstribosphenids alongside monotremes.Rebecca Pian; Michael Archer; Suzanne J. Hand; Robin M.D. Beck; Andrew Cody (2016). "The upper dentition and relationships of the enigmatic Australian Cretaceous mammal Kollikodon ritchiei". Memoirs of Museum Victoria. 74: 97–105.


Notes


References

* * * {{Taxonbar, from=Q783432 Australosphenida, Mammal taxonomy