- Australosphenida
Taxobox
name = Australosphenida
fossil_range =Middle Jurassic - Recent
image_width = 200px
image_caption =Short-beaked Echidna
regnum =Animal ia
phylum = Chordata
classis =Mammal ia
subclassis = Australosphenida
subclassis_authority = Luo, Cifelli, and Kielan-Jaworowska, 2001
subdivision_ranks = Divisions or Orders
subdivision =
* †Ausktribosphenida
*Monotremata The Australosphenida are a
clade of mammals. Today, living specimens exist only inAustralia andNew Guinea with only five surviving species, but fossils have been found inMadagascar andArgentina . The surviving species consist of theplatypus and 4 species ofechidna .This grouping includes the following animals:
*
Monotremata , divided into familiesKollikodontidae †,Ornithorhynchidae (platypus),Steropodontidae † andTachyglossidae (echidnas) and the genus "Kryoryctes ";
*Ausktribosphenida †, including the genera "Ambondro", "Asfaltomylos ", "Ausktribosphenos ", "Bishops", and "Henosferus " from the MiddleJurassic to LowerCretaceous of Argentina, Madagascar and Australia.'†' denotes anextinct taxon .The clade Australosphenida was proposed by Luo "et al." (2001, 2002) and was initially left unranked, as the authors do not apply the Linnaean hierarchy. In Benton (2005) it is ranked as a 'superdivision', i.e. one or two levels below the
infraclass .The grouping embodies a hypothesis about the evolution of 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 cusps arranged in a triangle or V shape, which is known as the
tribosphenic type of molar. Tribosphenic molars have long been held to characterize the subclassTheria (marsupial s,placental s and their extinct relatives) while monotremes were thought to be related to fossil groups with a linear alignment of cusps:morganucodon tids,docodont s,triconodont s andmultituberculate s, all of which were united with the monotremes into the 'subclassPrototheria '. 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 aMesozoic 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 , orBoreosphenida (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. [Benton 2005: 300, 306-308.] If this were confirmed, it would entail abandoning the clade Australosphenida.Notes
References
* Benton, Michael J. 2005. "Vertebrate Palaeontology". 3rd ed. Oxford: Blackwell Publishing. ISBN 0-632-05637-1
* Luo, Z.-X., R.L. Cifelli and Z. Kielan-Jaworowska. 2001. Dual origin of tribosphenic mammals. "Nature" 409: 53-7.
* ———. 2002. In quest for a phylogeny of Mesozoic mammals. "Acta Palaeontologia Polonica" 47: 1-78.
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