The Australosphenida are a proposed infraclass
of mammals within subclass Yinotheria
. Today, there are only five surviving species, which live in Australia
and New Guinea
, but fossils have been found in Madagascar
. The surviving species consist of the 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.
This grouping includes the following taxa:
, divided into the families Ornithorhynchidae
(echidnas) and the genus ''Kryoryctes
, including the genera
'', and ''Henosferus
'' from the Jurassic
of Argentina and Madagascar.
, including the genera ''Ausktribosphenos
'' from the Middle Jurassic
to Lower Cretaceous
, formerly included in true monotremes but now thought to form an outgroup.
'', sometimes recovered as an australosphenidan (when not inversely considered a cladotheria
'', 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 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 subclass Theria
s and their extinct relatives), while monotremes were thought to be related to fossil groups with a linear alignment of cusps: morganucodon
s and multituberculate
s, 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 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.]