Australosphenida

The Australosphenida are a clade of mammals, containing mammals with tribosphenic molars, known from the Jurassic to Mid-Cretaceous of Gondwana. They are thought to have acquired their tribosphenic molars independently from those of Tribosphenida. Fossils of australosphenidans have been found from the Jurassic of Madagascar and Argentina, and Cretaceous of Australia and Argentina. Monotremes have also been considered a part of this group in some studies, but this is disputed.

Taxonomy

This grouping includes the following taxa:

*†Henosferidae, including the genera '' Ambondro'', ''Asfaltomylos'', and ''Henosferus'' from the Jurassic of Argentina and Madagascar.
*†Ausktribosphenidae, including the genera ''Ausktribosphenos'', '' Bishops'' from the Lower Cretaceous of Australia and Mid-Cretaceous of Argentina
*†''Vincelestes'', sometimes recovered as an australosphenidan (when not inversely considered a cladotherian).Nicholas Chimento, Frederico Agnolin, Agustin Martinelli, Mesozoic Mammals from South America: Implications for understanding early mammalian faunas from Gondwana, May 2016
*†'' 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 hierarchy. In Benton (2005), it is ranked as a ' superdivision', i.e. one or two levels below the infraclass.

Australosphenid teeth

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 (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 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. However in a 2022 review of montreme evolution, it was noted that the most primitive monotreme ''Teinolophos'' differed substantially from other non-monotreme Australosphenidans'','' having five molars as opposed to three in all other non-monotreme australosphenidans, and having non-tribosphenic teeth, which appears to have only evolved in more derived monotremes, meaning that the two groups were likely to be unrelated. Later, the same authors suggested that the core grouping of australosphenidans (excluding monotremes) were actually stem-therians.

Notes

References

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Mammal taxonomy