Subclass of mammals
The Australosphenida are a clade of mammals, containing mammals with tribosphenic molars, known from the Jurassic to Mid-Cretaceous of Gondwana. Although they have often been suggested to have acquired tribosphenic molars independently from those of Tribosphenida, this has been disputed. 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 many studies, but this is also disputed.
Taxonomy
This grouping includes the following taxa:
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.
Evolution
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 ausktribosphenids (two families of the Australian Cretaceous tribosphenids) in fact belong to the placentals and were therefore true tribosphenids, but unrelated to the ancestry of the monotremes.[5]
Most recent phylogenetic studies, lump henosferids and aukstribosphenids alongside monotremes.[6][7] However in a 2022 review of montreme evolution noted that 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 molars, meaning that monotremes and non-monotreme australosphenidans were likely unrelated.[8] Later, Flannery and coauthors suggested that the core grouping of australosphenidans (excluding monotremes) were actually stem-therians as members of Tribosphenida, with the group representing a paraphyletic grade, with Bishopidae more closely related to Theria than to other australosphenidans.[2]
