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 JurassicofMadagascar 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.
| Australosphenida
It survives through monotremes, if the group is polyphyletic then it also survives through Boreosphenida | |
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| Jaw fragment of Ambondro mahabo | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Clade: | Australosphenida Luo, Cifelli, & Kielan-Jaworowska, 2001 |
| Taxa | |
And see text | |
The australosphenids Steropodon and Teinolophos of the Early Cretaceous are united with Monotremata in the Monotremaformes clade, here defined by the common ancestor of Steropodon and extant monotremes (echidnas, platypus). Major groups associated with
monotremaforms are the southern tribosphenic mammals according to the majority of recent phylogenetic studies.[1]
Despite this, some researchers choose to include Steropodon within Monotremata, making Monotremaformes a minor synonym. Flannery's study indicates that Australosphenida is biphyletic, dividing the group between a paraphyletic grade related to boreosphenida and Monotremaformes.[2] Another more recent study recovers Ambondro more related to monotremes than to therians.[3]
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.
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.[7]
| Mammaliaformes |
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Most recent phylogenetic studies lump henosferids and aukstribosphenids alongside monotremes.[8][9] 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.[10] Later, Flannery and coauthors suggested that the core grouping of australosphenidans (excluding monotremes and others australosphenidans with Steropodon) 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]
| Mammaliaformes |
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On the other hand, the study that cites Flannery's article, given in 2024, maintains Ambondro as a relative of monotremata. Implying support for the Australosphenida-Boreosphenida hypothesis.[3] It may also be that both studies are right, consequently Tribosphenida would be a monophyletic group that contains the crown group Mammalia.[11][3][12]. Note: The names highlighted in pink are the specimens that initially defined Australosphenida[1].
| Mammaliaformes |
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