Monotremes have also been considered a part of this group in its original definition and in many subsequent studies, but its relationship with other members has been disputed by some scholars.

In 2001, Luo et al proposed the clade Australosphenida to include monotremes as well as Jurassic and Cretaceous mammals with tribosphenic molars known from the Southern Hemisphere, based almost exclusvely on characters of the skull and lower jaws.

[5] Cladogram after Luo et al 2001:[6] Basal mammaliaforms Shuotherium Ambondro Steropodon (Monotremata) Ausktribosphenos Eutriconodonta Zhangheotherium Henkelotherium (Dryolestida) Peramus Kielantherium Eutheria Metatheria 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.

[7] Most recent phylogenetic studies lump henosferids and aukstribosphenids alongside monotremes.

[10] 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.