The family Henosferidae, comprising the genera Henosferus, Ambondro, and Asfaltomylos, has been found in the southern hemisphere at locations in Argentina and Madagascar.

This suggests that this family could have been more widespread and diverse in Gondwana during that time; however, due to their fragile state, some fossils might have been destroyed by geological events.

[9] All these dates are more recent than the oldest known platypus fossils, suggesting that both the short-beaked and long-beaked echidna species are derived from a platypus-like ancestor.

The threefold division of living mammals into monotremes, marsupials and placentals was already well established when Thomas Huxley proposed the names Metatheria and Eutheria to incorporate the two latter groups in 1880.

[10] Prototheria, on the other hand, was generally recognised as a subclass until quite recently, on the basis of a hypothesis that defined the group by two supposed synapomorphies: (1) formation of the side wall of the braincase from a bone called the anterior lamina, contrasting with the alisphenoid in therians; and (2) a linear alignment of molar cusps, contrasting with a triangular arrangement in therians.

Therefore, neither of these states can supply a uniquely shared derived character that would support a 'prototherian' grouping of orders in contradistinction to Theria (Kemp 1983).

Some peculiarities of this dentition support an alternative grouping of monotremes with certain recently discovered fossil forms into a proposed new clade known as the Australosphenida, and also suggest that the triangular array of cusps may have evolved independently in australosphenidans and therians (Luo et al. 2001, 2002).

[13] As a result, some taxonomists (e.g. McKenna & Bell 1997) prefer to maintain the name Prototheria as a fitting contrast to the other group of living mammals, the Theria.

[16] Future analysis and better fossil remains could affect the membership of Yinotheria as well as rearranging and revising the relationships of stem-monotremes and crowned monotremes.