This hypothesis is contrary to the conventional view that marsupials and placentals form a clade (Theria) that excludes monotremes.

Marsupionta was proposed in 1947 by the American zoologist William King Gregory[1] and has since been the subject of multiple studies.

[2][3][4] The majority of researchers prefer the Theria hypothesis, in which marsupials and placentals form a clade to the exclusion of monotremes.

Theria is characterised by a number of common derived characteristics (synapomorphies), which include among others, viviparity (the birth of live young), the presence of teats, and several features in the skull and shoulder girdle structures.

It can therefore be assumed that the epipubic bones were an ancestral trait of mammals that has been reduced in today's placentals, and that no morphological evidence exists for the Marsupionta hypothesis.