| Marsupionta | |
|---|---|
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Mammaliaformes |
| Class: | Mammalia |
| Subclass: | Marsupionta Gregory, 1947 |
| Subgroups | |
Marsupionta is a hypotheticalclade ofmammals containingmarsupials andmonotremes, but not theplacentals. 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 zoologistWilliam King Gregory[1] and has since been the subject of multiple studies.[2][3][4]
The majority of researchers prefer theTheria hypothesis, in whichmarsupials andplacentals form aclade to the exclusion ofmonotremes. 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.
Potential synapomorphies of Marsupionta exist only in the epipubic (pouch) bones. The twoepipubic bones that protrude from the pelvis bone, are present in both monotremes and marsupials, but are missing in placentals. However, some primitive mammals, as well as fossil ancestors of theCretaceous higher mammals also exhibit these bones. It can therefore be assumed that the epipubic bones were anancestral trait of mammals that has been reduced in today's placentals, and that no morphological evidence exists for the Marsupionta hypothesis.
Genetic findings regarding the correct classification scheme for marsupials and monotremes are contradictory. Comparisons of mitochondrial DNA support the Marsupionta hypothesis,[5] while genome sequencing[6] speaks for the Theria hypothesis. Other studies do not come to a clear conclusion.
