Marsupionta is a hypothetical

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s containing

marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...

s and

monotreme Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...

s, but not the placentals. This hypothesis is contrary to the conventional view that marsupials and placentals form a clade (

Theria Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...

) that excludes monotremes. Marsupionta was proposed in 1947 by the American zoologist William King Gregory and has since been the subject of multiple studies.

Evidence

Morphological evidence

The majority of researchers prefer the

hypothesis, in which

s and placentals form a

to the exclusion of

s. Theria is characterised by a number of common derived characteristics (

synapomorphies In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to ...

), which include among others,

viviparity In animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juv ...

(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 two

epipubic bone Epipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials, monotremes and fossil mammals like multituberculates, and even basal eutherians (the ancestors of placentals, who lack them). They first occur i ...

s 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 the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

higher mammals also exhibit these bones. 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.

Molecular evidence

Genetic findings regarding the correct classification scheme for marsupials and monotremes are contradictory. Comparisons of mitochondrial DNA support the Marsupionta hypothesis, while genome sequencing speaks for the Theria hypothesis. Other studies do not come to a clear conclusion.

References

{{Taxonbar, from=Q1902874 Mammal taxonomy Marsupials Monotremes