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Marsupionta
Marsupionta is a hypothesised subclass within the Mammalia group. The existence of Marsupionta is a postulation by some researchers as a category devolving upon a notional unification between marsupial Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a ...s (Marsupialia, Metatheria) with the egg-laying monotremes (Monotremata, Protheria). Under this suggested classification, placental mammals (Placentalia, Eutheria) would be the sister subclass to Marsupionta. The Marsupionta hypothesis was proposed in 1947 by W.K. Gregory and has since been the subject of multiple studies. This merging of marsupials and monotremes into the hypothesized subclass of Marsupionta is contrary to the widespread belief that pouch and placental mammals share the common subclass Theria that excludes monotremes. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Theria
Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. Characteristics Therian mammals give birth (''see viviparity'') to live young without a shelled egg. This is possible thanks to key proteins called syncytins which allow exchanges between the mother and its offspring through a placenta, even rudimental ones such as in marsupials. Genetic studies have suggested a viral origin of syncytins through the endogenization process. The marsupials and the placental mammals evolved from a common therian ancestor that gave live birth by suppressing the mother's immune system. While the marsupials continued to give birth to an underdeveloped fetus after a short pregnancy, the ancestors of placental mammals gradually evolved a prolonged pregnancy. Therian mammals no longer have the coracoid bone ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |