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The traditional "Theria hypothesis" states that the divergence of the monotreme lineage from the Metatheria and Eutheria lineages happened prior to the divergence between marsupials and placentals, and this explains why monotremes retain a number of primitive traits presumed to have been present in the synapsid ancestors of later mammals, such ...
The latter subclass is divided into two infraclasses: pouched mammals (metatherians or marsupials), and placental mammals (eutherians, for which see List of placental mammals). Classification updated from Wilson and Reeder's "Mammal Species of the World: A Taxonomic and Geographic Reference" using the "Planet Mammifères" website. [1]
Marsupials' reproductive systems differ markedly from those of placentals, [10] [11] though it is probably the plesiomorphic condition found in viviparous mammals, including non-placental eutherians. [12] During embryonic development, a choriovitelline placenta forms in all marsupials.
Placental mammals are anatomically distinguished from other mammals by: a sufficiently wide opening at the bottom of the pelvis to allow the birth of a large baby relative to the size of the mother. [4] the absence of epipubic bones extending forward from the pelvis, which are found in all other mammals. [5]
Due to the fact that placental mammals and marsupials nourish their developing embryos via the placenta, the ovum in these species does not contain significant amounts of yolk, and the yolk sac in the embryo is relatively small in size, in comparison with both the size of the embryo itself and the size of yolk sac in embryos of comparable developmental age from lower chordates.
This could be explained by an original feature of mammals, as these epipubic bones are also found in monotremes. Marsupial reproductive organs differ from the placentals. For them, the reproductive tract is doubled. The females have two uteri and two vaginas, and before birth, a birth canal forms between them, the median vagina. [7]
The class Mammalia is divided into two subclasses based on reproductive techniques: monotremes, which lay eggs, and therians, mammals which give live birth, which has two infraclasses: marsupials/metatherians and placentals/eutherians. See List of monotremes and marsupials, and for the clades and families, see Mammal classification ...
The monotremes have a sex-determination system different from that of most other mammals. [131] In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal. [132] Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses.