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In March of 2004, Wikipedia user posted a comment on the discussion page of the article Marsupial, asking whether Marsupials should be placed in the subclass Metatheria or Marsupialia. Acting upon this, I decided that since neither nor myself are qualified to make the final decision, I would attempt to get input on this topic from users of Wikipedia.
In their comment, , cited two reasons for switching from Marsupialia to Metatheria. Firstly, the article on the taxon Metatheria seems to suggest it is the more modern taxon. Secondly, University of Michigan Museum of Biology uses Metatheria rather than Marsupialia.
Metatheria is part of a classification system proposed by Thomas Henry Huxley in 1880, in which the three main groups of mammals were classified into three subclasses, Prototheria, Metatheria, and Eutheria. The older system was essentially the same, but used the names Monotremata, Marsupialia, and Placentalia.
The older system referred to the unique features of each group's reproductive system. Monotremes are unique because their urinary, excretory, and reproductive systems share the same opening (monotreme means 'single opening' in Greek). Marsupials are named after the pouch, or marsupium , in which their young spend most of their foetal development. Placentals are named for the placenta, a system in placental mammals that protects the developing organism from harmful substances such as toxins and bacteria during pregnancy. The advantage of the older system is that the animals within each taxon are called by the same name in everyday conversation: monotremes and marsupials are rarely, if ever, called prototheres and metatheres.
Huxley's Prototheria-Metatheria-Eutheria is almost a renaming of the older system. Prototheria means 'first beasts', a reference to the fact that monotremes were the first mammals and the closest to mammals' reptilian ancestors. Metatheria can be roughly translated as 'intermediary beasts', referring to a common belief at that time that marsupials were ancestors of placentals. Eutheria means 'true beasts', reflecting a feeling that placentals were 'superior' in the evolutionary line over Prototheria and Metatheria. One potential criticism of this system is that it seems to suggest that evolution is a march from 'primitive' to 'advanced' forms of life, which is both arrogant and completely untrue.
The Prototheria-Metatheria-Eutheria system has a number of miniscule differences from the Monotremata-Marsupialia-Placentalia system. One of these is that the groups involved are somewhat more inclusive than their predecessors. For example, Metatheria also includes the non-marsupial ancestors of the marsupials, after their divergence from placentals; Eutheria includes the non-Placental ancestors of placentals, after their divergence from marsupials.
A variant of the Prototheria-Metatheria-Eutheria system is the Prototheria-Theria system. Under this system, two subclasses exist: Prototheria 'first beasts', and TheriaTheria is a subclass of the biological class level taxon of Mammalia. Theria means "beasts". These are the advanced mammals which give birth to live young without using a shelled egg. They have external ears, can suckle on a nipple, and have an ankle spec beasts. Theria is divided into two infraclasses, Metatheria and Eutheria. This system is supported by the fact that the marsupial and placental groups diverged from each-other more recently than the monotremes separated. The Prototheria-Theria system is consistent with 'evolutionary classification', in which the organisms in a taxon are more closely related to each other than those in other taxa.
The two infraclasses of Theria in the Prototheria-Theria system may also be Marsupialia and Placentalia. This provides the connexion between the groups and the names of their taxa seen in the pre-Huxley system. (This particular variant can go even further, using Monotremata-Ditremata, rather than Prototheria-Theria).