Wikipedia:WikiProject Tree of Life/Marsupial poll

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In March of 2004, Wikipedia user PlatinumX 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 PlatinumX nor myself are qualified to make the final decision, I would attempt to get input on this topic from users of Wikipedia.

General information[edit]

In their comment, PlatinumX, 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 Theria 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).

Choosing a system[edit]

Now that the relevant information has been presented, I will move to the purpose of this article. The goal in making it was to get input from Wikipædia users on which system of classification should be used. A number of different options are listed below. To vote, add an entry under each subsection (after the explanation paragraph). If you don't feel you're qualified to make a decision, please wait until you've read the descriptions below, and then decide if you're qualified to chose.

Please include a fairly detailed explanation of your reasoning. Any information on your qualifications (schoolteacher/professor, biologist, taxonomist, etc.) would also be useful. Please include a signature with timestamp (just type -- ~~~~ or press the 'signature and timestamp' button on the wiki edit bar on the top of the edit box) at the end of your entry. Votes without a signature and timestamp will not be counted.

To see the voting system, please look under Voting on the talk page. Thanks for your input. -- 03:19, 30 May 2004 (UTC)

System 1: Monotremata-Marsupialia-Placentalia[edit]

The classic system, with three subclasses: Monotremata, Marsupialia, and Placentalia.

  1. I'm not big on mammals, but I'd suggest this. First, everyone who recognises the groups will recognise these names, which may not be the case for the other system. Placing the latter two in a separate subclass Theria gives only a very small amount of evolutionary information, the price of which is listing both subclass and infraclass everywhere. Further, one should keep in mind the Multituberculata, an extinct group of mammals that are not "therian" but are still closer to them than to the monotremes. The Prototheria include them, and as such are a paraphyletic group, which are rejected by many authors and so should be avoided where possible. It would be better to treat them as a fourth subclass, which supports at the very least using Monotremata. -- Josh 05:40, 30 May 2004
    1. Multis are probably not closer to therians than to monos; higher-level phylogeny of mammals is highly uncertain. They're probably members of the separate subclass Allotheria. I think a subclass Theria is better, as that could also include fossil mammals such as zatherians, tribosphenidans and dryolestidans. Ucucha 08:27, 16 Apr 2005 (UTC)
  2. I'm voting for this one, too. ToL Web uses Monotremata/Marsuialia/Eutheria, but I prefer Placentalia to Eutheria. Colin Tudge, in The Variety of Life (2000 ISBN 0-19-850311-3) page 438, uses order Monotremata, Metatheria (unranked clade) and order Marsupialia, and Eutheria. - UtherSRG 02:51, 1 Jun 2004 (UTC)
  3. This is the generally recognised classification. There are good reasons to consider the others, but even better reasons to support this one, I think. My qualifications: I have no formal training in biology, however I live in Australia (where most monotremes and marsupials come from) and wrote maybe three-quarters of the Wikipedia monotreme and marsupial articles that exist at present, using the standard sources as recommended by the leading experts I have been lucky enough to meet. Tannin 07:43, 1 Jun 2004 (UTC)
    1. McKenna & Bell uses the subclass Theriiformes. I think we should follow them. Ucucha 08:27, 16 Apr 2005 (UTC)

System 2: Prototheria-Metatheria-Eutheria[edit]

Huxley’s system, in which The three subclasses are Prototheria, Metatheria, and Eutheria, rather than Monotremata, Marsupialia, and Placentalia. The three subclass using the Proto-, Meta-, and Eu- prefixes emphasizes the utility of these latin terms making classroom instruction easily and more mememorable.


System 3: Prototheria-Theria[edit]

Prototheria Theria
Prototheria Metatheria Eutheria

This is the system in which two subclasses exist: Prototheria and Theria. Theria is divided into two infraclasses, Metatheria and Eutheria. (Prototheria would also have an infraclass corresponding to these. This could be the infraclass Prototheria or Monotremata, though it could also be Ornithodelphia listed in the article Mammalia.) Explanation in table form:

  1. This is really the only way it's ever presented in the classic modern texts to my knowledge. Wilson and Reeder (1993) use this system and that's what most mammalogists take until proven otherwise. Likewise, most textbooks and mammalogy courses use this. To my knowledge, McKenna and Bell (1997) is the only major text that uses the term Placentalia (and Marsupialia in a non order or superorder sense) and they call it a cohort, not an infraclass. I'm also fairly certain McKenna and Bell (1997) place the cohorts Placentalia and Marsupialia within the infraclasses Eutheria and Metatheria respectively, though I don't have the text in front of me at the moment. Also, it's important to note the distinction between the egg-laying prototherians and the live-bearing therians. Another thing is that if Placentalia, Marsupialia, and/or Monotremata are used at a subclass or infraclass level, when articles for fossil mammals that are prototherian but not monotreme (or even ornithodelphian), metatherian but not marsupial, and/or eutherian but not placentalian, we won't have a place for them. The subclasses for extant mammals should be Prototheria and Theria (Theriiformes is also acceptable). The Theria should be divided into two infraclasses: Metatheria and Eutheria. Once extinct mammals are included: the infraclass Ornithodelphia should be used for all members of the order Monotremata, Cohort (Grandorder, Mirorder, or Superorder rank may be worthy) Marsupialia should be for for extant metatherians, and Cohort Placentalia for extant Eutherians. A final note: all marsupials have a placenta, but not all marsupials have a marsupium (pouch). The marsupial placenta is a chorioallantoic (spelling?) placenta, while the eutherian placenta is choriovitelline. The exception is the Peramelia (bilbies and bandicoots), which also have a choriovitelline placenta. The difference between eutherians and metatherians regarding this character is in the degree of transfer from mother to offspring. My point is that although they should certainly be redirected to the sites, common names should not be driving this, because the common names are misnomers. Also, common names shouldn't really be driving taxonomic arguments anyway. --Aranae 07:28, Dec 22, 2004 (UTC)
Update: I still think a monotreme group vs. live-bearing mammals is the appropriate division at the level of subclass. I'm not particular in how they're named, but Ucucha's proposal for Australosphenida seems to be more encompassing and probably best. That hypothesis is still a bit new and only in a few papers, so we may want to wait and see if there's dissension or if mammalogists start using it. I think Theriiformes is the way to go for the subclass of live bearing mammals, but that should include the Allotheria (such as in McKenna and Bell 1997). The best interpretation of fossils indicates multituberculates probably gave birth to live young. So that would be subclass Australosphenida and subclass Theriiformes. The Theriiformes contains the three infraclasses Allotheria (multituberculates and kin), Tricondonta, and Holotheria. Holotheria contains the cohorts Marsupialia and Placentalia, which contain all extant taxa. Note that extant taxa can just simply be referred to as being in the subclass Theriiformes and the appropriate cohort without listing the infraclass and other ranks. Those other ranks will come into play for prehistoric mammal articles. Without those ranks, though, the prehistoric mammals simply won't fit into the resy of wikipedia's mammal taxonomy. --Aranae 01:27, Apr 20, 2005 (UTC)
I think the system currently in use (subclass Monotremata, subclass Marsupialia, and subclass Placentalia) is about the worst option on this page.
    1. It fails to account for the long period of evolutionary history shared by marsupials and placentals.
    2. It invents new ranks for these terms. According to McKenna and Bell (1997), the term Monotremata has been used as an order before, but never as a subclass. Technically ICZN doesn't apply above superfamily, but if it did, this type of new rank would mean that wikipedia should (perhaps - people argue about what constitutes publishing) get cited by all comprehensive discussions that involve the term. Marsupialia has been used as a family, class, superorder, order, cohort, and supercohort, but never as a subclass. The same thing applies to using subclass rank for Placentalia.
    3. Many many fossil mammals just don't fit into this scheme. --Aranae 01:27, Apr 20, 2005 (UTC)
I mostly agree, however, I don't think that Allotheria and Triconodonta are Theriiformes (multis may have been given birth to live young, but that's probably convergent evolution). In that case, Holotheria has become synonymous with Theriiformes, as there are no other groups in that subclass, so that my Cladotheria-Metatheria-Eutheria system could be used. Ucucha See Mammal Taxonomy 12:56, 20 Apr 2005 (UTC)

System 4: Prototheria-Theria Variant[edit]

Prototheria Theria
Monotremata Marsupialia Placentalia

This is a variant on the Prototheria-Theria scheme. Rather than have the two infraclasses of Theria be Eutheria and Metatheria, they would be Placentalia and Marsupialia. The infraclass of Prototheria would be Monotremata. Table explanation:

  1. I am casting the vote for this system, because I feel it is the most logical. Since Monotremes, Marsupials, and Placentals are always referred to by these names, the taxa that are similar should be retained, since it makes taxonomy easier. The reason why I chose the two-sublclass/three-infraclass system is because I feel evolution is very important in taxonomy, so two subclasses with three infraclasses makes the most sense in organising mammals. Theria and Prototheria makes the most sense because Prototheria avoids confusion with its infraclass/order Monotremata. --Ingoolemo 06:29, 30 May 2004 (UTC)

System 5: Monotremata-Ditremata[edit]

Monotremata Ditremata
Monotremata Marsupialia Placentalia

This is the same as Variant 1, except that the two subclasses are Monotremata 'single opening' and Ditremata 'dual opening'. Ditremata is used in a scholarly lecture, On the emergence of mammals by B. Lukács of Hungary, in which 'Atheria [read: ''prototheria''] vs. Theria' is equated with 'Monotremata vs. Ditremata'. Table explanation:


System 6: Australosphenida-Allotheria-Theriiformes[edit]

Allotheria Australosphenida Theriiformes
Multituberculata etc. Ausktribosphenida and Monotremata Metatheria Eutheria

This system includes a number of subclasses, as following:

  • Class Mammalia
    • Incertae sedis, for example Symmetrodonta, Shuotherium and Morganucodonta
    • Subclass Allotheria, including Multituberculata, Gondwanatheria and Haramiyida
    • Subclass Australosphenida, including Ambondro, Asfaltomys, ausktribosphenids and monotremates.
    • Subclass Theriiformes
      • Infraclass Cladotheria (paraphyletic) including Zatheria, Boreosphenida, Dryolestida and others.
      • Infraclass Metatheria including marsupials and extinct relatives (Marsupialia does only include the last common ancestor of living marsupials)
      • Infraclass Eutheria including placentals and extinct relatives (Placentalia does only include the last common ancestor of living placentals)

This is the system I use on Mammal Taxonomy. I think it gives the best impression of evolutionary relationships. Ucucha 08:33, 16 Apr 2005 (UTC)

  1. Ucucha 08:33, 16 Apr 2005 (UTC). I think this is the only system in which all mammals can be placed. When you use the "Marsupialia", deltatheroids will no longer have a place, as they are metatheres but not marsupials. The Australosphenida, including Ambondro, Asfaltomylus, Ausktribosphenos and Bishops is probably better than a Prototheria which only includes the Australian "real" monotremates and Monotrematum (if you want to know what kind of beast these are, please use Mesozoic Mammals). The Cladotheria is necessary to let those beasts belong somewhere. Ucucha See Mammal Taxonomy 12:11, 19 Apr 2005 (UTC)

Ornithodelphia, Monotremata, or Prototheria?[edit]

Logically, under the Prototheria-Theria scheme, the infraclass of Prototheria corresponding to Eutheria and Metatheria should also be called Prototheria. But under Systems 4 and 5, should Monotremata be used, as described, or should Ornithodelphia be used, as in the mammal article?

  • I'm going to go for Monotremata. The reason why is because it fits in with the Mammalian taxonomy I voted for. Having a separate infraorder Ornithodelphia is just redundant, because Monotremes are the only order in it. -- Ingoolemo 06:33, 30 May 2004 (UTC)
  • Monotremata. - UtherSRG 02:52, 1 Jun 2004 (UTC)
  • Class Prototheria, Infraclass Ornithodelphia, Order Monotremata. There are mammals to build articles about that are not around today. --Aranae 07:28, Dec 22, 2004 (UTC)
  • Subclass Australophenida, Infraclass Prototheria (Ornithodelphia and Monotremata are synonyms), orders Tachyglossa and Platypoda. Ucucha 08:34, 16 Apr 2005 (UTC)

Total Votes[edit]

Mammal System vote, as of June 1:

System 1 System 2 System 3 System 4 System 5 Total Votes/Total Necessary
3 0 0 1 0
75% 0% 0% 25% 0% 4/10

Egg-laying Mammal Infraclass, as of May 30:

Monotremata Prototheria Ornithodelphia None of them
2 0 0 0