Talk:Eutheria

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I have moved the information in Eutheria back to this page. Some time ago, a user posted a question on the talk page for Marsupial, wondering if the subclass should be changed to Metatheria. Since neither I nor the User (PlatinumX) was qualified to make such a decision, I decided to solicit users for their input. Unfortunately, after four opinions were voiced, no one else took any notice. At the request of UtherSRG, I decided to go ahead with the changes recommended following my input solicitation. The results were as follows: three of the four votes for a system dividing the class Mammalia into three subclasses, Monotremata-Marsupialia-Placentalia. Josh argued that this system should be adopted for clarity, because derivatives of the three taxa are the names used in everyday speech (when was the last time you heard a marsupial called a metathere?). The only other system that received a vote (one vote from me), was a system in which there are two subclasses (Prototheria, consisting of Monotremata, and Theria, consisting of Marsupialia and Placentalia). The vote for this was cast on the grounds of evolutionary taxonomy, in which the groups most closely related are grouped most closely. To paraphrase Josh again, this provides very little evolutionary information, but the cost is that one must list both subclass and infraclass in every taxobox. Suffice it to say, this is the purpose I have in moving it back to Placentalia.--Ingoolemo 17:31, 2004 Jun 17 (UTC)

Since nothing happened in the last three years, I decided to use the system that is most commonly used among systematic biologists. That means that Mammalia is split in three: Prototheria (which is known to be polyphyletic, but only includes Monotremata in this context), Metatheria (which is inclusive of, but not the same as, Marsupialia) and Eutheria (which is inclusive of, but not the same as, Placentialia). DaMatriX 21:40, 26 April 2007 (UTC)

[edit] Fusion

I've been searching in taxonomic databases and nowhere there's the differentiation between "placentalia" and "eutheria". My conclusion is that this differentiation is false and the two taxons are the same. The two articles must be fused. Llull 14:22, 1 March 2006 (UTC)

[edit] Edentata

The edentata griup has been added when the genetic analisys have demostraed that is a poliphylenic group. Then it must be deleted. Llull 09:41, 11 March 2006 (UTC)

[edit] difference between Eutheria and Placentalia

Although the membership of Eutheria and Placentalia are very similar, there is a difference between how the two names are used by many paleontologists. Placentalia is a crown group embracing all extant placentals. Eutheria is a total group (or "panstem") that includes all creatures more closely related to placentals than to their next closest living kin (presumably marsupials).

The difference in membership is small because few fossil species are known from the stem group (that is, belong to Eutheria but not Placentalia). So it may not be worth it to separate the two articles. On the other hand, future discoveries may uncover many eutherians that aren't placentals, providing much food for a separate article about Eutheria. In either case, it would be good to explain the distinction in this article. Cephal-odd 04:18, 27 October 2006 (UTC)

[edit] This article needs updating

Terminology, facts and new hypotheses are really challenging mdoern classification of mammals.

"Eutherians are distinguished from other mammals in that the fetus is nourished during gestation via a placenta while, in general, this is not the case with other mammals (Bandicoots are a conspicuous exception to this rule.)" -- Almost all marsupials have some form of placenta that is essential for gestation.

Also the realtionship between Metatheria, Prototheria and Eutheria is under scrutiny as conflicting reports suggest that Meta- and Proto- may be sister clades to the exclusion of eutheria.

--ZayZayEM 02:05, 26 April 2007 (UTC)

To my knowledge, metha- and eutheria are sisterclades, with the exclusion of prototheres. Do you have a source for that statement you make? Mesozoic genera like Eomaia are regarded basal Therians and ancestral to both metatheria and eutheria. Besides, the placenta is what make eutherians distinct from other mammals, so I do not understand your doubt on the accuracy of this article....DaMatriX 21:27, 26 April 2007 (UTC)

[edit] They reproduce sexually

The following sentences are confusing:

Eutherians are distinguished from other mammals in that the fetus is nourished during gestation via a placenta while, in general, this is not the case with other mammals (Bandicoots are a conspicuous exception to this rule.) They reproduce sexually, and the offspring are carried in the mother until fully developed. Eutherians, like the related metatherians, are viviparous.

Problems:

• One can infer that the closing parenthesis should perhaps be located at the end of the following sentence; that is, that the intent of the sentence after the parenthesized one ("They reproduced sexually . . .") is intended to discuss bandicoots. But that can't be right as bandicoots are marsupials, which means they're born before being fully developed.
• Therefore the inference is that "They reproduce sexually . . ." is intended to discuss the main subject, eutherians. But then, the sentence just before the parenthesized one says ". . . this is not the case with other mammals" so one expects that this sentence should explain how eutherians differ from other mammals. But all mammals reproduce sexually, even if all mammals aren't carried in the mother until fully developed. One way to help would be simply to remove the comma after sexually, but perhaps someone knowledgeable can further improve on this. Also note that "reproduce sexually" literally means that reproduction is not asexual, that is, it requires male and female gametes. But it could also be taken to mean that a copulatory act is required. Anomalocaris 02:43, 3 June 2007 (UTC)

I agree, and I've clarified this with an edit. Now the eutherians are clearly distinguished between other mammal groups, and the comment about sexual reproduction was removed since it wasn't especially relevant.Twir 18:25, 24 August 2007 (UTC)

[edit] Placentalia and other high-ranking taxa

Some recent literature has recognized stem Eutherians (that is, non-placentals), so I added a few to the taxobox.

Also, there is much disagreement about how the major placental groups are related, and the "Epitheria" theory (that xenarthrans are the earliest-branching group) faces some strong contenders (for example, Afrotherians may have branched earliest). So I removed Epitheria from the taxobox and listed Xenarthra as equal to the other super-orders.

Cephal-odd 01:44, 3 July 2007 (UTC)

[edit] Placentalia is the old term for Eutheria

Placentalia is the old name for Eutheria. Placentalia was used because the young grows in the womb connected to the placenta. This definition is misleading because marsupialia young also develop this way to begin with but are born at a much earlier stage in developmental, hence their major development in their mother's pouch. Because of its misleading nature, placentalia was changed to eutheria. This is the definition currently taught in zoology courses in university in Australia, like the University of Melbourne.—Preceding unsigned comment added by 220.239.209.158 (talk) 07:51, 18 September 2007 (UTC)

[edit] nomenclature

Absolutely, this all needs to be fixed. Firstly, I object to the assertion that bandicoots "develop small placenta-like structures during gestation". They are not just "placenta-like" - they are bone fide, highly invasive, chorio-allantoic placentae. Even sharks have placentae - and they are not just "placenta-like", but fit the full, functional definition of a placenta, and are accordingly termed such. Furthermore, all marsupials are in fact "placental", although in most cases the placenta is rather superficial. Nevertheless, there is interdigitation between endometrial and trophoblast tissues, which means they are quite "sticky" when you pull them apart (I've done it myself). Nevertheless the term "Placental Mammal" has unfortunately become entrenched for eutherians. Even the term "Marsupial" is not entirely appropriate, as many marsupials do not have a pouch, and indeed echidnas (monotremes) have a pouch-like structure that holds the egg. There are other aspects of reproductive anatomy that are in fact universally diagnostic of marsupials, although unfortunately they are not incorporated into any existing nomenclature. "Eutheria" and "Metatheria" have their own problems as terms, as they imply that marsupials are somehow not "proper" mammals, which of course they are. So there is no perfect resolution to the nomenclature problem, however I think it's important they these issues are incorporated into article so that readers understand them.

--Sfbergo (talk) 03:53, 30 December 2007 (UTC)

From the experiences we had at birds, and briefly having browsed into the Ferungulata/Cetartiodactylata dispute (not as clear-cut as Ferungulata puts it BTW as it is a Linnean taxon - meaning it can always be made monophyletic, but might become a synonym or not make sense anymore afterwards) -

it is not wise to use too many of the proposed phylogenetic taxa. The guideline that works very well with birds - if it has been proposed "traditionally", check monophyly and adjust as needed. If support is close to unequivocal (Cypselomorphae is one of these cases), add and adjust systematics as needed.

Any "clade" with less than multiple lines of evidence and 70-80% bootstrap support or with 1-2 lines of evidence and less than 95% (ML) to 99% (MP) bootstrap support - forget about it until better data comes along.

The main problem is: what the studies present is usually one or two "best-fit" phylograms. We never get to see the others. Phylogeny software is so computationally intensive (and PAUP* is not even parallel processing-capable) that competing hypotheses rarely get tested/discussed in detail. Add to that cytochrome b (and probably other mitochondrial sequences) in small mammals accumulating 2% sequence change on average every million year and homoplasy levels of up to 25% and possibly more in the supposedly "close to infallible" transposon-based phylogenies, and you're in for a big mess.

Another problem is: phylogenetic software tries to break down everything into dichotomies. But these are simply more common, not exclusive in nature (phylogenetic systematics IMHO tends to grossly overestimate how common polytomies are. Speciation and lineage sorting don't happen instantaneously, they take time. And whatever else happens during this time happens de facto simultaneously.

So any interordinal relationships going back into the Mesozoic and based on mtDNA are likely to be utter guesswork - educated guesswork, but still a far shot from robust. (It would only be proper to add - as they are based on assumptions on transition-transversion ratios that may or may not be correct - error bars around each node. Isn't done either.)

The Monotremata-Marsupialia-Placentalia lineup discussed in the intro is such a case. Last year's new fossils basically make every proposed scenario seem shaky in one way or another. Odds are the monotremes are a bit more distant, but there is really no good data on which to base a decision. We need fossils closer to the split than we have.

In a nutshell, it helps to remember that a cladogram is inference (assumption), whereas an identifiable fossil is evidence (fact). Dysmorodrepanis (talk) 06:33, 30 January 2008 (UTC)