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Subgroups of Mammalia[edit]

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)
Rather than do a reboot of these various old comments (I'm not quite couragous enough for that), I would like to point out that various sources, such as Benton's Vertebrate Paleontology (2005) the Tree of Life, and the phylogeny archive are all consistent with the phylogenetic grouping of Theria to the exclusion of Prototheria (of which Monotremata is the only known-for-sure member). Theria is united by synapomorphies including live birth, chochlear coiling, and others. There is little current debate, and the arguments over this on this talk page are not even up-to-date. With regards to the eutheria node in particular, the node is currently synonymous with placentalia, but includes extinct outgroups to placentalia, which should be emphasized and more heavily discussed in the article. Metatheria is in even greater need of this discussion, as the taxobox doesn't even list extinct metatherians. While Dysmorodrepanis is right in that phylogenetic systematics will underrepresent polytomies, the most parsimonious trees will have features being evolved the fewest number of times, thus providing the bifurcating trees we currently use. Until much finer resolution and much better records can be established (things that aren't easy even with extant species), it is wise to accept the current most parsimonious tree with the knowledge that further data can occassionaly shift relationships. Hedging one's bets for possible future information is rather pointless! Tigerhawkvok (talk) 05:55, 25 July 2008 (UTC)


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)


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)

difference between Eutheria and Placentalia[edit]

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)

This article needs updating[edit]

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)

They reproduce sexually[edit]

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.


  • 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)

Placentalia and other high-ranking taxa[edit]

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)

Placentalia is the old term for Eutheria[edit]

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 (talk) 07:51, 18 September 2007 (UTC)


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)
Dysmorodrepanis, can you supply a citable ref for "a cladogram is inference (assumption), whereas an identifiable fossil is evidence (fact)"? I get annoyed with people who place excessive emphasis on phylogenetic definitions, e.g. "Dinosauria = Triceratops + Aves", which are just summaries of cladograms (inferences). -- Philcha (talk) 08:01, 24 September 2008 (UTC)

Sources, etc.[edit]

  • Eutheria appar assumes Eutheria = Placental Mammals. Most sources they cite are about placentals.


-- Philcha (talk) 07:57, 24 September 2008 (UTC)

Please review[edit]

I've had a go at getting Eutheria into good shape for Wikipedia Version 0.7. I doubt whether there's time to get it up to A-class or GA, but I'd hope B-class would be easy enough. Please comment.

Then we can consider the V 0.7 offer of free copyediting.

N.B. The previous version assumed Eutheria = placentals, which is untrue (see "epipubic bones"). So we need a new article on placentals. I think I have enough info to do this in an elapsed time of about, if anyone's interested enough to comment review in time for V 0.7. -- Philcha (talk) 13:19, 29 September 2008 (UTC)


It's strange how "placentalia" redirects to Eutheria. Placentalia is meant to embrace all true placentals, but Eutheria is meant to include placentalia as well as their closest relatives+ancestors.545lljkr (talk) 16:20, 25 October 2008 (UTC)

See previous post. So far no-one seems interested in an article about placentals, which are a distinct sub-group of eutheria. But then a lot of Wikiprojects stay away from articles about high-level taxa, because these are difficult and / or because members have their pet critters.-- Philcha (talk) 17:03, 25 October 2008 (UTC)

:: My two cents from my reading of modern marsupial genetics literature suggests ~theria is regarded as obsolete and misleading.--ZayZayEM (talk) 06:54, 26 October 2008 (UTC)

Hi, ZayZayEM, could you pease explain a bit more:
  • Which modern marsupial genetics literature.
  • Which "~theria" taxa are regarded as obsolete and misleading, and why, -- Philcha (talk) 07:54, 26 October 2008 (UTC)
Sorry. No I am confused. They are talking about the Theria hypothesis. (ie. is it [Prototheria{Meta+euth}] or [{proto+meta}Euth]). Ignore my folly. I think a seperate article for Placentalia should be considered. This article should deal with characteristics of Eutheria and known pre-placental Eutherians and their relationships.--ZayZayEM (talk) 04:43, 27 October 2008 (UTC)

stuff about Placentalia[edit]

The edits from last September look good and added useful information about the fossil forms. Now the article is about stem-eutherians, and pretty much ignores Placentalia. For instance, the list of Placental orders has disappeared from the taxobox. That would be an improvement if there were an article for Placentalia, but there isn't one - Placentalia currently redirects back to Eutheria.

May I suggest we restore the information about Placentalia until that taxon has its own article? Cephal-odd (talk) 06:58, 16 February 2009 (UTC)

I think it would be better to create a real article at Placentalia. I'm not sure how much there is to add to Eutheria, but a decent article on Placentalia would be pretty big - definition, how the placenta forms and works, pros and cons of this method of reproduction compared with egg-laying and marsupial methods, summary of modern placentals, evolutionary history. I can't offer to do it myself, as I've just fished a major revamp of Ctenophore and for a little light relief am now working on a subject not rleated to zoology or paleontology. However if anyone develops a proper article on Placentalia and asks me to comment, I will. --Philcha (talk) 08:25, 16 February 2009 (UTC)
I don't think making such an article would be a good idea as Eutheria is often used (when not discussing paleontology) for the placentals, so it would be better to discuss Placentalia in this article. Narayanese (talk) 20:44, 18 March 2009 (UTC)
I think that would be confusing as well as technically incorrect. The big difference between placental and non-placental eutheria is that the non-placental ones, like all earlier therapsids, had epipubic bones which stiffened the abdomen during locomotion. This would have limited the size of the young, so non-placental eutheria probably gave birth to very "premature" young, as marsupials so. OTOH the normal idea of a placental is of an animal with a well-developed intra-uterine nutritional system that allows for the production of fairly well-developed young (especially in precocial species like the plains herbivores, whose young can stagger about immediately and can run within a few hours). --Philcha (talk) 23:00, 18 March 2009 (UTC)

"Contradiction" with Theria[edit]

I undid the "contract" banner. I don't know whether Theria contradicts Eutheria, which has only 1 citation, to a simple link of eutherian taxa. However I produced most the current content of Eutheria and it's correct according to paleontology articles in top-class journals. --Philcha (talk) 21:19, 27 October 2009 (UTC)

Link to fossil in the lead[edit]

Philcha has undone a revision of me by which I unlinked the word fossil in the following sentence of the lead:

One of the major differences between placental and nonplacental eutherians is that placentals lack epipubic bones, which are present in all other fossil mammals and living mammals (monotremes and marsupials).

From a rough point of view, it might appear justified to link to fossil, here, because many eutheria are extinct. But the issue of eutheria is, regarding the categories of issues, not connected to the issue of fossils and fossilization, as such. Justified may be links from the article at hand to other articles on certain classes of living things, like those on mammals or monotremes, on certain palaeobiological epochs, like Mesozoic, to certain single species, like eomaia, and to all kinds of other issues which are specifically connected to the article`s catchword. For example, if eutheria should be the best or, at least, one of the very best examples of how a certain way of fossilization may take place, one could also link to the respective article. The article Fossil can, on the other hand, be linked to, for example, from the articles geology or palaeobiology. It is a too general term to be linked to from any arbitrary article on a palaeobiological issue. --Hans Dunkelberg (talk) 05:52, 28 April 2011 (UTC)

One could, for example, link to fossil, from life, plant, animal, or evolution.
If nobody should contradict me, I would go to unlink fossil, in this article, again, till about the end of May. --Hans Dunkelberg (talk)
The way you are apparently approaching linking is inconsistent with the way pretty much everyone else does it on Wikipedia. The question we should rather ask is whether a link to "fossil" is useful to readers of this article. I think it is, since readers may be unfamiliar with the concept and perhaps want to read more about it. Ucucha 14:07, 14 May 2011 (UTC)
What You say can widely be rejected on the basis of the assessment that Your formulation "the way pretty much everyone else does it on Wikipedia" is a) an exaggeration, and b) that a clearly palatable tendency of the kind You mean should not lead us to give up our goal of creating an encyclopedia as good as possible. There will always be somebody who does not understand the one or the other term, and it is always possible to type any term one is interested in into the search field and press "return". I am prettily much convinced that a more strict approach to the issue of linking could help many people to survey the matter they are currently searching information on, and also the knowledge which mankind has accumulated, so far, as a whole, much better. Trying to get as popular as possible, regarding academic principles that can not reasonably be justified to be changed, would mean to receive a poisoned chalice, for a lot of young people, in this issue. If we want to get / to stay popular, we should be polite and justify what we do by sentences of the according length and accuracy. Also check Wikipedia:Other stuff exists! --Hans Dunkelberg (talk) 14:27, 14 May 2011 (UTC)
I'm not sure quite what you're saying here, but it does seem to me that linking fossil here is reasonable, relevant, and helpful to the reader. I can't see any obvious reason not to do it, so what's the problem? I agree "other stuff exists" isn't a valid argument, but, if I understand Ucucha correctly, his argument is actually "this is useful", and I'm inclined to agree with him. Anaxial (talk) 15:10, 14 May 2011 (UTC)

We have indeed seen that Ucucha has advocated linking to Fossil, from Eutheria. I did not want to say Ucucha had been unpolite, or so, but only wanted to utter my view of what we could, generally, pay more attention to, first, to get / stay popular (statements of the sterile kind of those generated by a computer can easily be misunderstood as unpolite if they are just a little short, and Wikipedia is just dealing with a widespread impression that newcomers were treated roughly, on the page, and so), because that seemed to me to be his approach of the thing.

What I mean is that there are certain categories of things. The categories within Wikipedia quite clearly illustrate what I mean. It is always recommended to put a page into the respective subcategory, if there is a fitting subcategory. Within this categories system of Wikipedia, there is never linked to a category that would be two or even more categories higher or lower. It is a little more difficult with linking in articles, because things are not so well put into a clear arrangement, there, so that a link is not just a thing, but also an association, to a certain degree, there. Things are not put into a strict order, here, as strongly as that is the case within the categories system, which is, by its very nature, a simplification. But one can widely say that one should, generally, only link to things which are either in the same (rough spiritual) category or one category higher or one category lower, also in articles. Linking to Fossil from Eutheria would appear to me as linking to something from a completely different branch of thinking and therefore, though such links are, perhaps, not very clearly defined / described, and stigmatized, on any Wikipedia-rules page, as inappropriate.

Such links appear in little encyclopedias for children, where every foreign word is explained. Within Wikipedia, they can divert the attention for example of a student who is learning for an examination, and make it more difficult for him to concentrate onto what he is working about. Similar problems can arise, through inappropriate linking, for any other user of the Wikipedia.

For example pupils, but also adult students, can often also have certain problems with categorizing things, and we can help them by avoiding to link to and fro, all-too wildly. --Hans Dunkelberg (talk) 16:27, 14 May 2011 (UTC)

Okay, I think I understand you now. I can see the reasoning behind your argument, but I'm afraid I don't agree with it. If there is an official WP policy somewhere that supports your position, then so be it, but I'm not aware of one. In lieu of a WP policy, I would suggest that we proceed by consensus. Anaxial (talk) 17:26, 14 May 2011 (UTC)
Here is a good formulation:
"Overlinking should be avoided, because it makes it difficult for the reader to identify and follow links that are likely to be of value.[1]"
Of course, now that two people have contradicted me, here, I am not going to remove the link to Fossil, any more, unless my view should meet with the approval of a majority.--Hans Dunkelberg (talk) 20:14, 26 May 2011 (UTC)

Lead and Non-placental Eutherians[edit]

The opening sentence of the Lead is doesn't make sense, as it fails to define eutherians. I suggest it be re-worded. I would do this, but I suddenly find myself doubting my own knowledge of mammals. The article makes several mentions of non-placental eutherians, but then fails to list more specific taxa, or even provide examples. What is a "non-placental eutherian"? Boneyard90 (talk) 09:28, 24 July 2012 (UTC)

I've changed "a clade" to "the clade" in the opening sentence. Will you now agree that the sentence provides a definition? By this definition, an animal is a eutherian if and only if it is either a rodent, a primate, or a mammal more closely related to them to the marsupials. Admittedly, "to them" is ambiguous; it could mean "to the rodents and to the primates" or it could mean "to the rodents or to the primates." The ambiguity is harmless, though, since exactly the same animals are designated as eutherians whichever way you read it.
Per your request for examples, I have noted that Eomaia is a nonplacental eutherian. Since the general reader will not be familiar with the names of any taxa in this category, all of which are extinct, further examples would just be tiring. Adding the Asioryctitheria would not improve the article.
Peter M. Brown (talk) 14:31, 24 July 2012 (UTC)
Hi, and thanks for responding so quickly. Regarding the article, I think my main issue then, if eutherians are defined in terms of rodents and primates, then the relevant common feature(s) of rodents and primates is/are not defined. Based on the lead, a mammal is a eutherian if it is like a rodent or primate; off the top of my head I recognize that both are warm-blooded, fur-bearing, live-birthing, omnivorous, manually dextrous, polydactylous mammals. Which of those, if any, are the common features that define eutherians? After researching and comparing among other articles, I gathered that "polydactyly" is not a defining feature, as I believe Artiodactyla is part of Eutheria. But why rodents and primates? I thought they were just arbitrary examples, but it sounds like they embody diagnostic features of Eutheria. Last, the phrase "nonplacental eutherian" is quite helpful. It confirms what I was suspecting, but it should be made plain in the article. I would not have thought such an animal possible otherwise, probably only because I've had no reason to encounter one (as they're all extinct). One or two examples should be sufficient, if only to show the reader that examples existed in the past, and it is not simply an academically conceptual animal. Suggested opening: Eutheria is the clade of mammals that give birth to live young, and is primarily composed of mammals that nourish their young with a placenta prior to a live birth, though there are some extinct groups that did not form a placenta during gestation. The live birth of eutheria is in contrast to other mammalian modes of gestation, such as birth from an egg (Monotremata) or extended development in a maternal pouch (Marsupialia). < -- Is my understanding accurate? Boneyard90 (talk) 16:50, 24 July 2012 (UTC)
What is provided is a stem-based definition (sometimes called a branch-based definition). See the discussion at Cladistics#Phylogenetic definitions of a clade. Since statements like this do determine membership in a group, they are definitions, though they say nothing about the characteristics common to the group. For another example, see the Massopoda article: this group consists of the animals more closely related to Saltasaurus loricatus than to Plateosaurus engelhardti. The definition is silent as to what is common common to all Massapoda species. While the second paragraph of the Eutheria article does start with a sentence hinting, very vaguely, at the sorts of features that distinguish the eutherians, you will not find a precise list of distinguishing characteristics on Wikipedia. As you point out, polydactyly is not common to all eutherians, but neither is live birth; marsupials are viviparous. Peter M. Brown (talk) 18:49, 24 July 2012 (UTC)
Your explanation sounds fair enough, and were I looking at a single book where I could flip the pages back, I should agree with you, but I have come to believe that every article should be as "self-contained" as possible. A gloss is usually sufficient, with a link to an article if I want more detail; but as this article stands, because it is vague, I am required to go to other pages, and, as we've seen, I have to construct the definition I'm looking for, or come to the Talk page and ask questions. If you say this article's style is standard practice in taxonomic articles, I will not pursue the matter further; however, I do not agree that it is the best way to present an encyclopedia article. Thanks again for your considered responses. Boneyard90 (talk) 19:06, 24 July 2012 (UTC)
The sentence is defining a taxon as equivalent to a particular part of the phylogenetic tree ("every species more closely related to A than to B"; alternatively, one could use "the last common ancestor of A and C and all its descendants"). Definitions like that are increasingly common practice in taxonomy. However, the definition of Eutheria in this article is uncited, and I don't remember reading it anywhere. As far as I know, people tend to define Eutheria more informally as "everything more closely related to living placentals than to living marsupials". See, for example, Wible, J.R., Rougier, G.W. and Novacek, M.J. 2005. Anatomical evidence for superordinal/ordinal eutherian taxa in the Cretaceous. Pp. 15–36 in Rose, K.D. and Archibald, J.D. (eds.). The Rise of Placental Mammals. Johns Hopkins University Press, 259 pp. Ucucha (talk) 14:55, 25 July 2012 (UTC)
If we start defining Eutheria in terms of Placentalia, we cannot retain the definition of Placentalia in terms of Eutheria. What could we use as a definition of Placentalia? The least inclusive clade containing Xenarthra, Afrotheria, and Boreoeutheria? Peter M. Brown (talk) 17:12, 25 July 2012 (UTC)
To the best of my knowledge, the usual definition of Placentalia is the placental crown group. Anaxial (talk) 18:19, 25 July 2012 (UTC)
Placentalia defined in terms of "placental"???? And how is "placental" defined? Peter M. Brown (talk) 21:29, 25 July 2012 (UTC)
For these purposes, the living members of the Eutherian clade (i.e. those creatures commonly called 'placentals'), which is itself defined by the possession of a set of genetic and morphological synapopomorphies. That is, Placentalia is the crown group encompassed within the broader Eutherian clade, and thereby excluding the stem taxa. Similarly, Marsupalia is defined as the crown group within the Metatherian clade. Anaxial (talk) 20:13, 26 July 2012 (UTC)

Since the first sentence of your most recent contribution uses the singular pronoun "itself", the reference must be to the "Eutherian clade"; you are saying that Eutheria is defined by the possession of certain synapomorphies. That puts you at odds with Ucucha, above, who provides a stem-based definition of Eutheria as well as a source. Do I have that right? Incidentally, you surely don't mean to limit the placentals to living animals, thereby excluding, e.g., the wooly mammoth. Peter M. Brown (talk) 21:44, 26 July 2012 (UTC)

I don't have a source immediately to hand that contradicts Ucucha (well, there's this, but I don't how reliable it is, and many of the synapomorphies listed wouldn't fossilise), so I'm happy to go with his definition, to the extent that it contradicts mine. As for limiting the Placentalia to living animals, you must be misunderstanding me, so I'll try and put it a different way: the Placentalia is the crown group within the Eutheria. This, naturally, includes mammoths, among other things. I've been remiss in not sourcing any of this, so see, for example, Mammals from the Age of Dinosaurs (Kielan-Jaworowska, Cifelli, & Luo, 2004), or this online source. Anaxial (talk) 19:07, 27 July 2012 (UTC)
I have no difficulty considering Dr. Elizabeth Dumont, the professor responsible for the first page you link to, to be a reliable source. Her publication list is most impressive; really top-notch peer-reviewed journals. The page only lists a few synapomorphies, however, and there is no claim that the list is comprehensive. The synapomorphies are not presented as a definition. Kielan-Jaworowski et al. define "Eutheria" in terms of "placental", while Armstrong (your other link) defines "Placentalia" as the crown group of Eutheria. In my view, "placental" and "Placentalia" are so close in meaning that including both definitions would be circular. The Eutheria article certainly assumes that they are close, defining "Placentalia" and then proceeding to use "placental" as though no further definition is needed. Notwithstanding Ucucha's valid complaint that the definition of "Eutheria" is unsourced, I can't see replacing it without some way of avoiding circularity. Peter M. Brown (talk) 20:57, 27 July 2012 (UTC)
That's pretty much what I was trying to say, yes. Anaxial (talk) 23:13, 27 July 2012 (UTC)

The opening sentence ("the clade consisting of primates and all other mammals—in many orders—that are more closely related to them than they are to marsupials") remains problematic. Without a more clear disclaimer that the primates were named merely as an example of eutherians, this wording suggests that primates are especially important in the definition of Eutheria, as opposed to other orders such as carnivores or cetateans which by implication are not. Additionally, the phrase "many other orders" is somewhat misleading, because the word "many" downgrades the reality that eutherians make up almost ALL of the extant animals in the mammalian class: they comprise 20 of the approximately 26 classes of mammals, and 92-95% of the species. (Apologies if this restates an issue that was already resolved, but when the opening paragraph of the entry was discussed in the conversation above, the discussion seemed to veer away from the question of the use of the word 'primate' in particular.) — Preceding unsigned comment added by (talk) 04:31, 25 April 2013 (UTC)

There is certainly a problem. Eutheria is standardly given a stem-based definition, but this requires specifiers, and the general reader is too likely to think that the anatomical or behavioral characteristics of the internal specifiers are characteristic of the clade. Kielan-Jaworowska et al. (2004) have the same problem, defining Eutheria as consisting of
. . .a clade consisting of all animals more closely related to extant placentals (such as humans) than to extant marsupials (such as kangaroos).
The parenthetical phrases are really essential; since Placentalia is defined as the Eutherian crown group, it cannot without circularity be used as an internal specifier in a definition of Eutheria. Humans and kangaroos are therefore mentioned, but that suggests that humans and kangaroos are somehow better representatives of Eutheria and Metatheria, respectively, than elephants and opossums.
Sometime in the past, the phrase read "primates and armadillos"; I don't know why "and armadillos" was deleted. Should that wording be restored, to make clearer that any of a broad range of animals will do for the internal specifier? Another possibility would be to use very inclusive taxa: "more closely related to Boreoeutheria, Afrotheria, and Xenarthra than to either Ameridelphia or Australidelphia". Hitting the reader with five techical taxonomic terms, though, seems unconstructive.
Unless someone has a better idea, I'd suggest restoring "and armadillos".
I don't see a particular problem with "many other orders". Would "most other extant orders" be better? Peter Brown (talk) 21:34, 25 April 2013 (UTC)
  • Kielan-Jaworowska; et al. (2004). Mammals from the Age of Dinosaurs. Columbia University Press. p. 463. ISBN 0-231-11918-6. 
Reading back over what we previously wrote here, I can't help but think that you're giving too much importance to precise definitions. In truth, everyone involved in this issue knows what it means if Eutheria is defined as everything closer related to extant placentals than to extant marsupials and if Placentalia is defined as the placental crown group—there is no dispute whatsoever about the classification of any extant taxa within or outside Placentalia. We don't need to give a precise anchor taxon-based phylogenetic definition if our sources don't do that. Ucucha (talk) 00:15, 26 April 2013 (UTC)
What sources do you have in mind? Kielan-Jaworowska et al. certainly do give a precise anchor-based definition. And defining "Placentalia" in terms of "placental" does seem circular; a very tight circle indeed. I'm also not sure whom you include among "everyone involved in this issue"; those who have contributed to this thread? The idea is to produce something useful to the general reader. Peter Brown (talk) 01:01, 26 April 2013 (UTC)

I have attempted to clarify the status of Eutheria and Placentalia to the general reader in the lead by adding a definition :"extant placental mammals plus extinct taxa more closely related to placentals than to marsupials." The definition comes from Hu, et al. (2010). New basal eutherian mammal from the Early Cretaceous Jehol biota, Liaoning, China. Proceedings of the Royal Society B 277 (1679): 229–236. and references therein. --Animalparty-- (talk) 02:22, 8 December 2013 (UTC)


I note with considerable interest that the Early Cretaceous mammal Acristatherium has recently been added to the subgroup list. According to O'Leary et al. (2013), "The oldest age of Eutheria in our study is constrained by taxa such as Maelestes and is 91 Ma"—Late Cretaceous. What reason is there to doubt the conclusions of O'Leary et al.? Peter Brown (talk) 22:17, 5 July 2013 (UTC)


  • O'Leary; et al. (2013). "The placental mammal ancestor and the post-K–Pg radiation of placentals". Science. 339: 662–667. doi:10.1126/science.1229237. 

O'Leary et al. (2013) do not mention or discuss Acristatherium, and the statement you cite merely states that among the eutherians sampled by O'Leary et al., Maelestes and contemporaneous taxa are the oldest. I don't remember seeing any study that doubts that Acristatherium is a eutherian (then again, I don't remember reading any other paper than O'Leary et al. that doubts that Eomaia is a eutherian).
If we're discussing the age of eutherians, the elephant in the room is of course Juramaia, which is Jurassic and thus far older than Acristatherium. Archibald (2012) doi:10.1002/9780470015902.a0001555.pub3 says it may prove a stem therian, but I haven't seen anyone else doubt its eutherian affinities. Ucucha (talk) 06:45, 6 July 2013 (UTC)
From the O'Leary supplement material - "Juramaia, a new Mesozoic specimen (19) was not available for study by our team." It will be up to future studies to determine the affinities of these purported Jurassic and EK eutherians. It should be noted that O'Leary et al. did apparently accept the metatherian status of the EK Sinodelphys. If metatherians existed in the EK, then by definition eutherians had to have been contemporary. MMartyniuk (talk) 14:38, 6 July 2013 (UTC)
Thanks, both of you. So the point is that, since Sinodelphys lived at the time, there had to be eutherians around, and MMartyniuk is sure enough that Acristatherium was one of them to add it to the list, in the company of Montanalestes. So long as the metatherian status of Sinodelphys is undisputed, there's no reason to doubt that these other genera were eutherian.
It seems a little odd to think of Juramaia as an elephant. . . Peter Brown (talk) 15:53, 6 July 2013 (UTC)
"MMartyniuk is sure enough that Acristatherium was one of them" Not quite. I have no idea if it is or isn't a eutherian. But that's what it was described as and nobody has explicitly challenged its identification, so excluding it from the taxon list would be original research.
Personally, I wouldn't be at all surprised if it and many of these other things are stem-therians rather than therians. O'Leary et al present a pretty decent case for the origin of Eutheria at our around the K-Pg boundary, so it seemed odd to me that they included Sinodelphys as a therian/metatherian in their tables without comment in any of the actual text. I wonder if this was an oversight of some kind. They only state "These optimizations may change once taxa such as Deltatheridium, Sinodelphys, and Vincelestes are added to this large combined matrix", which implies they did NOT analyze the affinities of Sinodelphys and only took previous workers' word for it. Either way, more work needs to be done on these things beyond extended abstracts published in Nature with puny phylogenetic analyses. MMartyniuk (talk) 16:46, 6 July 2013 (UTC)


The taxobox seems to be getting a bit unwieldly, and descends through about half of the article. Might I suggest a simplification of the taxa within the box, with more detail given within the article body? The ?question mark taxa are confusing for one (e.g. by which source are they enigmatic?), and might better be explained in text. Additionally, Zhelestidae could essentially be collapsed for now since no descendent taxa have articles, and if Zalambdalestidae is monotypic it could be wikilinked to Zalambdalestes. I realize that the descendent articles for many non-placental eutherians are incongruent (e.g. Leptictida and Asioryctitheria seem to variously share genera), but I think a taxobox should be as concise and non-controversial as possible, and in some cases even "see text" is superior. Lastly, I'm in favor of a replacing the taxobox picture to a collage of modern eutherians (I can make a new one if need be, including non-placentals), rather than having a single species (Juramaia) as representative of the whole. This might better thematically "set" the article in the mind of the casual reader. The Juramaia image could be placed lower in the article within the discussion of evolution. --Animalparty-- (talk) 03:02, 8 December 2013 (UTC)

I think that there's often an over-use of "See text" in taxoboxen, but this seems exactly the sort of case where it's appropriate and desirable. The present version is, I suspect, somewhat confusing for more casual readers. I'd agree with the use of a collage as well, much as we have at Laurasiatheria, since the article is about the group as a whole, rather than just the stem taxa and their evolution. Anaxial (talk) 07:54, 8 December 2013 (UTC)

term "Placentalia" parantetical[edit]

the parenthetical in the first section may need further review: (technically a misleading name, since all mammals develop a placenta[2]) The article cited seems to be arguing for an expansion of the term "placenta" rather than describing a structure fitting the more common usage of the term. From the article, "Nutrients can also be transferred between embryonic compartments that are not in direct contact with the endometrium. Therefore, the definition of a placenta should include any participation of fetal membranes in intra-uterine embryo–maternal physiological exchange." However, a more typical definition of placenta would be, "The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply." (from the wiki page Placenta). However, in monotremes no such attachment to the endometrium is ever formed. I'm snipping it out. Dantose (talk) 20:23, 3 February 2014 (UTC)

That's fine with me, I'm not a fan of such editorial pendantics. Placentas and placental-like structures occur in sharks as well, but this article is clearly not about them. Placental is a common name for this group of animals, and the reproductive aspects of placentals (extant eutherians) are in desperate need of coverage here.--Animalparty-- (talk) 22:44, 3 February 2014 (UTC)