Talk:Crown group

Diagram needed

This article would be substantially easier to understand if it included a diagram illustrating the relationships that it describes. Harold f 18:10, 2 March 2007 (UTC)

Added 6/&03/07Grahbudd 10:14, 6 March 2007 (UTC)

The diagram's a bit weird, though. I've never seen the term "scion" used like that, and the "zygotaxon" is just another (larger) crown group. Neither term is even in Wikipedia. 75.30.150.158 00:18, 24 April 2007 (UTC)

Just to express a degree of amusement that the lack of a Wikipedia page can be considered proof of the non-existence of a term!

The term zygotaxon, and indeed all 'crown group terminology', is relative to which crown group we're talking about. So if we're talking about the birds, it can be useful to consider Archaeopteryx as part of the zygotaxon, and other dinosaurs as part of the stem group - this provides an extra degree of discrimination. W

Whilst the seemingly excessive terminology does seem to cloud the issue at first, it is logical (to a Greek speaker at least!) and makes it easier to visualise evolution in a meaningful framework, reducing the desire to pigeon-hole fossils into modern crown groups that has plagued pre-Ordovician palaeontology for so long!

Verisimilus T 18:19, 9 May 2007 (UTC)

"The" crown group

Is it not the case that biologists sometimes use the term "crown group" or "the crown group" to refer to the crown group of all living organisms? If so, the article does not seem to mention it. -- Dominus (talk) 17:40, 6 October 2008 (UTC)

This term is really overused and I am sick of seeing it everywhere. Do people just like saying crown before every word? — Preceding unsigned comment added by 166.137.100.23 (talk) 07:37, 15 April 2013 (UTC)

Read the article before commenting on it! In discussing birds, for example, writing "crown-group birds" is essential if, for example, one wants to exclude animals like Archaeopteryx. Peter Brown (talk) 16:38, 15 April 2013 (UTC)

Orphaned references in Crown group

I check pages listed in Category:Pages with incorrect ref formatting to try to fix reference errors. One of the things I do is look for content for orphaned references in wikilinked articles. I have found content for some of Crown group's orphans, the problem is that I found more than one version. I can't determine which (if any) is correct for this article, so I am asking for a sentient editor to look it over and copy the correct ref content into this article.

Reference named "Budd2000":

• From Amiskwia: Budd, G.E. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla". Biological Reviews. 75 (02): 253–295. doi:10.1017/S000632310000548X. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• From Cambrian explosion: Budd, G.E. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla" (abstract). Biological Reviews. 75 (02): 253–295. doi:10.1017/S000632310000548X. Retrieved 2007-06-27. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

I apologize if any of the above are effectively identical; I am just a simple computer program, so I can't determine whether minor differences are significant or not. AnomieBOT⚡ 19:28, 25 October 2008 (UTC)

Feedback request

{{Cambrian explosion calling card}} I hope that this article is now up to "B-class" standards; can anyone suggest a way that it could be further improved? Thanks, Martin (Smith609 – Talk) 19:38, 25 October 2008 (UTC)

The first order of the day for making this a better article is dropping Archaeopteryx as an example. "Stem birds" include everything from dinosaurs to a host of Thecodonts and possibly Pterosaurs. Thus "stem bird" is not a very precise categorization and does not really help in understanding Archaeopteryx. I'd rather use stem terapods or stem mammals in stead, as these points to groups that are fairly well defined and are often discussed as units. Petter Bøckman (talk) 09:10, 24 March 2010 (UTC)

One problem is the appearance of the terms "plesion", "scion" & "zygotaxon" in the related diagram -- none of which are defined or even alluded to in the article text. (And after a search thru Wikipedia, I couldn't find them mentioned in any other article, either.) What do these terms mean? How do they relate to the general theory & practice of cladistics? If they are simply synonyms of evolutionary grade, then perhaps the diagram ought to be simplified. -- llywrch (talk) 23:57, 11 June 2010 (UTC)

The crown-concept has not been universally adopted, and the other groups (with the exception of stem-groups) has largely remained constructs so far. I have restructured the article a bit so that these groups now can be given their own short descriptions. --Petter Bøckman (talk) 07:42, 12 June 2010 (UTC)

Your changes do help explain. Thanks. -- llywrch (talk) 06:18, 13 June 2010 (UTC)

Thanks! Now it's just finding better examples and trying to make the tone a tad more neutral...Petter Bøckman (talk) 21:27, 13 June 2010 (UTC)

This is a useful article which I hadn't been aware of when editing articles which use phylogenetic terminology (e.g. Kingdom (biology), Eukaryotes); MUCH clearer than the article on monophyly which is also needed to explain ideas to readers. I've taken the liberty of (a) making a note that not everyone insists that all members of a crown group be extant, although this does semm to be the original and most widely used definition (b) revising the diagram so that the stem and crown groups don't overlap: the original version suggested that the basal node of the crown group might also be in the stem group. Comments? Peter coxhead (talk) 09:23, 4 December 2010 (UTC)

I can't relate the diagram to the text at all. The text implies the scion and plesion are part of the crown group, since they are descended from the basal node (the most recent common ancestor), but the diagram shows them as distinct from the crown group (in red). Arrrgh! -- cheers, Michael C. Price ^talk 13:00, 20 June 2011 (UTC)

Uh, the text rather clearly says the plesion grpup is splitting off before the most recent common ancestor of the crown group, so yes, they are outside (and hence extinct by definition). A scion group is supposed to be a crown group with some extinct ancestors, typiaccly anchored in a crown species and a representative for an extinct side branch (e.g. birds as sparrow + Archaeopteryx). I'm more than happy to accept suggestion for improvements and clarification. Petter Bøckman (talk) 13:15, 20 June 2011 (UTC)

The "basal node" is the MRCA of the red area, therefore the scion group and plesion are part of the crown group - according to the lead definition. Or are the two red triangles different crown groups? If so this must be indicated, 'cus this confused the hell out of me. -- cheers, Michael C. Price ^talk 13:19, 20 June 2011 (UTC)

Yes, they are supposed to be. You can always define a "crown within a crown", e.g. crown big cats within the crown felides. I think we need a new diagram. Petter Bøckman (talk) 13:26, 20 June 2011 (UTC)

Diagram is fine - it just needed an explanation! -- cheers, Michael C. Price ^talk 13:32, 20 June 2011 (UTC)

The Stem groups section contains the sentence:

"All organisms more closely related to crown-group birds than to any other living group (i.e. crocodiles) are referable to the bird stem group."

This is incorrect. Dodos were more closely related to crown-group birds than to any other living group, but they are not referable to the bird stem group. What the sentence actually defines is the total group. With "total group" substituted as the definiendum, the sentence could profitably be used as the total-group definition. Such a definition is needed as the term is not otherwise defined, although Total group redirects to Crown group. Stem group can readily be defined in terms of total group and crown group.

Why don't you just call them birds? Why do you have to say crown birds? — Preceding unsigned comment added by 166.137.100.23 (talk) 07:42, 15 April 2013 (UTC)

The parenthetical "i.e. crocodiles" is not clear. If the idea is only to give an example of a group to which stem- or total-group birds are less closely related than they are to crown-group birds, "e.g." should be used. Mammals and slime molds are also examples, though. These groups differ from crocodiles in that each is less closely related to crown-group birds than is some other extant group, lizards for example. What is distinctive about extant crocodilians is that they are more closely related to extant birds than they are to any extant group that contains neither birds nor crocodilians. If the intent is to call attention to this distinctiveness, some rewording is in order.

Crocodilians, not crocodiles. Crocodiles are no more closely related to birds than are caimans. Peter M. Brown (talk) 01:16, 9 August 2011 (UTC)

I have replaced "crocodile" by the more specific "crocodilian", as it is clearer in the context of this already very technical and quite difficult to understand article. However, care is needed in cases like this. The Crocodilia article says 'The correct vernacular term for this group is "crocodilians"'. However, in Wikipedia the "correctness" of common/vernacular terms is not to be determined by what editors think is correct, but by what is used in appropriate sources. (There are some quite long-running battles over this issue elsewhere; e.g. attempts to say that the term "ape" includes humans not because sources consistently use "ape" this way, but because this makes "ape" equivalent to a clade). A quick Google search suggests to me that "crocodile" is regularly used in both senses, i.e. for extant members of both the order Crocodilia and the family Crocodylidae. It would be good if common use was more precise, but Wikipedia's principles require us to follow, not lead. Peter coxhead (talk) 08:29, 9 August 2011 (UTC)

Sigh, the pitfalls of foreign languages... Thanks for helping make this article better chaps! Petter Bøckman (talk) 08:49, 9 August 2011 (UTC)

The stem group section isn't very good to begin with (it is mostly composed of hallelujah sections salvaged from the original article), I think a full rewrite is in order. The section should start with a the definition followed by examples, as with the other expressions. Petter Bøckman (talk) 08:54, 9 August 2011 (UTC)

Next time just say crown crocodiles. Then everybody will know what you mean. — Preceding unsigned comment added by 166.137.100.23 (talk) 07:39, 15 April 2013 (UTC)

Alternative definition of crown group

The alternative definition of "crown group" ("An alternative definition does not require all members of a crown group to be extant, only to have resulted from a "major cladogenesis event") doesn't quite capture the way the term is used, particularly by paleontologists. At present I can give examples of the way it's used, but can't find a source for a definition. The Hennig definition excludes the use of the term within groups that are wholly extinct. However, when discussing such a group, a distinction can be made between the "crown group" – the 'end group' before extinction – and earlier lines, i.e. the "stem group". It seems to me that current usage simply makes these terms relative to some defined total group, i.e. any total group can be decomposed (in more than one way) into a crown group plus a stem group. As I say, I can give examples of this usage, but not a sourced definition. Anyone have any ideas or comments? Peter coxhead (talk) 08:45, 9 August 2011 (UTC)

The article mention this in the section:

An alternative definition does not require all members of a crown group to be extant, only to have resulted from a "major cladogenesis event".[3] The first definition forms the basis of this article.

Perhaps this section can be expended with some examples? Petter Bøckman (talk) 08:48, 9 August 2011 (UTC)

(By the way, I included that quote in what I wrote above.) Yes, examples could be added, but my real point, which was perhaps not clear, is that the less common alternative usage isn't captured by the definition given. Is there a better one – just for this alternative usage – in the literature? I don't know of one. Peter coxhead (talk) 08:54, 9 August 2011 (UTC)

Additions (cladograms, rewriting)

I've added a couple of cladograms to the article. I think that whenever a specific term is being defined/discussed in the context of an example, it's helpful to have a cladogram showing it. (I understand things much better from diagrams, and I'm sure I'm not alone.) Cladograms could also be added for the other kinds of groups defined in the article.

I've also altered the stem group discussion. It implied that there is a unique stem group for birds. However, this doesn't seem to me to be correct in the way the term is used in the literature. What is meant by "stem-group birds" depends on what the total group of birds is in the context of the discussion – at least this is how I interpret the way the term is used in the literature. There is a unique maximum stem group of birds, namely the clade whose most distant ancestral member is not an ancestor of crocodilians, but I don't think that the literature always equates "stem group" and "maximum stem group". However, this view and the changes I made aren't sourced, and they should be. Peter coxhead (talk) 09:58, 9 August 2011 (UTC)

You are right, I have seen "stem group" (apparently) been used for Aves (trad) minus Aves (Crown). Like you I have not seen any definitions, just examples. I think we here again are seeing the slow diversification of what was originally a very sharply defines concept. Petter Bøckman (talk) 11:33, 9 August 2011 (UTC)

Yes I think this is true of many concepts in cladistics. Reverting to a topic we've discussed before, "paraphyly" had a sharply defined meaning in some early literature, but now seems to be used much more loosely.

All this does make following WP:VERIFIABLE difficult though, because no-one ever seems to say that they are using a looser meaning than the original, they just do it. So if you write something like "Originally concept X was defined as ... but it is now used in different ways", all you can do is add references which show some of the "different ways". However, editors have argued that this is WP:SYNTH, since there is no source which directly says "it is now used in different ways". Peter coxhead (talk) 16:52, 9 August 2011 (UTC)

A big improvement, but there is still some stuff that needs work. For one thing, "total group," though used in the Stem groups section, is only defined in the subsequent Pan group section. For another, "living group" is ambiguous. I think that it is supposed to refer to a group with any living members, but it could equally well mean a group consisting entirely of living organisms.

Further, the Stem groups section says that a wide definition might consider

"all organisms outside crown-group birds but more closely related to them than to any other living group (i.e. crocodilians)."

Are the crown-group birds and the owls different groups? I would say so, as there are crown-group birds that are not owls. But no Archaeopteryx was more closely related to crown-group birds than it was to owls. So, under the proposed formula, every Archaeopteryx is excluded from consideration as a stem-group bird. This is not the intent.

Again, the section says:

"All organisms more closely related to crown-group birds than to any other living group (i.e. crocodilians) are referable to the bird stem group."

This rule will include all crown-group birds, since every crown-group bird is very closely related to another.

It is not clear what is contributed by the mention of crocodilians. To be sure, a crocodilian, of whatever species, is excluded because it is less closely related to crown-group birds than it is to a different extant crocodilian species. The same could be said of snails, though. Is there something special about crocodilians?

I would propose that the bird total group be defined in the Stem groups section as follows:

"The bird total group consists of every organism more closely related to every crown-group bird than to any living organism that is not a crown-group bird."

Stem group can then be defined as total group minus crown group and the Pan group section modified appropriately. Peter M. Brown (talk) 13:49, 10 August 2011 (UTC)

You formating is a bit hard to follow, Peter Brown. I also have problems following the logic of your bird-owl example. As far as I can see, the definition in the sentence "all organisms outside crown-group birds but more closely related to them than to any other living group (i.e. crocodilians)" is watertight. The whole stem group section should be changed a bit though, starting with definition followed by examples as the other sections. Petter Bøckman (talk) 07:20, 11 August 2011 (UTC)

I have given the section a full re-write. I hope it is clearer now. Petter Bøckman (talk) 08:23, 11 August 2011 (UTC)

Yes, excellent! Peter M. Brown (talk) 13:30, 11 August 2011 (UTC)

Looking at the reformulation again, though, I don't understand the association of the stem group with "primitive relatives of the crown groups, down to (but not including) the last common ancestor of the crown group and their nearest living relatives." What is the relation between "the crown group," singular, and "crown groups," plural? Also, "down to" seems to mean "toward the present." As the formulation imposes no limit in the other direction, toward the past, these relatives include ancestors going back billions (thousands of millions) of years. Is that what is intended? If so, how do all those ancestors relate to the stem group? Peter M. Brown (talk) 02:18, 17 September 2011 (UTC)

Ah, the pitfalls of foreign languages! The "down" in this case is in reference to a cladogram or tree analogy. In my language, directions in time are "back" and "forth", not up and down. To the degree the vertical is used, it is in relationship to a vertical time-line, which by convention have the present at the top. Using "down" to describe forward in time never struck me. I'll make the reference to he tree clear. Petter Bøckman (talk) 07:58, 17 September 2011 (UTC)

What is a plesion group?

I have never been clear on this, and the Plesion-group subsection is not helpful. A plesion group is defined as a "side branch on the phylogenetic tree splitting off before the most recent common ancestor of the crown group itself." The trithelodontids are a side branch on the phylogenetic tree that split off from the lineage of the crown eutherians prior to their most recent common ancestor so, by this definition, they are apparently a plesion group of the eutherians. Phylogenetically, though, the metatherians are in the same position. So are the sauropsids. We are told that "by the very definition, all members of a plesion-group are extinct" but, since I can see live birds out my window, I am pretty sure that sauropsids are not extinct. Apparently I have not properly applied "the very definition." Could someone clarify? Are the trithelodontids, for example, a plesion group?

Again, the subsection says, "all fossils found not to be in a crown group will be considered members of a plesion group." Have we ever established, for any fossil, that the organism is not in any crown group: not in crown eukaryota, crown bacteria, or crown archaea?

Peter M. Brown (talk) 01:58, 30 December 2011 (UTC)

I agree that it's not always a clearly used concept. The intention, I think, is as per page 649 of this article: Forey, Peter L.; Fortey, Richard A.; Kenrick, Paul; Smith, Andrew B. (2004), "Taxonomy and fossils: a critical appraisal", Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 359 (1444): 639–653, doi:10.1098/rstb.2003.1453 {{citation}}: Unknown parameter |lastauthoramp= ignored (|name-list-style= suggested) (help). The concept is to be used in relation to a specific phylogenetic tree in which the extant species are placed into taxa of various Linnaean ranks. When fossil groups are added, they may not fit into these taxa and indeed may disturb the logic by which the taxa were constructed. One solution is to leave the fossil groups "floating" as plesion taxa. Their position is defined by the classified taxon to which they are the sister. I think that the problem is that in your first paragraph above, properties of plesion groups are incorrectly treated as defining characteristics. All plesion groups are by definition sister groups to crown groups, but not all sister groups to crown groups are (or are treated as) plesion groups. Peter coxhead (talk) 03:11, 30 December 2011 (UTC)

The first sentence of the subsection is phrased as a sufficient condition for something's being a plesion group; it does not purport to identify properties that such a group must possess. Relative to the eutheria, the condition is satisfied by the metatheria, the tritheledontidae, and the sauropsida. Peter coxhead apparently would agree that the metatheria and the sauropsida are not plesion groups; the sentence should therefore be modified. From his account, it seems that a plesion group is an extinct sister clade of a crown clade with a Linnaean rank. Could the sentence be replaced with that characterization? It is clearer than the definition in Forey et al.

There is still the problem of the last sentence of the subsection. As the second of my paragraphs above indicates, this also requires clarification.

With the concept of a plesion group in hand, we can move on to the definition of a scion group in the next subsection. A scion group, the text says, is composed of a crown group and one or more plesion groups. Aves is given as an example. Surely, though, Aves includes the last common ancestor of Archaeopteryx and extant birds. For that ancestor to be in a scion group as characterized in the subsection, however, it would have to be either in a crown group or in a plesion group. It is certainly not in a crown group. Since it is in the actual lineage of the Aves crown group, though, it is also not in a plesion group since those are, by definition, side branches. If the definition is to stand, therefore, Aves cannot be used as an example.

Peter M. Brown (talk) 18:55, 30 December 2011 (UTC)

Well, it's really all quite simple; A plesion group is any extinct side-branch. The problem here is the funny phylogenetic way of expressing relationship, which somehow requires the reference to a crown group. The idea is to be able to "label" a group like Morganucodonta as a plesion group relative to "Crown mammals". As a term it is not particularly relevant, as the term "extinct group" will usually convey the meaning.

Any suggestion for better wording is welcome. Petter Bøckman (talk) 20:21, 30 December 2011 (UTC)

This isn't just a matter of wording. Petter B. thinks that Morganucodonta is a plesion group, and I am guessing that he would say the same of Archaeopteryx. Peter C. holds that a plesion group has to be sister to a "classified taxon." I don't know quite what that phrase means but surely, to be classified, a taxon at least has to be named. Does the sister group of Archaeopteryx have a name? I don't know of one. If it doesn't, and if I have interpreted both these gentlemen correctly, Petter B. would hold that the Archaeopteryx genus is a plesion group and Peter C. would hold that it is not. Peter M. Brown (talk) 22:06, 30 December 2011 (UTC)

Technically, I should have written of Archaeopterygidae sensu Xu instead of Archaeopteryx. The point should be clear, though. Peter M. Brown (talk) 01:18, 31 December 2011 (UTC)

Peter C. cites Fortey & al, but the figure used in the text is Craske, A.J. & Jefferies, R.P.S. (1989) A new mitrate from the Upper Ordovician of Norway, and a new approach to subdividing a plesion. Palaeontology 32, pp 69–99. I haven't been able to track down the article online (I'll see what I can do at work next week), but I found this one Kluge 1990, which indicate the rabbit hole goes quite deeper that our article indicates (e.g. a paraphyletic definition of a plesion). As for a plesion group being restricted to the closest sister group of a crown group, I am fairly sure that is not the intended meaning, as it would make the plesion-concept useless. Since the fossil record in inadequate, we may never know all side-branches. The closest sister group to e.g. crown Aves or crown Mammalia is likely unknown, and could well consist of a single species. Petter Bøckman (talk) 09:20, 31 December 2011 (UTC)

The Craske and Jefferies article is available here. Peter M. Brown (talk) 17:39, 31 December 2011 (UTC)

According to an account by Marc Ereshefsky, the proposal is to give a fossil group the pseudo-rank of "plesion" whenever its sister group is known. It is not required that the latter be a crown group. The reference is Ereshefsky, Marc (1997). "The Evolution of the Linnaean Hierarchy". Biology and Philosophy. 12: 493–519.

We could use this account in the article. Petter Bøckman will object that, so defined, the concept is useless; there is no Wikipedia policy, however, that claims must be unobjectionable.

On the other hand, if crown groups are not involved, the account arguably does not even belong in the section on Other groups under the crown group concept. Peter M. Brown (talk) 18:45, 31 December 2011 (UTC)

Indeed, even useless terms need explaining. I must admit I hadn't looked too closely into all this crown group derivatives. I assumed the one making the figure knew what he was doing, and that the terms were consistent. Only now do I discover there are several uses of some of these terms and that the cladistic terms are far from the perfect tools of description they are claimed to be by their proponents. It seems to me we have several interpretations of the term "plesion group", some useful, others not so much. We may perhaps erect a new article on group names in cladistics and just leave a summary here. We should report on all we can find reliable sources for. Petter Bøckman (talk) 19:06, 31 December 2011 (UTC)

Edit: I've put in some other meanings that I have found references for. Petter Bøckman (talk) 19:55, 31 December 2011 (UTC)

Just to point out re what's written above, that it's not a question of what "Peter C would hold". It's clear that the term is variously and inconsistently used, but I believe equally clear that the original intent was as per Fortey, Ereshefsky and others, i.e. to use "plesion" as a pseudo-rank for fossil groups to avoid disturbing the classification of the 'extant groups'. The point was not the position in the cladogram (although this is a necessary condition) but the purpose. The following example serves to illustrate my understanding. Suppose we had this satisfactory system:

Order A	Family B Family C

with all the taxa nicely monophyletic, etc. B and C could contain extinct species but are not extinct families. Then we find one extinct species, X, which appears to fit like this:

Order A	Family B Species X Family C

where it's clear from apomorphies that Species X does not belong in either Family B or Family C. Rather than erect a new (extinct) family for X, it can be treated as a plesion group, i.e. outside the main classification. Cladistically, Plesion X is defined as that clade which is sister to Family B.

I agree that in this sense "plesion" doesn't belong under the Other groups under the crown group concept section. Peter coxhead (talk) 23:26, 1 January 2012 (UTC)

I don't see, in either Forey or Ereshefsky, the restriction that neither B nor C be extinct. Suppose that another fossil species, Y, is subsequently discovered, with

Order A	Family B Species X Species Y Family C

As I read Ereshefsky, the approach would be to call Species Y a plesion as well. This would run contrary to Peter C.'s statement, above, that "All plesion groups are by definition sister groups to crown groups." Peter M. Brown (talk) 01:02, 2 January 2012 (UTC)

I should definitely not have used the term "crown group", given that this is used in different ways. I'm not sure about the diagram above, but in this diagram

Order A	Species X Species Y Family B Family C

I think both X and Y would be called "plesion" groups by those who use this terminology. Peter coxhead (talk) 01:54, 3 January 2012 (UTC)

Sister group

Part of the problem is that "sister group" is ambiguous. In a strict sense, clade F is a sister of clade G if and only if (a) neither includes the other and (b) every clade that is larger than F and includes F also includes G. Condition (b) is so strong, though, that we can seldom be at all confident that any two groups are sisters in this sense. We cannot definitively rule out on morphological grounds the possibility that there is an unknown species S, in neither F nor G, such that some clade contains F and S but not G.

In concrete applications, the term is more widely used. Percy M. Butler has written that, if docodont dentition is similar in certain respects to that of Kuehneotherium and Woutersia, then the docodonts must be considered a sister-group of the Kueneotheriidae. Surely, he meant only to rule out that any known clade includes the Docodonta and not the Kueneotheriidae. He was not predicting that such a clade will never need to be defined.

The reference is P. M. Butler (1997). "An alternative hypothesis on the origin of docodont molar teeth". Journal of Vertebrate Paleontology. 17 (2): 435–439.

Sisters in the narrow sense can be identified only by molecular sequencing, which requires that both candidates be extant. This is irrelevant in the context of the present discussion.

Peter M. Brown (talk) 17:21, 2 January 2012 (UTC)

Since the phylogenetic nomenclature specifically refers to a phylogenetic tree, I think we can rather safely assume that there's a "secret" condition built into the definition, i.e. restricted to groups included in this study. Often, a second condition, that of having extant members seems to be assumed, again without specifically stating it (birds-crocodiles being the standard point in question). The third (and also usually not communicated condition) is of course known. On a very fine scale (i.e. species scale), even DNA sequencing won't help you if you if the sister group isn't known and you don't have a sample to work from. Another thing is that when we are down to the species scale, paraphyly starts to become an acute problem and the phylogenetic conditions of bifurcating branches and ancestral species going extinct are not met any more. Petter Bøckman (talk) 18:25, 2 January 2012 (UTC)

Absolutely! All definitions only make sense in relation to a specified tree. Peter coxhead (talk) 01:54, 3 January 2012 (UTC)

We probably can't say that enough times when dealing with these kinds of articles. Also, there probably should be an article on sister groups. Petter Bøckman (talk) 07:35, 3 January 2012 (UTC)

Edit: Sister group is now up. Petter Bøckman (talk) 08:01, 3 January 2012 (UTC)

It's also often important to be clear what kind of tree is being considered. Definitions which apply to a true phylogenetic tree, in which the interior nodes are species or whatever, may not apply to a cladogram, in which the interior nodes simply serve to indicate branching. Thus in a phylogenetic tree, the definition of sister group needs to be clear about the status of the basal node; in a cladogram this is irrelevant. I didn't fully understand this until I read this: Podani, J. (2010), "Taxonomy in Evolutionary Perspective : An essay on the relationships between taxonomy and evolutionary theory", Synbiologia Hungarica, 6: 1–42. Peter coxhead (talk) 13:39, 3 January 2012 (UTC)

Good point! Petter Bøckman (talk) 13:20, 4 January 2012 (UTC)

The "common definition"

A source has been asked for regarding the "common" definition of birds and mammals. What will be adequate sources for the common definition? The best I can come up with is the definitions given in non-specialist woks like encyclopedias. Here's what UXL Encyclopedia of Science (2002) has to say on birds:

Birds are warm-blooded vertebrate (having a backbone) animals whose bodies are covered with feathers and whose forelimbs are modified into wings.

This definition is obviously trait-based. Would this be suitable? Petter Bøckman (talk) 12:41, 13 August 2012 (UTC)

Oh, good enough, though early members of Aves probably weren't warm-blooded. Peter M. Brown (talk) 17:27, 13 August 2012 (UTC)

I suppose that depends on how you define "warm-blooded". The dino-people are adamant that the whole Archosauria had some degree temperature control. The coelosaurids appear to have been fairly warm blooded. Whether they all were running at an avian 42 degrees centigrade is anyones guess, but it seems reasonable to expect they at least had a constant temperature in the 30ies, probably in the upper range. Research on growth in Archaeopteryx indicate they grew slowly (reptilian style), so the full modern 42 degree temperature was probably beyond them. We do on the other hand include the spiny ant eater in the warmblooded cohorts, and it has a temperature fluctuation around 31 degrees. I'm fairly sure Archaeopteryx could beat that.

I'll source the "common definition" at once. Petter Bøckman (talk) 07:31, 14 August 2012 (UTC)

Unsourced statement

This sentence has an "unsourced" tag to it. Exactly what is unsourced here?

This has led to a confusion over the exact extension of well known taxa like birds and mammals.

Petter Bøckman (talk) 10:41, 20 August 2012 (UTC)

It is claimed that the use of the prefix "stem" to indicate a stem group, e.g. "Stem-Aves" or "Stem-Arthropoda", has led to a confusion. Neither source uses or discusses "stem" as a prefix, however, except in the context "stem-based", which is not the usage under discussion. Peter M. Brown (talk) 14:08, 20 August 2012 (UTC)

Ah, I think I know what has happened. When I wrote that section, it was about the use of crown groups. Somehow it has been moved and the term "crown" has been replaced with "stem". I'll reword it and you can see if it makes more sense now. Petter Bøckman (talk) 14:28, 20 August 2012 (UTC)

Removed sections

I see you have cleaned up the article and removed some not much used terminology. Would it be an idea to have a sentence devoted to these terms, like "Further elaboration on the concept like zygon-, plesion- and scion groups have been proposed, but have not seen much use.", or something to that effect? Petter Bøckman (talk) 07:28, 6 September 2012 (UTC)

The plesion does have sufficient notability to be discussed somewhere, though not in a section entitled " Other groups under the crown group concept". Perhaps it should have an article of its own. If you can source the statement you suggest by specifying who proposed scion and zygon groups, by all means include it. Peter M. Brown (talk) 13:36, 6 September 2012 (UTC)

Stem group in extant taxon

I get the feeling that these clearly exist, though usually unacknowledged b/c they are seen as undesirable from a phylognetic viewpoint, yet are useful from a taxonomic one for organizing the organisms in question. I've run in such a case in the latest classification of Apocynaceae (see my sandbox for details), but I'm not sure how to insert a mention of this in the article, which focuses strongly on the existence of stem groups where all the taxa forming the stem are extinct. Circéus (talk) 21:50, 27 February 2014 (UTC)

Misnamed article?

This article not only talks about "crown groups" but also of many other derived concepts such as "stem-groups", "basal groups", "zygon", "plesion", etc. They are all collectively named "accessory groups" in Aubert 2015. A formal analysis of phylogenetic terminology: Towards a reconsideration of the current paradigm in systematics. Phytoneuron 2015-66:1–54. Shouldn't we rename this article so that the title is more relevant? BernZ (talk) 11:21, 12 December 2015 (UTC)

I think there's a general problem with articles about phylogenetics/cladistics terminology. On the one hand, I don't think it really works to have separate articles on each of a set of closely related terms, as we have for Monophyly, Paraphyly and Polyphyly, for example, since this inevitably involves re-explaining the same things, potentially inconsistently. On the other hand, the individual terms have much better-known names than any collective name that fits. Although "accessory group" might be a more accurate article title, it's not exactly a "common name" as per WP:AT. I'm not sure what the answer is. I'd like to see a broader discussion of this issue. Peter coxhead (talk) 13:27, 13 December 2015 (UTC)

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Pan-group = Scion?

Of the two terms ‘pan-group’ and ‘scion’, the former is only explained in text, and the latter only occurs in the diagram. The ‘scion’ in the diagram seems to fit to the description of ‘pan-group’. Are the two synonymous? ◅ Sebastian 07:23, 31 December 2021 (UTC)

From a comparison between the diagram in this article with the one found in the German article (showing total group (aka ‘pan-group’), stem group and crown group), it appears that the difference is that the ‘pan-group’ begins right at the basal node, while the ‘scion’ begins at some later time. Could that be an explanation? ◅ Sebastian 07:41, 31 December 2021 (UTC)

@SebastianHelm: possibly, but the real point seems to me to be that the diagram does not reflect the article. The important concepts in the article are crown group, stem group, and total group, and it's these that should be shown clearly, and aren't. Peter coxhead (talk) 19:19, 31 December 2021 (UTC)

I agree, the diagram does not reflect the article. In addition to “scion”, it also displays “zygotaxon”. Both of these terms are “weird”, as already been remarked on this talk page 15 years ago, and for which still no article, not even a redirect exist. As if that weren't enough, there are more problems: 1. It seems to use the term “plesion” differently from this article. 2. The red color for “Crown group” is distributed over two triangles, which appears to indicate that the crown group consists of their combined area.

So, how about replacing the diagram with the one from the German article? One possible problem might be the copyright. As IANAL, can someone with the necessary legal knowledge say whether it's OK to use that here? ◅ Sebastian 14:50, 3 January 2022 (UTC)

File:Crown n Stem Groups 01.svg is in Commons, so is available to use. Just needs explanation added, maybe on the diagram? Peter coxhead (talk) 16:16, 3 January 2022 (UTC)

That would at least be an improvement; the explanation could be taken from its caption at Opabinia. However, it has the same problem as #2 above – the disjunct crown group. The German picture avoided that by distinguishing A and B. So I'd suggest to color each of the two crown groups differently; and then we could write in the caption something like “• crown group • another crown group – the two are [something like sister groups]”. Now, but what to write for the brackets? I would have simply written “sister groups”, but as sister group#Example states, that isn't correct. Unfortunately, that article doesn't say what is the correct term. ◅ Sebastian 19:34, 3 January 2022 (UTC)

There's some relevant discussion above; part of the point of concepts like stem group and plesion was to keep them separate from the extant groups. The point is that if we didn't know about the extinct units we would call the two crown groups sisters: they are "sister crown groups" but not sister clades. I would avoid this here; just say they are two crown groups. The yellow area showing the stem group should not include the crown node (nor the basal node, I think, but this is less clear to me). (Incidentally, there's a similar problem with the German diagram: which of A or B does the very basal node belong to? It appears to belong to both, but this is impossible.) Peter coxhead (talk) 22:32, 3 January 2022 (UTC)

To generalize the explanation in Budd & Jensen (2000): "The crown group of a [taxon] consists of the last common ancestor of all living forms in the [taxon] and all of its descendants; the stem group consists of a series of entirely extinct organisms leading up to the crown group away from the last common ancestor of this [taxon] and the most closely related [taxon]." I think the "up to" and "away from" do make it clear that neither the crown node nor the basal node are included in their use of the term stem group. So the diagram needs a bit of re-drawing. Peter coxhead (talk) 22:48, 3 January 2022 (UTC)

This is my possible version. The diagram shows two crown groups of extant species, C1 and C2, which would be sisters within the clade T, except for the extinct species that form the stem group S1. The crown group C1 and the stem group S1 form the total group T1. T1 and C2 are sisters (at least until we find extinct species forming a stem group to C2).
@SebastianHelm: thoughts? Peter coxhead (talk) 07:36, 4 January 2022 (UTC)

Wonderful! This addresses all problems mentioned above, so I added it to the article, with a description taken from your explanation.

Since you colored the basal node black, we could also add that to the legend (as in ‘Crown n Stem Groups 01.svg’) with a link to basal node. I stopped short of doing that when I realized that basal node is a redirect to Basal (phylogenetics), which instead uses the term “root” without ever mentioning “basal node”. While in my own impression the two are synonyms, I don't feel qualified to state so in an article.

Just out of curiosity: Would it also be correct to say “S1 and C2 are sisters”? ◅ Sebastian 11:48, 4 January 2022 (UTC)

I had a few more thoughts about Peter's picture, but since they're off topic here, I posted them at File talk:Crown n Stem Groups 02.svg#Further improving this picture. ◅ Sebastian 12:28, 4 January 2022 (UTC)

Crown group versus clade

The introduction states that a crown group is one way to define a clade. Does this mean that clade and crown group are synonymous? The other way I can understand the lead is that a crown group is one kind of clade; but it remains unclear to me what would be an example of a clade that is not a crown group. Kent G. Budge (talk) 16:06, 8 July 2022 (UTC)

It is my understanding that crown groups are specifically in regards to living species (in that it contains all of the extant/living members of the group, plus any extinct species that share the common ancestor). If a clade is entirely composed of extinct species, then it would not be called a crown group. So yes, a crown group is a type of clade, but they are non synonymous. Cougroyalty (talk) 17:11, 8 July 2022 (UTC)

Thanks. That makes sense. Let's find a reliable source and put it in the article. --Kent G. Budge (talk) 17:16, 8 July 2022 (UTC)

I think this is yet another example of a term that biologists often use differently while assuming there's a single common definition. See, for example, the last sentence of the section "Contents of the crown group". In a way, it's the complementary concept "stem group" that is more distinct – it's a way of defining and talking about a group that is not a clade. The real contrast is perhaps between the "crown clade" and the "total clade". This article begins with a sharper definition than my understanding of Hennig's original concept, and has some useful discussion points. Peter coxhead (talk) 13:35, 9 July 2022 (UTC)

"phylogenetic telescoping"

This concept which abrupt rears its head in the "Palaeontological significance" is completely unexplained. A quick google search suggest this is not a broadly or commonly used term and even so, it clearly needs an explanation here as a layman's understanding of the basic concepts provides no help whatsoever understanding what it actually being said. Circéus (talk) 16:02, 14 October 2023 (UTC)

Diagram Links

Both cladograms containing Aves show Aves as being in the place of Avialae, but when you click on the Aves links, they lead to a Wikipedia page defining Aves as being synonymous with Neornithes, despite said cladograms showing Neornithes as being inside of Aves. I know that Aves has a disputed definition, but this seems like it would be extremely confusing to people unfamiliar with the topic, so I suggest that Aves should just be replaced with Avialae on both cladograms, and the links should lead to the Avialae page as well.

Also, the link in the caption of the first diagram saying "total group or pan-group" does nothing when I click on it. Is the link broken, or is it just leading me back to the same page, as it seems like it would? Why would there be a link to the page I'm already on? Either way, I feel like it should be fixed. Thunderhawk256 (talk) 14:19, 11 April 2024 (UTC)