Talk:Chordate
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In Taxonomy, should not tetrapodes (and below) be moved one step to the left? Bold text
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Can someone please evaluate the recent papers by vienne, et al (metaphylogeny of 82 genes sheds new light on chordate evolution) and Delsuc, et al (Tunicates and not cephalochordates are the closest living relatives of vertebrates)- I am not a professional biologist and so really don't feel I should put it in, but I think it should be commented on.
According to http://www.ucmp.berkeley.edu/chordata/chordata.html, the four characteristics of all chordates are the following:
pharyngeal slits - a series of openings that connect the inside of the throat to the outside of the "neck". These are often, but not always, used as gills.
dorsal nerve cord - a bundle of nerve fibers which runs down the "back". It connects the brain with the lateral muscles and other organs.
notochord - cartilaginous rod running underneath, and supporting, the nerve cord.
post-anal tail - an extension of the body past the anal opening.
I looked at tackling Vertebrata. Rather than redirect to Chordata I would have considered a separate page for Vertebrata to deal only with lower level taxonomy. Chordatea would then be reserved for the higher level discussion of the phylum into its sub-phyla.
We really need to decide on a set of vertebrate classes for use in wikipedia. Obviously a perfect (maybe even a satisfactory) scheme is impossible because the classification is currently unsettled, and different sources emphasize different priorities in its construction. However, at the moment we have pages using various older and newer schemes, and this only adds confusion. I would like to try tackling the problem of working out a standard system. Hopefully we can come up with something which is at least tolerable.
As a general approach, I think we should start with the traditional classes and try modifying them into holophyletic groups as much as is possible without completely reinventing the system. Also, we should work by picking and choosing from extant classifications, i.e. not introduce any new innovations. As always, the variant systems should be explained on the relevant pages; the important thing is not to have to do this for every order and family.
Rank of Vertebrata
The Vertebrata are traditionally ranked as a subphylum of the phylum Chordata. Many newer sources, however, distinguish the Craniata from the Vertebrata, the difference being that the former includes the hagfish. Such sources may assign ranks in a few different ways:
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It is hard to tell which of these systems has the most support, but I think it is the second one. However, it is still far more common not to distinguish craniates and vertebrates, and so to have Chordata and Vertebrata as phylum and subphylum. Given this, I think the best option would be to keep these ranks, but acknowledge the new distinction by treating the hagfish as a subphylum, i.e. the first system. The name Myxini appears to be the most common for the rank of subphylum or class.
Jawless fish
Earlier classifications group jawless fish together as a class Agnatha, but they are not closely related to each other, and the idea of treating them as several different classes has been around for a long time. Sometimes the Agnatha is retained as a superclass, but I don't think there is much reason to do this, especially if the hagfish are being excluded from the vertebrates altogether. Exactly what classes should be used is a less obvious matter. The only extant forms, which we should worry about first, are the lampreys. It looks like they are most often placed in a class Cephalaspidomorphi. However, I imagine such a class would also include the extinct Cephalaspidiformes, which are no longer considered especially close to lampreys. As such a less common group, like Petromyzontida.
Jawed fish
The class Chondrichthyes and the extinct classes Placodermi and Acanthodii do not appear controversial, and I don't think we should not hesitate to use them. The Osteichthyes are more problematic. Traditionally they are treated as a class, with subclasses Actinopterygii and Sarcopterygii, but the latter is paraphyletic to the land vertebrates. As such, the land vertebrates are sometimes included as sisters or children of the Sarcopterygii, with various promotions or demotions of ranks to allow them to fit. Some sources, for instance ITIS, do not do this but still promote the Osteichthyes and its subtaxa. Alternatively its children may be promoted, and the Osteichthyes themselves abandoned.
I think the last would work best for us. Most fish would then be placed in the class Actinopterygii. Aside from the slight adjustment in rank, this is straight out of the traditional system (so should be fairly recognisable) and is entirely stable, unlike the Osteichthyes. It's not quite so clear what to do with the Sarcopterygii, whether to keep that as a paraphyletic class or to further divide it. Some sources list classes Actinistia (coelocanths) and Dipnoi (lungfish), but these don't cover all the extinct members, and I doubt anyone has erected separate classes for all of them. We may want to try treating them as orders without definite classes for the time being, as is done with Collembola and the like; this could apply to the living forms as well, since there are relatively few. I am not sure how this could be done in the parent taxobox, however.
Land vertebrates
Living amphibians probably form a monophyletic group, and so their treatment as a class Amphibia is no problem. The fossil ones are more difficult. Traditionally they were all included in the Amphibia, with living forms and their closer relatives comprising the subclass Lissamphibia. However, that makes the group paraphyletic, and so some have either restricted it to those forms closer to the Lissamphibia than the amniotes, or abandoned it in favour of a class Lissamphibia. I think we should keep Amphibia as a class. I would suggest giving it the narrower meaning, and so treating forms like Ichthyostega as tetrapods without a definite class.
The amniotes are especially hard to deal with. The traditional system divides them into classes Mammalia, Aves, and Reptilia. As the last is paraphyletic, varying other systems have been proposed with some support, especially among palaeontologists. The generally accepted relationship among living forms is as follows:
- Mammalia (mammals)
- Anapsida - Testudines (turtles)
- Diapsida
- Lepidosauria
- Archosauria
- Crocodilia (crocodiles and alligators)
- Aves (birds)
As far as these go, the mammals do fine as a separate class. However, this leads to problems considering the mammal-like reptiles. Traditionally these are placed in class Reptilia, subclass Synapsida, but they are closer relatives to mammals than they are to any other reptiles, and as such are excluded from phylogenetic definitions of reptiles. If the mammals are demoted to a subclass, they may be grouped with them as a class Synapsida, but otherwise these forms are difficult to place. It doesn't help that orders and families are confused as well. I think that at the moment they are probably not worth altering the traditional class Mammalia, and should probably be treated as having uncertain placement, for classes and for lower level ranks.
The other forms are more complicated, because birds fit right in the middle of the extant reptiles. A small but increasing number sources take the Reptilia, or Sauropsida as it may then be called, to include them. The same is even more true for the Diapsida and Archosauria, either of which could be a reptile subclass in traditional systems. The idea that the birds belong among the Dinosauria, traditionally a superorder, has made some entry into popular science. Some time ago Bakker had proposed changing the class Aves for a class Dinosauria, with Aves as a subclass, but this does little for the other reptiles and never really caught on.
The straightforward approach would be to demote Aves to a subclass, and include it in the class Reptilia. This, however, does not lend itself to making groups among the other reptile orders, and I have not seen it done. I have seen the Aves included in a class Diapsida, with a separate class Anapsida. This may stem from the idea that the mammals and diapsids are closer to each other than to the anapsids; things like the tree of life reject this, but I don't know if it has lost all support. Exactly how to organize the internal structure of this class isn't clear, but generally involves further demoting the birds. Another system which appears more common is to have three sauropsid classes, Anapsida, Lepidosauria, and Archosauria. The birds are usually a subclass of the last - occasionally an infraclass in the subclass Dinosauria, but I think we should stay away from infraclasses for such important groups, especially when all it accomplishes is a better treatment of the dinosaur groups (some of which are bound to be paraphyletic to birds no matter what).
As such, I would support dividing the amniotes into four classes, i.e. Anapsida, Lepidosauria, Archosauria, and Mammalia, with the mammal-like reptiles left tentatively classless. However, this system is contingent on the demotion of Aves to subclass, and I don't know enough about birds to know if this would be reasonable for ornithologists. Failing that, some have suggested promoting each of the reptile orders to classes, but this does not work at all for fossil forms. Leaving groups like dinosaurs, pterosaurs, and ichthyosaurs unclassified does not seem to me nearly as reasonable as doing the same with basal amphibians, so if this is the case I think we would need to retain the Reptilia.
Please let me know what you think about these. If someone wants, I would be happy to make up a couple of possible taxoboxes.
Interesting, Josh. I have to say that outside the birds, I haven't really had much idea what the current state of play is. Birds are a mess. Although Sibley et al have done valuable work on clarifying relationships, but their work is not without controversy, and is often felt to ignore non-DNA factors. The Americans have embraced Sibley whole-heartedly, but Europe has generally been more cautious.
The present state of the bird articles are something of a compromise, but, since the large majority are written by Europeans and Australians, tend to be fairly traditional. Exceptions, however, abound. Albatross follows a very recent BOU guideline, and many southern species are based on HANZAB. This probably doesn't help at all. jimfbleak 06:31 29 May 2003 (UTC)
One thing that has struck me about categories of fish is that some are very stable, and others under constant revision. I've been leaning towards pruning taxoboxes down to not include much that seems likely to change - it reduces their value a little as navigational devices, but not that badly. Then articles like Actinopterygii and Chordate can list out old and new taxa schemes. If a scheme becomes unfashionable, then all you have to do is annotate it with "no longer used, but possibly a useful reference when reading old books". (One of FishBase's amusing features is the list of previously-used names for species, often clarifies other mystifying sources.) Trying to develop and use a single scheme seems crazy-making, when we've got the room and the freedom to describe multiple competing schemes.
To take a specific example, Craniata is worth an article describing how and why it might be used, but I don't think it's useful to readers to include it in taxoboxes, and if it falls out of favor, that would be a lot of taxoboxes to edit. Stan 17:12 29 May 2003 (UTC)
On the other hand, if ranking Vertebrata as a subphylum fell out of favour, we would have just as much work ahead of us. That particular one doesn't seem especially likely, but in general there is a problem in that many of the standard groups seem likely to change. If you have some idea about which ones are worth keeping, I would be interested.
I agree with you that wikipedia can and should review all the different systems, or as many as possible. But when it comes down to it, things like guppies, grass snakes, and pigs will end up listing something in their taxoboxes, with minimal room for comment (I might note that I initially objected to taxoboxes for exactly this reason). It would be best if we had something standard to put there to avoid confusion, even something that might change later.
-- Josh
- I think I'd rather try to invent a perpetual motion machine than develop/choose a single, stable taxonomic scheme. :( We just have to blunder along as best we can, I think. I won't make a specific comment on most of the above, as I have yet to learn much about anything other than birds and mammals - and there is enough confusion and ambiguity in those two alone to keep us busy for quite some time! Tannin
Ok, then. Instead of pointing out the obvious, that the classification of chordates is incredibly variable and that there is no good way of settling on a single system, maybe you could suggest an alternative. Should we omit taxoboxes on vertebrate pages, should we include them but omit classes, should we include classes but not worry about any kind of consistency between different pages, or what? The need for a single system is a problem which we could have avoided, but we adopted it anyways, and it doesn't do much good to say it's unfeasible now.
- Here's an idea: I don't think it's likely that Vertebrata will go away anytime soon (feels around in back - yup, backbone still there :-) ), and it's always going to be a leading differentiator in the minds of our audience, so the problem with its taxobox inclusion is not so much its presence, as what we call it. To make that easier to mass-change we could have special redir pages - Vertebrata as phylum, Vertebrata as subphylum, and piped to "Phylum" etc, then it's not too hard to find all the refs and rewrite them with a script at some point. Another option could to include the group name but simply leave the phylum/class/order word blank - yes, they're all clickable for consistency, but I bet statistics would show that readers almost never take that route from a random taxobox, so for troublesome groups simply don't try to assert what kind of taxa they are. Next round of fish additions, I'll play around a bit, see how it looks in practice. Stan 19:16 29 May 2003 (UTC)
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- The Actinopterygii aren't much of a problem, we just have to decide whether to use that or Osteichthyes as the class. I would suggest just going ahead and using class Actinopterygii, since that doesn't impact most other groups. If nobody balks, I'll probably end up changing the currently listed orders and families to it. Other groups are more of a problem. Trying things out is a good idea. It might be better to, instead of introducing ranked redirect pages, simply standardize on a table format without white space, e.g.
Class:<\td> Chondrichthyes<\td><\tr>, so that it could be searched for. Josh
- The Actinopterygii aren't much of a problem, we just have to decide whether to use that or Osteichthyes as the class. I would suggest just going ahead and using class Actinopterygii, since that doesn't impact most other groups. If nobody balks, I'll probably end up changing the currently listed orders and families to it. Other groups are more of a problem. Trying things out is a good idea. It might be better to, instead of introducing ranked redirect pages, simply standardize on a table format without white space, e.g.
- (Via edit conflict.) A fair comment, Josh, but rather pessimistic, I think. Is the glass half full or half empty?
- I have no particular opinion to offer on the best way to tackle the fundamental categories, and in reality, it doesn't really matter from the point of view of the bird and mammal entries that I mostly deal with. (It's not that I'm not interested in (e.g.) trees or fish, by the way, just a matter of not having time to do everything at once. Quite often I wish I was triplets!) In these areas, the top half of the taxobox is unimportant. In 99.9X% of cases, the reader can be assumed to already have a pretty fair idea of what a "bird" or a "mammal" is. However we normally can't assume that the reader knows the difference between the Diprotodontia and the Dasyuromorphia - and if you want to learn about marsupials, getting a handle on that difference is half the battle.
- In other words, the taxoboxes serve a really valuable function, and provide a very useful navigation purpose. The fact that they are (and will probably always be) imperfect does not make them useless, not by any means.
- Now, with regard to consistency: this is an issue, and will always remain an issue. For my part, I have been working on getting as many bird and mammal taxoboxes consistent with each other as possible, and intend to continue doing that. (Other contributors are doing the same, of course.) For the most part, this is not too difficult a task. Yes, there are times when it isn't possible, but these are relatively infrequent and, with care, can usually be dealt with fairly gracefully. I mean "consistent" in three different ways here: consistent in content - trying, for example, to have the individual raptors classified as either Falconiformes or Ciconiiformes and not a mixture of both - and consistent in style: bringing, bit by bit, the taxoboxes and the in-text listings into a form that, allowing for the unavoidable variation imposed by individual circumstances, is visually similar: which both provides a more professional look and feel to the 'pedia and makes it easier for the reader to find things and grasp the relationship between them; and consistent in table code: that means editing out the masses of useless extra spaces and bizzare line breaks that make it so hard to see what you are doing when you are altering a taxobox.
- I think it would be a serious mistake to insist on any particular uniform set of classification levels. For example, with some taxa it really does help to specify subfamilies, but with others that "extra" information only results in making the entry less informative. However, if there are improvememts to be made to the top half of the standard vertebrate taxobox, and these have broad consensus support, then I'll be happy to do my share of the article editing to bring the existing ones into line. Tannin 19:53 29 May 2003 (UTC)
I agree that, despite their limitations, the taxoboxes have turned out to be cool and useful. That's why I've added them to a number of groups, and why I would like to add them here. The problem is that doing that requires deciding on a particular higher level classification to work with, which is why I wanted to see people's opinions on the matter. I'm not trying to force much else onto the way the lower levels are treated - I would suggest leaving out intermediate ranks that are not standard or are variable, but both you and Stan obviously already know that.
So, then, in absence of specific suggestions, I'm going to go ahead and add classes which I think will work best for the time being. The rationale is that given above, plus me being to timid to try splitting the Reptilia and demoting the Aves without anyone's support. Please don't hesitate to suggest changes.
Does anyone have a source for "bands of muscles that go around the body" as a criteria for chordates? As far as I'm concerned, the slits, notochord, hollow nerve cord and post-anal tail are the "main" characteristics. jag123 17:11, 10 Dec 2004 (UTC)
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[edit] Images representative of the diversity in vertebrates and chordates
Rather than have tuna representing both vertebrates and chordates - which will leave people seriously confused at first - I propose we should include more than one picture, or possibly taxobox:
| Hagfish | ||||||||||||
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 Pacific Hagfish resting on bottom 280 m down off Oregon coast |
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- Samsara 17:51, 10 January 2006 (UTC)
- Addendum: I know it's not stricly correct to consider the less successful taxa like lampreys and hagfishes "basal" or "primitive", but I think focussing on them rather than the more familiar ones is at least illustrative from a human perspective. So I propose to make it a rule to always include a familiar and a "basal" example, with the basal example topmost, and ideally two examples that do not inhabit the same environment, so that the second picture is not assumed to be included due to its feeding relationship with the former, but clearly as another member of the group.
- There are many cases where this rule could be usefully applied, especially those taxa which include humans: animals, deuterostomes, chordates, vertebrates, mammals, primates... Similarly for the taxa leading up to flowering plants. - Samsara 18:00, 10 January 2006 (UTC)
- Including the tunicates as a better example (tuna-hagfish is probably more representative of the craniata).
| Tunicates | ||||||
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 Sea Tulips, Pyura spinifera |
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- Perhaps a less plant-like photo could be found? I know they look like that, but there are photographers gifted enough to portray them more animal-like? - Samsara 18:05, 10 January 2006 (UTC)
- The question now is how to put it in the article? Taxobox does not seem to support inserting a second image. - Samsara contrib talk 01:21, 1 February 2006 (UTC)
[edit] Tree in main text.
Until recently the classification tree in the main body of the text (not the one under the picture) was indented to indicate the ranks of the clades, with evolutionary relations indicated roughly by the order of the clades at a given "degree of indentation". Someone recently indented everything from tetrapod down inside dipnoi, on the (correct) grounds that tetrapods evolved from lungfish. But is the tree meant to replresent this fact in this way? If there's a convention, please state it. But the tree now looks as though it's saying that tetrapods are a kind of lungfish.
Also, I'm really worried here, because I don't want to be a lungfish.
- Yeah, this is really really messed up!! Someone needs to fix this ASAP. Maybe look back to around the revision as of 16:28, 25 September 2005 --AlanH 15:56, 2 March 2006 (UTC)
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- Actually, it isn't clear why Tetrapoda was indented under Dipnoi. The tetrapods did not (as far as I know) evolve from within the current lungfish clade, but rather were a sister group of the Dipnoi under Sarcopterygii, the lobefins. The tree was incorrect before the anonymous editor made the change: the tetrapods were attached two levels too high. But the anonymous editor was too exuberant, and pushed the tetrapods down three levels instead of only two. (I'm going by the NCBI taxonomy site here.) Anyway, I'll fix it.
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- This doesn't actually answer the anonymous objector above, who didn't want to be a lungfish, because although we are not lungfish, we are lobefins, which the objector may find just as upsetting. There's not much that we can do about that, I'm afraid, without lying. ACW 20:12, 2 March 2006 (UTC)
[edit] Osteichthyes
Bony fish aren't listed in the taxobox. I'd change it, but I have no idea how to edit a taxobox. Bony fish are the class Osteichthyes, subphylum Vertebrata.
erm...are you sure all chordates have a musculer tail? humans don't plus a million other vertebrates...or am i missing something.
- In the system we're using, bony fish are divided into two classes, Actinopterygii and Sarcopterygii. Osteichthys is something of a superclass.Dinoguy2 21:06, 8 May 2006 (UTC)
[edit] Hagfish a vertebrate?
The placement of hagfishes in this tree is a problem because opinion is divided whether or not they are related to lampreys. Two centuries ago both hagfishes and lampreys were classified together as "Cyclostomi" based on some shared features, and the cyclostomes were taken to be the sister group to jawed vertebrates (Gnathostomata). The many simple features of hagfishes were taken to be the result of degeneration owing to parasitic life.
Later, it was realized that hagfish is an active predator, not just a parasite, so it may not have simplified so much after all. Perhaps it retained its simple features from an ancestral state. In that case, it must have diverged from vertebrates long before the split between the lampreys and gnathostomes.
Linnaeus apparently used the names Craniata and Vertebrata interchangeably. But since hagfishes lack actual vertebrae but do have a cranium, it seemed logical to apply the name Vertebrata to the (lamprey + gnathostome) group, and Craniata to the more inclusive group of (vertebrates + hagfishes). (See <http://tolweb.org/Craniata/>)
Now the tables may have turned again. Many molecular studies have concluded that hagfishes and lampreys may be sister groups after all, which implies that the ancestors of hagfishes once had vertebrate and other vertebrate characters, but lost them for whatever reason. In that case, hagfishes are true vertebrates, and the taxon Craniata becomes redundant.
It's not clear that the dispute between morphology and molecules has been resolved. One way to keep the classification neutral until the dust clears would be to list hagfishes, lampreys, and gnathostomes as equal groups under Craniata, without implying a closer relationship among any of the three. Fossil groups of jawless fishes could also be listed the same way under craniates.
The main problem with that solution is it leaves Vertebrata in limbo. If hagfishes are sister group to lampreys, the more familiar Vertebrata should be used in preference to Craniata, and the most recent edits to the Chordate article would be the best approach. Regardless of what we decide, the discussion of the subphyla should address this controversy.
Cephal-odd 07:45, 10 May 2006 (UTC)
- I just added links to the Tree Of Life and NCBI Taxonomy nodes for "Chordata". These were the first external links, and in fact were this article's first references of any kind. Now, I agree with Cephal-odd that there is a taxonomic controversy involving the branching order of the Craniata. That having been said, both the sites I referenced agree that the Hyperotreti are craniates but not vertebrates. So I have taken the liberty of editing our taxonomy to show this. I have no objection to somebody moving the Hyperotreti back down under the vertebrates, but if you do so, please add a reference supporting this view. I think we can't go wrong as long as we make sure we back up our tree with reputable references. ACW 02:20, 11 June 2006 (UTC)
[edit] list of sub classes etc
I've made a few changes to the list - my aim was to add the Superclass Osteichthyes with the classes Actinopterygii (ray-finned fish) and Sarcopterygii (lobe-finned fish) within them. Though I ended up altering format a bit. Have I got this right?HappyVR 18:35, 2 July 2006 (UTC)
- The addition of Osteichthyes is a good idea.
- The change in format aligns all the classes of gnathostomes, but that's a break from how it's been formatted before. The prior version was using something akin to Michael Benton's system of indenting taxa to show sister relationships, hence the heading "Taxonomy & Phylogeny".
- Our table should show that Osteichthyes here is a paraphyletic group that excludes its tetrapod descendants. It might be a good idea to include Euteleostomi, a newer name for the clade that includes bony fish and tetrapods. Some references have used the name Osteichthyes to refer to the whole clade, but Euteleotomi was proposed to avoid the confusion of having a name associated with bony fish (and literally meaning "bony fish") include the tetrapods.
- Cephal-odd 15:48, 4 July 2006 (UTC)
- Yes, rather stupidly I didn't notice that the heading included phylogeny. So the tetrapod classes (and amniota ?) need to be tabbed to the right? Before I correct this could I draw your attention to Wikipedia:Village pump (technical)#How to add structure to a list where I and other people have been attempting to find a way to show relationships in a list. Obviously the list can be easily sorted out - but would using 'lines and T junctions' be a better way to show the phylogenetic relationships. Maybe we could use both. Or split the table into two. These are just suggestions.
- Unfortunately I think I've altered a similar list in a similar way, I'll have to go and check whether that list was also supposed to show phylogeny as well. It was vertebrate, luckily that list was labeled just 'taxonomy', but it did previously show the evolutionary 'rank' as right indent - I'll try to sort that out.HappyVR 16:43, 4 July 2006 (UTC)
- OK I've made an attempt at redoing the list - please check. Note I've assumed that the tetrapods are decended from an (extinct) member of the lobe-finned fishes. I'm quite certain that I've made a mistake here - please correct or let me know. (I wouldn't be surprised if I've made other mistakes)HappyVR 18:00, 4 July 2006 (UTC)
I've tried to do what I suggested above - adding lines to show evolutionary relationships - unforunately can't get rid of the box round the section at the moment. Is this ok?HappyVR 18:24, 4 July 2006 (UTC)
- Perhaps because I've taken so long to check on it, the link you provided to the Village Pump no longer seems to lead to a discussion about list structure. Is it archived somewhere? Regardless, I like the diagram with lines and T-junctions. The format somewhat resembles that used on sites such as Mikko's Phylogeny Archive <http://www.fmnh.helsinki.fi/users/haaramo/Metazoa/Deuterostoma/Chordata/Chordata.htm>.
- It should help clear up ambiguity about the phylogeny.
- The main drawback I see is that it makes the Linnean ranks harder to read, because a taxon's place in the traditional hierarchy often doesn't match its place in the phylogeny. Maybe we should have a separate table showing the Linnean classification? The advantage to having two separate tables is that we can leave traditional classifications alone but add phylogenetic trees, article by article.
- You say you're certain you've made a mistake, but the placement of tetrapods looks right to me. Tetrapods are indeed desceneded from a group of lobe-finned fishes. The only error I see is that Acanthodii is shown outside Teleostomi, but acanthodians are actually considered to be teleostomes -- I'll correct this. Otherwise, the tree looks good.
- Cephal-odd 04:27, 13 August 2006 (UTC)
[edit] Lack of consistency
Could someone merge the list in a table on the right side, like every other page I saw (for example Animalia or at least like Carnivora) for consistency?
[edit] Paedomorphy
On 7 March 2006, 24.61.98.230 wrote:
- It has been theorised that the phylum originated as a result of paedomorphy occurring in a primitive animal, like a sponge or coral.
On 7 May 2006, M Alan Kazlev altered this to:
- It has been theorised that the phylum originated as a result of paedomorphy occurring in a primitive ancestral form.
This comment must be substantiated. I am removing it; either of the above users may feel free to add it back with an appropriate citation. --Peter Farago 20:38, 3 September 2006 (UTC)
[edit] Hemichordata
User:Freepsbane just added Hemichordata as a subordinate group to Chordata. I had thought that the chordates and the hemichordates were sister groups. I'm going to revert this, because I can't find any source for the claim.
The sources I usually depend on here are the NCBI Taxonomy Homepage and the Tree of Life site. Neither of these has hemichordates as a daughter group of chordates. ToL, in fact, says that the hemichordates are more closely allied to the echinoderms than either are to the chordates.
Furthermore, the Wikipedia article on the Hemichordata calls it a phylum, not a subphylum, and does not give Chordata as a parent group. And the text of this article itself says things "... is broken up into three subphyla ..." and then does not list hemichordates.
If we decide to change this, we should change it everywhere.ACW 14:59, 24 October 2006 (UTC)
