Talk: Chordate
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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)
- 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. <tr><td>Class:<\td><td>Chondrichthyes<\td><\tr>, so that it could be searched for. Josh
- (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.
