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Date: Wed, 16 Jun 2004 08:44:01 -0700 (PDT)
From: "T. Michael Keesey" <mightyodinn@yahoo.com>
To: Mailing List - PhyloCode <phylocode@ouvaxa.cats.ohiou.edu>
Subject: Crowns, Panstems, and their Correspondence to each other
I notice that a number of the abstracts follow de Queiroz and Gauthier 2002 in creating panstem/crown pairs, in each case with the name of the panstem formed by adding the prefix "Pan-" to the name of the crown clade. I've commented on this and discussed it on another forum, and thought I'd summarize some thoughts here. CORRESPONDENCE It is, of course, interesting to note that crowns and panstems have a 1:1 correspondence with each other, and it does seem that this would warrant at least a recommendation with regards to the definition and/or nomenclature of these clades, possibly having them always be named in pairs. I'm curious; does anyone think a rule would be warranted? There is one instance where the correspondence is not really 1:1, at least for practical purposes, and that is, of course, if the extant outgroup to a crown clade is also the immediate ancestor. In this case the panstem and the crown clade become heterodefinitional synonyms. Does anyone think that this means certain crown clades should not have panstems named for them? I personally think that it's fine for the two clades to be heterodefinitional synonyms, as our understanding may shift. (Of course, in every instance, the crown clade should be senior to the panstem.) "EXTANT" This has been discussed before, and is mentioned in Note 9.4.1, which states that the author should clarify their exact meaning for the word "extant". It seems to me it may be preferrable for the code to define a consistent meaning. The most stable and easily applicable definition I can think of is "living and published at the time of the definition's publication", but I'd certainly like to see others' suggestions. Should "...and published..." be included? It would help maintain the stability of previously established crown and panstem clades, I think. DEFINITIONS I note two schools of thought in the abstracts for the upcoming meeting as to definitions for crown and panstem clades. In Sereno's definitions, there really are no panstems, only stem-based clades that happen to have extant specifiers. The crown clades, then, are defined in terms of the stem-based clades: Stem = Clade(A <-- B) (A and B are extant) Crown = Clade(extant Stem) However, in other abstracts, such as the one for Gauthier et al.'s paper on major amniote clades, it goes the other way: the panstem is based on the crown. Crown = Clade(A + B) (A and B are extant) Panstem = Clade(Crown <-- extant non-Crown) In the former, the stem-based clade is the more stable one, while, in the latter, the node-based (crown) clade is more stable. It seems preferrable to me that the crown be the more stable one, but I'd be very interested to hear other opinions. I will allow one exception, and that is Wagner's brilliant definitions for _Panbiota_ and _Biota_ which, while not exactly following the stem-modified crown approach (it would be impossible to do so, as there is no outgroup), are in the same spirit, with the crown clade being modified by a more inclusive clade. NOMENCLATURE While it seems a good idea to delineate the correspondence between crown and panstem clades, I see some issues ahead with using the "Pan-" prefix. (And I thought mandated affixes were something PhyloCode was trying to get away from....) A number of currently named taxa actually do start with "Pan-", and are not panstem clades. Is someone who is not familiar with carnivoran taxonomy to see the clade _Panthera_ and assume it is the panstem of "Thera"? (When, in actuality, by the philosophy being followed, it would be a crown clade, and "Panpanthera" the panstem!) Among dinosaurs, there is a _Panoplosaurus_ which is certainly not a panstem clade containing _Oplosaurus_ (which is an actual unrelated dinosaur genus). Then there are some existing "Pan-" taxa which have actually been named after other taxa, such as _Panarthropoda_ and _Pancrustacea_. These were not named as the panstem clades for _Arthropoda_ and _Crustacea_, respectively, but for more inclusive groups. Closer to home, if we were to have panstem clades separating our species from its closest living relatives, it seems they would be "Panhomo" and "Panpan" (_Pan_ being the crown clade of chimps, not the panstem clade for ""). "Panhomo" is rather confusing, as it looks like a combination of _Pan_ and _Homo_. (On another topic, while it may seem like there can be no crown clade called _Homo_, as there is only one extant species, I submit that there is one: "the clade stemming from _Homo sapiens_". Of course, this would necessitate removing _neanderthalensis, _erectus_, etc. from _Homo_, which would be highly disruptive, to say the least....) Another point to make about the proposed naming convention for crown/panstem pairs is that, while it may be obvious that _Panmammalia_ is the panstem of _Mammalia_ (even if, for example, _Panderichthys_ is not the panstem of "Derichthys"....), it's not obvious that _Mammalia_ is a crown clade from its name alone. If there is a mandated prefix for panstems, why not one for crowns as well? (I'm not saying I would like one, just why one and not the other?) One solution might actually be found in the traditional system, where taxa may optionally be written with a title, e.g. Kingdom Animalia, Class Reptilia, Order Primates, etc. Why not have a similar option for PhyloCode taxa, so that the names themselves can be free to be formed in any manner? Crown Clade _Amniota_, Panstem Clade _Synapsida_, Stem Clade _Saurischia_, etc. (I find it often handy to use just "Clade" as a title when contrasting with traditional taxa, e.g., Clade _Reptilia_ vs. Class Reptilia.) Of course, this has the disadvantage of not nomenclaturally linking panstems with their crowns. (Although it might be useful in any event.) Another thought would be to allow punctuation just in this case, namely, a hyphen: _Pan-Mammalia_, _Pan-Amniota_, _Pan-Vertebrata_, etc. Then it would be clear that _Panoplosaurus_ is not a panstem clade, while _Pan-Sauria_ is. Thoughts? ===== =====> T. Michael Keesey <http://dino.lm.com/contact> =====> The Dinosauricon <http://dinosauricon.com> =====> Instant Messenger <Ric Blayze> ===== __________________________________ Do you Yahoo!? Take Yahoo! Mail with you! Get it on your mobile phone. http://mobile.yahoo.com/maildemo