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Date: Sat, 11 Sep 2004 22:48:48 -0700 (PDT)
From: [unknown]
To: Kevin de Queiroz <Dequeiroz.Kevin@NMNH.SI.EDU>, phylocode@ouvaxa.cats.ohiou.edu
Subject: Re: REPOST: Crowns, Panstems, and their Correspondence to ea=
--- Kevin de Queiroz <Dequeiroz.Kevin@NMNH.SI.EDU> wrote: > The approach we have taken is not to have a rule specifying the mea= ning of > =93extant=94 but to have a rule stating that a specified meaning (t= he same one > that TMK suggested) is to be adopted if the original author did not= specify > the meaning. This gives the authors of definitions more freedom bu= t also > prevents ambiguity. Here is the rule: > 9.5. If the author of a crown clade definition (Note 9.4.1) did not= specify > the meaning of "extant" or "crown clade" or an analogous term used = in the > definition (e.g., "living", "Recent"), then subsequent authors are = to > interpret that definition as referring to organisms or species that= were > extant on its publication date (Art. 5). Great idea! But for stability, mightn't it be good to specify "extant AND ALREADY= PUBLISHED on its publication date"? > > I notice two schools of thought in the abstracts for the upcoming= meeting as > > to definitions for crown and panstem clades. In Sereno's definiti= ons, there > > really are no panstems, only stem-based clades that happen to hav= e extant > > specifiers. The crown clades, then, are defined in terms of the s= tem-based > > clades: > >=20 > > Stem =3D Clade(A <-- B) (A and B are extant) > > Crown =3D Clade(extant Stem) > >=20 > > 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. > >=20 > > Crown =3D Clade(A + B) (A and B are extant) > > Panstem =3D Clade(Crown <-- extant non-Crown) > >=20 > > 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 pre= ferrable to > > me that the crown be the more stable one, but I'd be very interes= ted to hear > > other opinions. >=20 > I found this comment very interesting, as I hadn=92t noticed this d= ifference > until TMK pointed it out. I=92m not sure what he means when he say= s that one > definition is more stable than another in this context. I meant that the specifiers are explicitly stated. The definition can= 't be upset by a change in topology outside the clade or discovery of new e= xtant species. Which I think is an important point. Suppose we went with the former = approach and made these definitions: _Pancrocodylia_ =3D Clade(_Crocodylus niloticus_ <- _Vultur gryphus_) _Crocodylia =3D Clade(extant _Pancrocodylia_) Under these scheme, if one of these two things happens, the content o= f _Crocodylia_ changes drastically: 1) an extant group not usually considered crocodylian (e.g. _Testudin= es_) is found to be closer to _C. niloticus_ than to _V. gryphus_ (as had bee= n proposed in at least one study) 2) a new extant species outside the clade of previously known crocody= lians, but closer to than than to _V. gryphus_, is found (e.g. a living ornithos= uchid) In case #1, turtles become crocodylians, as well as a number of fossi= l taxa. In case #2 a number of fossil taxa become crocodylians as well. (You could argue that the problem in #1 would go away if you also inc= luded _Testudo graeca_ in the external specifiers of _Pancrocodylia_, but c= an we always count on that kind of foresight? And that still wouldn't help = in case #2.) However, if the crown's definition is given primacy: _Crocodylia_ =3D Clade(_Crocodylus niloticus_ + _Alligator mississipp= iensis_ + _Gavialis gangeticus_) _Pancrocodylia_ =3D Clade(_Crocodylia_ <- extant non-crocodylians) Cases #1 and #2 would restrict _Pancrocodylia_, but leave _Crocodylia= _ unchanged. =3D=3D=3D=3D=3D =3D=3D=3D=3D=3D> T. Michael Keesey <http://dino.lm.com/contact> =3D=3D=3D=3D=3D> The Dinosauricon <http://dinosauricon.com> =3D=3D=3D=3D=3D> Instant Messenger <Ric Blayze> =3D=3D=3D=3D=3D =09=09 _______________________________ Do you Yahoo!? Shop for Back-to-School deals on Yahoo! Shopping. http://shopping.yahoo.com/backtoschool