Message 2004-10-0024: Re: REPOST: Crowns, Panstems, and their Correspondence to ea=

Sat, 11 Sep 2004 22:48:48 -0700 (PDT)

[Previous by date - Re: Panstems]
[Next by date - Another Possible Problem with Naming Conventions for Panstem=]
[Previous by subject - Re: REPOST: Crowns, Panstems, and their Correspondence to ea=]
[Next by subject - Re: Rankless classifications]

Date: Sat, 11 Sep 2004 22:48:48 -0700 (PDT)
From: [unknown]
To: Kevin de Queiroz <Dequeiroz.Kevin@NMNH.SI.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=
> 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=
> the meaning of "extant" or "crown clade" or an analogous term used =
in the
> definition (e.g., "living", "Recent"), then subsequent authors are =
> interpret that definition as referring to organisms or species that=
> extant on its publication date (Art. 5).

Great idea!
But for stability, mightn't it be good to specify "extant AND ALREADY=
on its publication date"?

> > I notice two schools of thought in the abstracts for the upcoming=
> > 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=
> > 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=
> > me that the crown be the more stable one, but I'd be very interes=
ted to
> > other opinions.
> I found this comment very interesting, as I hadn=92t noticed this d=
> 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=

Which I think is an important point. Suppose we went with the former =
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=
_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=

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=
_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

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=

=3D=3D=3D=3D=3D> T. Michael Keesey <>
=3D=3D=3D=3D=3D> The Dinosauricon <>
=3D=3D=3D=3D=3D> Instant Messenger <Ric Blayze>

Do you Yahoo!?
Shop for Back-to-School deals on Yahoo! Shopping.


Feedback to <> is welcome!