Message 2004-06-0033: Crowns, Panstems, and their Correspondence to each other

Wed, 16 Jun 2004 08:44:01 -0700 (PDT)

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Date: Wed, 16 Jun 2004 08:44:01 -0700 (PDT)
From: "T. Michael Keesey" <>
To: Mailing List - PhyloCode <>
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


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.)


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.


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

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


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

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

(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.


=====> T. Michael Keesey <>
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