Message 2004-10-0056: Re: Panstems

Mon, 13 Sep 2004 21:05:44 -0400

[Previous by date - Re: Panstems]
[Next by date - Nomenclatural Freedom IS the issue]
[Previous by subject - Re: Panstems]
[Next by subject - Re: Panstems]

Date: Mon, 13 Sep 2004 21:05:44 -0400
From: [unknown]
Subject: Re: Panstems

Michel Laurin wrote:

=09Here, I believe that Kevin is=3D20
disregarding a point that some of us tried to=3D20
make at the meeting and that is related to=3D20
Mickey's comment.  Namely, systematists want to=3D20
keep these names for stem-based clades because=3D20
this is more or less the way that they have been=3D20
conceptualized (or at least delimited) by=3D20
generations of systematists.  For about a=3D20
century, Synapsida has referred to the stem of=3D20
Mammalia, and in the last 20 years or so, it has=3D20
come to include mammals too, a necessary change=3D20
to make it monophyletic.  It is because of the=3D20
long history of this name that many of us want to=3D20
keep it, not because Kevin and Jacques defined it=3D20
twelve years ago (although I agreed with the=3D20
definition that they provided at the time and=3D20
have used the name in that sense consistently).=3D20
Parenthetically, other taxa have a lateral=3D20
fenestra but were not included in Synapsida (at=3D20
least in the last 50 years), such as some=3D20
parareptiles.  Indeed, the fenestra is so common=3D20
in early amniotes that Reisz raised the=3D20
possibility that it might be an apomorphy of=3D20
amniotes lost in the "anapsids", so redefining=3D20
Synapsida on the basis  of the fenestra would be=3D20
a really bad idea.  Similarly, when I proposed=3D20
(with Reisz) phylogenetic definitions of=3D20
Parareptilia (=3D3DPanTestudines) and Eureptilia=3D20
(=3D3DPanSauria), I used Everett Olson's terminology=3D20
(proposed in the 1940s), that had been used by=3D20
several other paleontologists (even in Russia).=3D20
If I want to keep these names, it is not only=3D20
(not even mostly) because we have used these=3D20
names in the last 10 years in PN but rather,=3D20
because these definitions reflect the use of=3D20
these names in the literature (rank-based and PN).

>Of course, the reason that Jacques and I defined the name Synapsida =
phylogenetically as referring to the total clade of Mammalia is that =
this =3D
use "approximates" the way the name had been used for many years befo=
re.  =3D
However, it's misleading to imply, as Michel does, that the name was =
uniquely associated with the stem, rather than with an apomorphy (or =
a =3D
node) for the approximatly one hundred years before Jacques and I =
formulated a phylogenetic definition for it.  The name has been assoc=
iated =3D
with both, and in this particular case the exact association is ambig=
uous =3D
because the known composition of the two clades has been identical (i=
.e., =3D
there are no clear examples of taxa that lack the character but are =
clearly more closely related to mammals than to birds and turtles).  =
In =3D
addition, potential homoplasy in the character is NOT a good reason f=
or =3D
avoiding an apomorphy-based definition (and in this case, Michel seem=
s to =3D
be disregarding comments that were made at the Paris meeting).  If ot=
her =3D
taxa have evolved the character convegently, those taxa can be exclud=
ed by =3D
specifying that the condition has to be homologous with that in mamma=
ls =3D
(or Varanops).  If instead the character arose earlier, then we would=
simply have to conclude that some other early amniotes that have not =
traditionally been considered synapsids are, in fact, synapsids (chan=
ges =3D
in hypothesized composition happen all the time).  Moreover, even if =
one =3D
doesn't like apomorphy-based definitions, one could also make the arg=
ument =3D
that the name Synapsida has been no more clearly associated (at least=
prior to the first phylogenetic definition) with the stem of Mammalia=
 than =3D
with the node representing the last common ancestor of Varanops, Eoth=
yris, =3D
and Cynognathus.  In this context, it should be clear that I was not =
simply disregarding Michel's point made at the Paris meeting. Instead=
, I =3D
consider that argument incorrect.  If one is going to argue that the =
name =3D
Synapsida was unambiguously associated with a particular clade, then =
one =3D
has to cite an explicit phylogenetic definition (e.g., that of Gauthe=
ir =3D
and de Queiroz).  Conversely, if one is going to use the work of earl=
ier =3D
authors to establish usage, then one is not going to be able to assoc=
iate =3D
the name unambiguously with a particular stem as opposed to a node or=
apomorphy.  The use of this name by earlier authors is highly ambiguo=
us =3D
with regard to these three different possible conceptualizations, bec=
ause =3D
the precise distinctions embodied in them generally were not made at =
that =3D
time.  Consequently, the ambiguous association of the name Synapsida =
with =3D
the stem in question by earlier authors is not a good reason for reje=
cting =3D
PanMammalia as the name for the total clade originating from that ste=
m, as =3D
an equally good case can be made for a historical association of the =
name =3D
Synapsida with either an apomorphy or a node.  Indeed, if one is goin=
g to =3D
follow the usage of earlier authors, Theropsida would seem to be a be=
tter =3D
choice than Synapsida, given that there is less reason to think that =
the =3D
former name does not describe an apomorphy and was originally associa=
ted =3D
unambiguously with thel clade in question (rather than with a paraphy=
letic =3D
group as in the case of Synapsida).<   =3D20

=09One of the most common critique of the=3D20
PhyloCode is that it will generate many new names=3D20
and disrupt continuity with the literature; that=3D20
is certainly not true of the PhyloCode as it now=3D20
stands, but it COULD be true if the use of the=3D20
Pan- prefix for total clades is made mandatory=3D20
for most such clades.  Let's not give critiques=3D20
of the PhyloCode such an obvious problem to point=3D20

>For the reasons given above, the argument about disrupting continuit=
y is =3D
false.  Regarding new names, this compaint has been raised repeatedly=
 in =3D
the history of taxonomy, yet the proposal of new names never ceases, =
nor =3D
does it cause any new problems beyond the general one of dealing wth =
ever =3D
increading information that characterizes all of human history since =
the =3D
invention of language.  Neither of these concerns is well founded, an=
d we =3D
should not therefore allow them to constrain us.<

Kevin de Queiroz
Division of Amphibians & Reptiles
Smithsonian Institution
P.O. Box 37012
NHB, Room W203, MRC 162
Washington, D.C. 20013-7012
Voice:  202.633.0727
FAX:  202.357.3043


Feedback to <> is welcome!