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Date: Tue, 14 Sep 2004 16:22:37 +0200
From: [unknown]
To: PML <phylocode@ouvaxa.cats.ohiou.edu>
Subject: Re: Apomorphy-based clades; was Re: Panstems
----- Original Message -----
=46rom: "Mickey Mortimer" <Mickey_Mortimer111@msn.com>
Sent: Tuesday, September 14, 2004 2:44 PM
> 1. A philosophical argument.
I think many people will ignore that. Few scientists care much about
philosophy, and many (including myself) hold a certain despise for it=
; after
all, apart from science theory, most if not all philosophy is unneces=
sary
for science.
> 2. Continuity again.
> 3. Good ol' incompleteness.
These, on the other hand, are VERY good arguments.
> 4. Confuciusornis and Carinatae.
> Indeed, such debates take up their traditional form due to yet anot=
her
> problem of apomorphy-based definitions- continuously variable chara=
cters
> being used as apomorphies. A good example is Carinatae, recently
redefined
> as (keeled sternum homologous with Vultur gryphus). The authors of=
that
> definition used Chiappe et al.'s wording to defend the structure in=
some
> Confuciusornis as a ridge and not a keel, thus confuciusornithids a=
re
> outside Carinatae.
This was a feeble attempt to get around the fact that the presence or
absence of that "ridge" is _individual, intraspecific variation_ in
*Confuciusornis sanctus* (as far as can be told -- perhaps the two di=
ffered
in feather color --, but even this is unlikely, because, judging from=
a
chicken on my dinnertable, the "ridge" is just the ossification of a
cartilaginous keel -- and cartilage _never_ fossilizes, except in fro=
zen
mammoths and the like).
I should mention that an earlier definition of Carinatae was node-bas=
ed,
*Ichthyornis* and the crown-group being specifiers. This is reasonabl=
y close
to much historical usage, although it does not correspond to any chan=
ge in
the shape of the keel.
> 5. M[=FC]ller and Diapsida.
> [...] Indeed, parsimony is ambiguous as to whether the lower arch
> of archosaurs and lepidosaurs is even homologous. Using DELTRAN,
> apomorphy-based Diapsida would include only archosauriformes and
> rhynchosaurs. Using ACCTRAN, it would include only Sauria.
I don't think this is ambiguous. Among lepidosaurs, only the most der=
ived
sphenodontids have an ossified lower temporal bar. The Late Cretaceou=
s
*Priosphenodon* still lacks one, for example.* This means Diapsida wo=
uld
shrink to a part of Archosauromorpha. -- But anyway, I have been told=
in
Paris (Gauthier, pers. comm.) that a temporal bar need not be ossifie=
d to be
a temporal bar. This brings Diapsida back to extend all the way to it=
s
traditional origin. But then a problem kicks in that you have mention=
ed in
June on this list. When is a bar a bar? When it's bordered by a fenes=
tra.
When is a fenestra a fenestra? Is the size of the hole important? If =
yes,
see "ridge". If no, we may one day find a species in which it is indi=
vidual
variation whether thosee three skull bones meet in the middle or not.
Methinks apomorphy-based definitions only work when discoveri=
es are
rare -- when we have the famous stability that equals ignorance.
* This was known long before anyone suspected the closed bar in archo=
saurs
and friends could be secondary, too -- but only to a few paleontologi=
sts, it
seems. The rest of the world continued to worship *Sphenodon* as the =
only
lepidosaur to have kept the plesiomorphic temporal configuration; it =
even
has a German name that translates as "bridge lizard", "bridge" as in
"temporal bar", lest people ever forget.
> Similarly, I have yet to see justification for why we should retain=
a
source
> of taxonomic ambiguity when it's very easy to eliminate that source=
.
I think I agree. But... we should make an exception for the clade of =
all
known life. That is impossible to define otherwise. :o)