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Well, I was inclined to agree with Dr. Schwimmer that we have been
babbling about this "Reptilia" topic far too long. However, I can't
just let Stan's comments on what makes something a "stem amniote" go
by. Stan is focusing on the "major adaptive breakthrough" argument
for maintaining higher-order paraphyletic groups, e.g., "reptiles"
are a class because they are jointly defined by having made one major
breakthrough (the amniotic egg, scaly skin, whatever) and failed to
make others. Now what are these "other" breakthroughs, and can we use
them as consistent criteria for defining "classes" or what have you?
I think not:
1) Is endothermy a "breakthrough"? Let's say it is. Then why are the
majority of dinosaurs - all the non-avian ones, to be specific -
excluded from the "class" Aves? Even if you did stick to the position
that ALL the terrestrial dinosaurs were ectothermic, even including
the small theropods most closely related to birds, no-one would agree
with you, making it impossible to use this derived "character" to
define a "class." What goes in the "class" and what doesn't would
depend on the whim of dinosaur paleophysiologists, a notoriously
2) How about flight? That makes matters even worse. If true flight is
a "class-level" breakthrough, then bats are a "class" within the
"class" Mammalia, and pterosaurs are yet another class!
3) How about both of the above criteria together? Okay, now we end up
with the "class" Aves corresponding pretty well with the traditional
concept of the group. But how about mammals being a "class" and
dinosaurs still being a miserable little sub-group of the "class"
Reptilia, no matter how endothermic they may have been?
My point here is that if we use "major breakthroughs" as criteria for
setting ranks, we lose any hope of coming to an objective consensus.
For example, I could argue (contra Friesen's claim, which is exactly
what set me off in the first place) that turtles HAVE gone through a
profound adaptive breakthrough, namely, evolving a true shell and
figuring out how to get their pectoral and pelvic girdles INSIDE
their ribs. If this Houdini-esque kind of a trick isn't "profound," I
don't know what is. It seems to me that I could argue for turtles not
only being a "class" of their own, but, say, a "subphylum" or a
"phylum" or even a "superphylum"! Systematists can't even make their
minds up about what "rank" the Arthropoda should have, so what's to
stop me, apart from YOUR personal opinion? -Personally- I think the
turtles are far more interesting than all of the arthropods put
together, all ten zillion of 'em.
I hope my point is well taken. I can see a place for paraphyletic
groups in certain situations, but if we stick with the Linnean
ranking system we are just going to cause ourselves needless
headaches (and wasted bandwidth). Like it or not, higher-order
classifications based strictly on branching order are a lot easier to
agree upon: all we have to do is make sure our cladistic analyses are
done properly to everyone's satisfaction, which is an eminently
realistic goal. Once we have an agreeable topology, all we have to do
to name the groups is apply the principle of priority (it's my
opinion that all this stuff about deciding on names by looking at
"stem-groups" or "crown-groups" is a smokescreen; priority is a
completely adequate criterion). The appropriate "ranks" can be put in
any way you want, because by virtue of all the groups being
holophyletic, it is not possible for equally-ranked groups to be
derived from each other, or for higher-ranked groups to be derived
from lower-ranked groups.