superphylum Chelonia

[John Alroy <jack@homebrew.uchicago.edu>]

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  

argumentative breed.

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.