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Apomorphy-based definitions




Dear All,
I'm going to try first to dispel this "postponing the inevitable" misconception. The apomorphy-based Mammalia continues (after many decades) to provide a clearcut boundary based on osteology (jaw and ear ossicles). When we do find transitional forms that fill in the remaining gap, we simply take the very same characters and refine them a bit in light of the new evidence. It's a very stable strategy.
Class Aves on the other hand has not enjoyed this same kind of precise osteological definition, and we are suffering the consequences. The "true" semilunate carpal block might not be quite as precise as mammalian ear ossicles, but as far as I can tell, it is the closest we are going to get to it. If we find incipient structures very close to a "true" semilunate, we can refine it a bit (just as we would do with the mammal definition).
Basing Aves on imprecise characters like "flight" or feathers has clearly muddled the whole debate, making it confusing and inefficient. But we now have enough fossil evidence to rectify this. And eventually we may learn that the "true" semilunate was a key step in the development of powered flight. This seems to me preferable to waiting a couple of decades to see how the debates turn out over BCF and other flight-related issues. And it gives a little bit of "wiggle room" to refine the definition, and still maintain stability without the "straight-jacketing effect" of node and stem-based definitions (which I think are even more arbitrary). All classifications are arbitrary, and the best we can do is to minimize it the best we can.
Mammalia has demonstrated the superiority of such an apomorphy-based strategy, and clearly shows how confusing and imprecise crown groups (in particular) are to deal with. We don't know if multituberculates belong to the crown group, but we do know they have the mammalian jaw and three ear ossicles.
So if we want to apply the "Sisyphus" analogy, I would have to point to the recent mammal classification book (McKenna and Bell, 1997). It not only adopts a crown group definition, but the whole eutherian classification is going to crumble when their polyphyletic Grandorder Ungulata is split up (as well as Grandorders Anagalida, Ferae, Lipotyphla, and perhaps even Archonta). Clade Altungulata seems to be particularly unnatural. And supersplitting Linnean ranks like that is why such ranks now have such a terrible reputation among cladists. Abuse a tool and of course it will break.
We have an opportunity to learn from the mammalian example, and avoid the mess that pure cladification has made with mammal classifications (which perfectly illustrate the problems that I described in 1994 as "the multiplicity of names and categories, hierarchical instability, etc."). The book by McKenna and Bell, 1997, is the kind of thing I was warning about (and the Sisyphus analogy fits it very well).
Anyway, I personally do not want to go down that road with the reptiles and birds. I obviously make my classifications as cladified as I can (the new AVES classification shows this clearly). But total cladification ultimately fails because it is based ONLY on branching order (and fails to reflect the reality of paraphyly). If someone can come up with a better character than the "true" semilunate, I am willing to listen, but I still haven't found a better one.
----Cheers, Ken Kinman
P.S. If the Feducciaries are right, and the "true" semilunate evolved twice, then we are ALL in very very deep (up to our necks) in trouble. However, if I thought they were right, I wouldn't have made my proposal, so I'm not going to lose any sleep over that. :-)





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