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Re: Sterna, Whales, and Evolution



>Do you suggest that the development of a simple ridge on a pre-existing
>structure necessitates a Class-level division?

>Thomas R. Holtz, Jr.

The trouble is, Tom, that there is really no such thing as a "class-level
distinction", and the more you know about intermediate forms the more
arbitrary the ones you pick will get.  After all, the traditional vertebrate
classes were defined by a strict creationist who thought all animals were
immutable and had no knowledge that fossil forms even existed, let alonewhat
they were like.  Thus the real distinctions between birds or mammals on the
one hand and reptiles on the other were originally made on the basis of a
whole suite of characters which we can now no longer rely on either because
we can't identify them in intermediate fossil forms or because they spread
across what  we like to think of as "obvious" phylogenetic gaps.  Thus
defining mammals comes down to distinguishing between something like
Diarthrognathus, which retains a very small and weak (at least from
pictures) dentary-squamosal articulation as part of the jaw apparatus, and
something like (say) Morganucodon in which the articulation persists but the
system is uncoupled from the jaw - a pretty trivial character difference, it
would seem.

I still am inclined to think the best move is to recognize the value of
convenience and to note that class distinctions will mean different things
to an ichthyologist, palaeontologist, ornithologist etc so that they can
keep their sub-categories straight.  As WS Gilbert said, "It really doesn't
matter, matter, matter, matter, matter, matter".

But you knew all that anyway, right?
--
Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
International Wildlife Coalition              Fax/Modem: (905) 569-0116 (home)
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