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Re: Removing segnosaurs from Theropoda



At 8:36 PM 8/30/95, Stan Friesen wrote:
>Not if you accept paraphyletic groups, as I do.
>
>The whole point of establishing a paraphyletic taxon for the stem
>group is to *avoid* the necessity of erecting numerous small, largely
>similar, groups for each of the early lineages.

        Avoiding necessity may make things neater and tidier and take up
less room in a file cabinet, but if they don't accurately reflect real
relationships, then the classification scheme isn't worthwhile.  Now, I'm
not saying that cladists are necessarily correct in shoving everything in
monophyletic groups -- I know little enough about anatomy to make any claim
to understanding about the traits in question well enough to make a
statement about the way they evolved -- but as a whole, the concept seems
more sound in terms of reflecting the way things occur than paraphyly does.

> > If birds are
> > dinosaurs, then crocs are thecodonts, but no one ever says that.
>
>But then I *don't* include the birds in my Dinosauria!  I put birds in
>the seperate class Aves (not even in Reptilia).

        But what is your reasoning for this?  If birds are descended from
dinosaurs, and dinosaurs from reptiles, then by the various logical
reasonings, birds are reptiles, although highly modified ones.  If A=B and
B=C, then A=C, too.  (But, as I've mentioned before, I don't _call_ birds
dinosaurs, just the same way that, even though it's technically true, I
don't call dinosaurs agnathans.)  So...what criteria (in general, not just
for the bird/reptile thing) allows one group of organisms to be _removed_
from another?  Where is the precise break in the fossil record (which,
clearly, would require only one single "transitional" form, which, as we
all know, never happens, despite what the creationists want to believe)?



Jerry D. Harris
Shuler Museum of Paleontology
Southern Methodist University
Box 750395
Dallas  TX  75275-0395
(214) 768-2750
FAX:  (214) 768-2701
jdharris@lust.isem.smu.edu
        (Compuserve:  73132,3372)

---------/O\------*     --->|:|:|>     w___/^^^\--o

"If it looks like a duck, walks like a duck, and
quacks like a duck, then it is the sister taxon to,
but cannot parsimoniously be, the direct ancestor
to all other ducks."

                                --  _not_ W. Hennig

---------/O\------*     --->|:|:|>     w___/^^^\--o