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Re: Fwd: Removing segnosaurs from Theropoda



At 4:05 AM 8/31/95, Paul Willis wrote:
>>
>>        You cannot determine whether or not a character state is convergant
>>between two taxa until after your analysis has been performed.  To state
>>that a character is homoplasious before hand is an a priori hypothesis,
>>which is the staple of religion, NOT science.  It is the analysis that
>>determines the status of a character.
>>
>>        Jason Head
>>        Dept. Geological Sciences
>>        SMU
>
>This is rediculously circular. Of course a detailed analysis of morphology
>can reveal convergence without a phylogenetic analysis ever being
>performed. In a crude analogy (and I love being crude), even a cursory
>study of the morphological convergent characters of sharks, ichthyosaurs
>and dolphins will reveal their convergent nature. In this sence, convergent
>characters _must_ be identified prior to analysis, particularly where a
>suit of characters can be demonstrated to be convergent. If this is not
>done then the analysis will be artificially weighted in favour of a
>paraphyletic group based on convergent characters.
>
>Cheers, Paul
>
>pwillis@ozemail.com.au


        Only if you're a bad cladist!!  as for circular arguements,
studying morphological characters that are convergant will lead OF COURSE
reveal their convergant nature (In the dictionary: for redundant, see
redundant).  Nobody performing an in-depth analysis of comparative anatomy
would confuse sharks, ichthyosaurs, and dolphins (not nowadays, anyway).
Overall, gross convergances between taxa are easily spotable, but the list
of convergant characters that were previously deemed homologous via apriori
reasoning is quite long.  Obviously, at a basal level some subjectivity
comes to play in analysis, it is the nature of the beast.  However,
deciding that pes structure or possession of interdentary plates is
convergant between two dinosaur taxa, before phylogenetic analysis, is an
apriori assumption, that smacks of a desire to see a particular
relationship instead of a desire to see whatever relationship unfolds in
the mix.  I say this w/out intending to "flame" anyone in particular, but
with the future of paleontology in potentially dire straits (a la
congress/Ralph Reed- told you I'd start talking  about religion and
politics), we all need to dot our "i"s, cross our "t"s, and perform the
most scientifically accurate work possible.  This means formulating our
ideas upon the results of analysis, not shaping analysis to conform to our
ideas.

        All My Best,






Jason J. Head
Dept. of Geological Sciences
Southern Methodist University
Dallas, Tx. 75275