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Re: Removing segnosaurs from Theropoda



At 10:54 AM 8/29/95, Dinogeorge@aol.com wrote:
>>  If the interdental plates in all three types of animals are virtually
>identical,
>>then we must state that they have arisen in at least two lineages (probably
>>more) separately, and are thus not viable characters for a cladistic
>>analysis.
>
>This would be more correct, in my opinion. But an apomorphy that has arisen
>convergently in two distinct lineages can be considered a synapomorphy within
>each lineage; it's just not a synapomorphy uniting the two lineages.

        Yes, they could...and this sort of thing happens a _lot_ in
cladistics, only because the cladist doesn't describe _specifically_ what
is unique about the trait they're using -- they just mention the trait
(e.g., saying "fused metatarsals."  So what?  Artiodactyls have fused
metatarsals, but they look different than those of, say, birds or theropods
-- just saying "fused metatarsals" doesn't cut it, IMHO).  The cladist
needs to delineate precisely what is unique about the feature they're using
for it to be truly a synapomorphy.  Thus, when one just says "interdental
plates," then I'd have to say it's a useless character because interdental
plates exist in a variety of organisms which are clearly distantly related.
There's nothing I hate more than scouring some cladist's list of
synapomorphies and saying "Well, this animals also has trait 9, and this
group also shows trait 14," because it negates the utility of what the
cladist is saying (or, at least, trying to say).



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