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Re: Removing segnosaurs from Theropoda



At 8:27 PM 8/29/95, Dinogeorge@aol.com wrote:
>In a message dated 95-08-29 18:33:52 EDT, jdharris@lust.isem.smu.edu (Jerry
>D. Harris) writes:
>
>>        That is correct, as per your BCF theory.  But that was also _my_
>>point:  if the interdental plates extend back to the lineage that presages
>>theropods, prosauropods, and therizinosaurs, then you can't, as Thom Holtz
>>pointed out, use interdental plates as a synapomorphy for just the
>>prosauropods and the therizinosaurs -- instead, it's a symplesiomorphy
>>that's shared in both those groups _plus_ the Theropoda.  Now, as you did
>>clarify, "small interdental plates with respect to tooth size" could be a
>>synapomorphy, if the ones in theropods are considered different...but I'd
>>like to know what the primitive condition (the one possessed by the common
>>ancestor of birds, theropods, prosauropods, and therizinosauroids) was
>>before I'd be satisfied with a label of whichever later condition as a
>>synapomorphy.
>
>And you can't use it as a synapomorphy uniting segnosaurs and theropods,
>either, as Clark et al. do. Hence my careful use of the words "may be" in my
>original posting.
>
>Incidentally, the more I look at the mandible of _Erlikosaurus_, the more
>convinced I become that had it been discovered in Triassic deposits, it would
>without hesitation have been referred to Prosauropoda.

        Yes, and if  crocodylian mandibles were found in Triassic deposits,
they'd be called phytosaurs.  The downturned Maxillae are very similar, but
the story is just not that simple.  Until we learn more about segnosaurs, a
generally accepted answer will have to wait (esp. the relationships between
segnosaurs and therizinosaurs:  I am not convinced by Russell's
descriptions and analyses).  As for "Thecodontians", you don't have to be a
cladist (and/or strictly adhere to monophyletic groupings), to recognize
that "thecodont" has NEVER described an actual group of related taxa.  It
was a garbage can ( a la megalosaur), for poorly understood archosaurs, and
has since been abandoned.


>






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


        "I usually just make sh*t up" - discussion on oral presentations.