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Re: Removing segnosaurs from Theropoda



At 6:08 PM 8/28/95, Dinogeorge@aol.com wrote:
>Excuse me--interdental plates are also present in _Plateosaurus_; see
>Galton's chapter in _The Dinosauria_, p. 327 (and illustrations of the
>_Plateosaurus_ skull therein):
>
>"The teeth are set in sockets that are bordered lingually by small
>interdental plates alternating with special foramina of varying sizes."
>
>Interdental plates may thus be a synapomorphy uniting advanced Prosauropoda
>and Segnosauria.

        OK.  If this is true, then there must be something specific about
the nature of the interdental plates in therizinosauroids and prosauropods
that is _not_ present in the interdental plates of theropods -- what is it?
Otherwise, I could just as easily make the statement that interdental
plates are a synapomorphy linking theropods (above the level of the
plate-less _Herrerasaurus_) and therizinosauroids, and it becomes just as
viable as your statement linking the latter and prosauropods.  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.

        Just out of curiousity, George:  _Archaeopteryx_ has interdental
plates, if I recall correctly, and if you're postulating that birds arose
from some "thecodontian" ancestor even earlier than _Herrerasaurus_, then
one could just as easily argue that this early appearance of interdental
plates is a synapomorphy linking therizinosaurs and birds, making them
sister taxa.  Do you want to go that far, as well?  ;-)



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