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Re: Fwd: Removing segnosaurs from Theropoda



At 11:15 AM 8/29/95, Dinogeorge@aol.com wrote:
>Subj:   Re: Removing segnosaurs from Theropoda
>Date:   95-08-29 10:55:38 EDT
>From:   Dinogeorge
>To:     jdharris@lust.isem.smu.edu
>
>In a message dated 95-08-29 10:43:58 EDT, you write:
>
>>       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?
>
>They're much smaller relative to tooth size in prosauropods and segnosaurs,
>for one thing.
>
>>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.
>
>My objection was to the statement that interdental plates are common only to
>certain theropods, which is demonstrably incorrect. Tony Thulborn just
>informed me that interdental plates are also present in certain
>proterosuchians(!).
>
>>  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.

        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 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.
>
>
>---------------------
>Forwarded message:
>Subj:    Re: Removing segnosaurs from Theropoda
>Date:    95-08-29 10:55:38 EDT
>From:    Dinogeorge
>To:      jdharris@lust.isem.smu.edu
>
>In a message dated 95-08-29 10:43:58 EDT, you write:
>
>>       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?
>
>They're much smaller relative to tooth size in prosauropods and segnosaurs,
>for one thing.
>
>>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.
>
>My objection was to the statement that interdental plates are common only to
>certain theropods, which is demonstrably incorrect. Tony Thulborn just
>informed me that interdental plates are also present in certain
>proterosuchians(!).
>
>>  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.