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Fwd: Removing segnosaurs from Theropoda



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.


---------------------
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.