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Re: Removing segnosaurs from ...



In a message dated 95-08-29 11:01:41 EDT, Thomas_R_HOLTZ@umail.umd.edu (th81)
writes:

>>18) Lack of the Avetheropodan "double-jointed" lower jaw.
>>19) Interdental Plates.
>
>Now, you really cannot use interdental plates as uniting therizinosauroids &
>prosauropods, as they are also very typical of theropods.  At worst, they
>are primitive for Saurischia.
>
>

Character 18 is cited by Clark et al as being NOT present in _Erlikosaurus_,
that is, _Erlikosaurus_ supposedly HAD the avetheropodan mandibular joint(!).
Well, the joint looks identical to those of prosauropods, not theropods, and
I think that's another suspicious character they've added to their list.

As I noted in my other posting of a few minutes ago, segnosaurs and
prosauropods share the character "interdental plates present, but small
relative to tooth size" (and the tooth size is small already!). In those
theropods that had them, the interdental plates were quite large and
prominent--they almost look like successional teeth arising in between the
real teeth in lateral view. In the advanced theropods that should have had
them but didn't, the interdental plates seem to have been lost by fusion with
the supradentary lamina, not by vestigialization. So they really still had
them; they were just not visible--at least in certain cases.

In any case, the jury is still out on this feature.

G.O.