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Re: Alxasaurus and segnosaurs



In a message dated 96-02-05 20:30:45 EST, endocrin@desktop.com.au (Graeme
Worth) writes:

>There have been some postings over the past few months strongly suggesting
>that the published description of Alxasaurus is at deviance from the actual
>type specimen. On the surface this seems rather strange - what would be the
>point of describing it incorrectly when the specimen is available for
>comparison? If it was a mistake, why has it not been corrected? Have those
>who claim that an error has been made contacted the authors? To have this
>sort of discussion still circulating seems not only odd, but unnecessary, in
>that it would not appear to be all that difficult to arrive at a conclusion
>one way or the other!

Exactly the questions I had. The measurements given in Russell & Dong seem
accurate and conform to what I've seen of the type specimen, but the figures
don't seem to match the measurements. And the mounted skeleton of _The
Greatest Show Unearthed_ definitely is at variance with the postcranial bones
exhibited alongside(!).

>Leaving all that aside, here are some hypothetical questions. If we had
>never heard of Alxasaurus, where would the segnosaurs be classified? Would
>Therizinosaurus still be included with the segnosaurs? And, finally, if the
>claimed errors in the Alxasaurus description are in fact errors, is that
>likely to mean that Alxasaurus is not a segnosaur, or that it is but the
>classification needs to be reviewed?

Segnosaurs started out as theropods, but when skull material started being
described, they were quickly moved over somewhere into the
sauropod-prosauropod clade, "sedis mutabilis." The recent description by
Clark et al. of the skull of _Erlikosaurus_ would have moved them back to
Theropoda even without _Alxasaurus_, but I think they stacked lots of very
minor, easily reversed and unreversed skull features against a few really
robust postcranial characters, thereby numerically winning the synapomorphy
battle but losing the phyletic war.

>Boy, what a mess!
>Please, can someone clear this up once and for all?

Not likely. I certainly wouldn't budge until someone showed me how to derive
a prosauropod-like foot from a tetanuran theropod foot and a prosauropod-like
dentary with prosauropod-like teeth from a tetanuran theropod dentary.