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RE: Stegosaur volume of Swiss Journal of Geosciences
Michael Mortimer <email@example.com> wrote:
> But I'm betting armatus CAN be distinguished from
> non-Stegosaurus stegosaurids, since it preserves a large
This is to some degree a circular argument, because one of the reasons
_Hesperosaurus mjosi_ was referred to _Stegosaurus_ by Maidment et al. (2008)
was the overall similarity in plate size. This is despite the fact that the
dorsal plates of _H. mjosi_ are of a different shape to those of _S. armatus_
and _S. stenops_, being "longer anteroposteriorly than tall dorsoventrally".
Don't forget that Maidment &c *assumed* that _Stegosaurus_ exhibited a great
deal of morphological variation, including the shape and size of the plates and
spikes. On the basis of this assumption, Maidment &c referred _Hesperosaurus_
and _Wuerhosaurus_ to _Stegosaurus_, and regarded _S. longispinus_ as a nomen
dubium. But again, all this is circular reasoning on the part of Maidment &c.
Also, individual phylogenetic analyses have differed in where _H. mjosi_ comes
up: either closer to _Dacentrurus_ or closer to (other) _Stegosaurus_. So
plate shape/size may not be a very good arbiter for establishing stegosaur
> Diagnosability is not an all or nothing concept. A taxon
> can be diagnostic at "genus level", yet be undiagnostic
> within that genus. In that case, it should be fine to have
> the type species be a nomen dubium.
Yes, but this seems to be a very bad idea. By definition, a type species
typifies the genus it belongs to. So it would seem to be unhelpful for a
'good' genus like _Stegosaurus_ to have a 'bad' type species like _S. armatus_.
This is because if any doubt emerges regarding the monophyly of _Stegosaurus_,
then _armatus_ will pull down the genus _Stegosaurus_ along with it.
> If armatus does prove
> undiagnostic relative to Stegosaurus' sister genera, then
> I'd agree we should make stenops the type species. But
> nobody's claimed that's the case yet.
I don't think we really need to make the case now that _arma
genera. This is because there is a non-zero possibility of this happening
some time in the future. Therefore, it is best to discard a nomen dubium like
_S. armatus_, rather than it being a potential source of instability in the
future. This is the crux of my argument, and why it is good practice to set
aside nomina dubia when defining genera.