RE: Stegosaur volume of Swiss Journal of Geosciences

[Tim Williams <tijawi@yahoo.com>]

Michael Mortimer <mickey_mortimer111@msn.com> wrote:

> But I'm betting armatus CAN be distinguished from
> non-Stegosaurus stegosaurids, since it preserves a large
> plate.  


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 
relationships.


> 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. 



Cheers

Tim