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RE: Stegosaur volume of Swiss Journal of Geosciences



Tim Williams wrote-

> 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".

So you agree Hesperosaurus has a different plate shape than Stegosaurus armatus 
or S. stenops then, leaving Stegosaurus' plate shape as distinctive.  Thus 
armatus can be referred to Stegosaurus and doesn't cause any difficulties for 
phylogenetic nomenclature if it remains the type species.

> 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.

As long as it's distinctive to one genus (Stegosaurus in this case), it doesn't 
matter how useful it is otherwise in the phylogeny.

> > 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.

Type species do not typify their genera in the sense of being representative of 
them diagnosability-wise, they're just the first species to be named in their 
genus.  If there is ever any doubt of Stegosaurus' monophyly, THEN it would be 
appropriate to discuss ensuring the type species is diagnostic.  For now it's 
just a hypothetical situation that certainly does not warrant the ICZN taking 
action.  Similarly, the fragmentary Diplodocus longus may pull down Diplodocus 
unless we make D. carnegii the type, but since it's just a hypothetical 
situation, we don't need to act on 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 _armatus_ is 
> non-diagnostic relative to non-_Stegosaurus_
> 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.

There's always a non-zero chance of this happening for any taxon.  That's 
science.  Maybe Noasaurus will prove undiagnostic since it's so fragmentary.  
Better get rid of Noasauridae in favor of "Masiakasauridae" (since Velocisaurus 
is hardly better).  Abelisaurus is pretty poorly known too.  Carnotauridae is 
the safer way to go.  Ornithomimus is based on a few metatarsals and 
metacarpals that may not even belong to the same taxon.  Let's go with 
Struthiomimidae just to be sure.

Much as with the Ceratops and Rapator examples, you don't seem to see the need 
to rigorously demonstrate a taxon is indeterminate before passing judgement on 
it.  It's quite frustrating that paleontologists now seem content to throw old 
taxa into the waste bin without trying to prove their validity, which goes 
against the science standard of trying to disprove your own hypothesis before 
you present it.  The easy way out will indeed result in a lot of obviously 
diagnostic taxa, but at the expense of our predecessors' legacy.

 Mickey Mortimer