[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

RE: Stegosaur volume of Swiss Journal of Geosciences



Tim Williams wrote-

> > Looks quite interesting.  However, in his abstract Galton
> > writes "Stegosaurus armatus Marsh 1877, based on a partial
> > tail and a very large dermal plate from the Morrison
> > Formation (Late Jurassic) of Morrison, Wyoming, USA, is a
> > nomen dubium." and "The International Commission on
> > Zoological Nomenclature (ICZN) will be petitioned to
> > designate S. stenops Marsh 1887 as the new type species of
> > Stegosaurus Marsh 1877 in order to conserve Stegosauria
> > Marsh 1877 and Stegosauridae Marsh 1880 (also
> > Stegosauroidea, Stegosaurinae)."  This seems like the same
> > flawed rationale used when Titanosauridae was killed by
> > Wilson and Upchurch (2003) because Titanosaurus was declared
> > a nomen dubium.
>
> I think the rationale is appropriate. If _Titanosaurus_ is a nomen dubium, 
> than it is perhaps best to exclude
> this taxon from coordinated family-level taxonomy.  

Perhaps best, sure.  But suggested or mandated by the ICZN, no.
I personally strongly feel a genus should be able to have eponymous taxa as 
long as it's a member of those taxa.  So as long as Titanosaurus falls within 
Titanosauria/idae, it should be fine regardless of whether we can distinguish 
it from other titanosaurians/ids.

> > Yet the ICZN says nothing about families
> > needing to be based on diagnostic taxa, it just says a taxon
> > should be "well known" if it has a family-level group named
> > after it.
>
> This is a distinction without a difference, surely. If a taxon is a nomen 
> dubium, than it can't be "well known".

I disagree.  "Diagnostic" and "well known" are not synonyms.  Titanosaurus is 
very well known regardless of its diagnosability, for instance.  I bet more 
people know about it than know about Isisaurus, Baurutitan, or most other 
sauropods for that matter.  "Well known" is not further defined by the ICZN 
(surprise!), so it's all subjective anyway.  Not only is it subjective, but 
it's not even a rule, it's only a recommendation (64A to be exact). 

> In the parlance of phylogenetic nomenclature, a nomen dubium is not a valid 
> operational taxonomic unit (OTU), and so effectively does not exist. So from 
> a phylogenetic taxonomic perspective, having families named after suspect 
> taxa can lead to instability. I recall having this discussion using _Ceratops 
> montanus_ as my poster child for why it was a BAD idea to maintain a family 
> called Ceratopsidae when the nominative taxon (_C. montanus_) was too poorly 
> known to be confident if it actually belonged in Ceratopsidae.

Er... OTU is a term used in phylogenetic analyses, not phylogenetic 
nomenclature.  Indeed, as we discussed before, a nomen dubium can be an OTU in 
an analysis, it will just have more than one equally parsimonious position 
(assuming the taxa it's indistinguishable from were also included).  In 
phylogenetic taxonomy, having clades named after nomina dubia CAN lead to 
instability, but won't always.  For instance, if we defined Stegosauridae as 
"Stegosaurus armatus and all taxa more closely related to it than to 
Huayangosaurus taibaii", it would be perfectly fine.  Even if we can't 
distinguish armatus from stenops or laticeps, it's obviously not Wuerhosaurus, 
Kentrosaurus, Dacentrurus, etc..  So in this case, neither the Phylocode nor 
the ICZN suffers by having Stegosauridae named after an indeterminate species 
of Stegosaurus.   

> Designating _stenops_ as the new type species for _Stegosaurus_ seems an 
> entirely sensible suggestion, IMHO.

I don't think it's necessary.  With Iguanodon, the referred species did not 
necessarily form a clade with I. anglicus, which was in turn difficult to 
analyze due to its incompleteness.  So knowing if other species were really 
Iguanodon was difficult as was knowing which taxa were iguanodontids.  But with 
Stegosaurus, S. armatus is deeply nested in Stegosauridae and forms a clade 
with referred Stegosaurus species.  Since there's no rule about type species 
having to be diagnostic within their genera, I don't think the issue merits an 
ICZN ruling.

It's better than sinking Stegosaurus in favor of "Eustegosaurus stenops" or 
something at least.

Note too that Galton's opinion on Stegosaurus taxonomy isn't representative of 
all stegosaur experts, as Maidment (2008) found S. armatus to be a senior 
synonym of Diracodon laticeps, Hypsirophus discursus, Stegosaurus stenops and 
S. duplex, while S. longispinus and S. sulcatus were nomina dubia.  Which is a 
fairly good argument in itself not to base taxonomic rules on subjective 
concepts like diagnosability.

Mickey Mortimer