RE: dracrorex and National Geographic

[Tim Williams <twilliams_alpha@hotmail.com>]

Denver Fowler wrote:

> Well.. the nodes are identical in arrangement, and dracorex, stygimoloch, and 
> pachycephalosaurus are all most likely from the same stratigraphic interval. 
> Dracorex and 
> stygimoloch are both non-mature.. which raises the question as to whether you 
> can erect a taxon 
> based on immature specimens. What happens if a taxon is first defined on a 
> juvenile, then an 
> adult morph is found, with quite different morphology... do you change the 
> holotype to the adult 
> but keep the juvenile name; or do you sink the juvenile (yet senior) name 
> into the adult name.

In this case, if the immature specimen is diagnostic at the species or genus 
level (and the two species can therefore be *demonstrated* to be conspecific or 
congeneric) then its name gets priority over the adult.  This is what happened 
with _Apatosaurus_ vs _Brontosaurus_; the former has a subadult individual as 
its holotype, but this didn't prevent _Apatosaurus_ from being awarded priority 
over _Brontosaurus_.

On the other hand, some genera and species that are based on juvenile or 
subadult material cannot be diagnosed at the species or genus-level, and 
therefore become nomina dubia.  This happened with certain ceratopsian and 
hadrosaur genera: the immature specimens lack diagnostic characters that can be 
used to tie them to taxa based on adult/diagnostic material.

>  I guess an example would be if stygimoloch was described before 
> pachycephalosaurus. If stygi 
> turned out to be pachy, which name gets priority; the first named, or the 
> adult morph.

Thankfully this is not the case (_Pachycephalosaurus_ was named first); but if 
it was, then it would depend upon whether _Stygimoloch_ and 
_Pachycephalosaurus_ shared the same species- or genus-level diagnostic 
characters that could demonstrate that the two are conspecific or congeneric.  
For establishing synonymy at the species level, a large sample would certainly 
help (like the _Triceratops_ example mentioned by Mike Taylor).  Uniting the 
two at the genus level, however, is more a matter of personal taste.

Mike Taylor wrote:

> As I recall that talk, it made it pretty convincing case that
> Dracrorex and Stygiomoloch are juveniles, but didn't lay out case that
> what they're juveniles _of_ is Pachycephalosaurus.

Sounds similar to the _Nanotyrannus_/_Tyrannosaurus_ story.  _Nanotyrannus_ is 
certainly immature - but is it necessarily a young _T. rex_?

Cheers

Tim


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