Re: Ceratopsine phylogeny questions

[Tim Williams <tijawi@yahoo.com>]

Guy Leahy wrote (quoting J. Scannella's abstract):


> Triceratops has been considered an unusual chasmosaurine for possessing 
> a short, broad unfenestrated cranial frill, whereas Torosaurus has an
> expanded, fenestrated frill. A study of comparative cranial morphology 
> reveals that the major changes which occur throughout Triceratops 
> ontogeny continue beyond what was previously considered the adult 
> growth stage and result in the parietal-squamosal frill morphologies 
> which diagnose T. latus. Torosaurus actually represents the mature 
> adult morphology of Triceratops.


Well, that certainly explains why _Diceratops_... I mean, _Nedoceratops_ has a 
morphology intermediate between _Triceratops_ and _Torosaurus_.  It's a growth 
stage too, perhaps.  


If true, the already-low taxonomic diversity of Maastrichtian N.American 
dinosaurs has been pared back even further.


Mike Keesey wrote:

> It's perfectly permissible to have a type
> specimen at any ontogenetic stage. Lots of species have subadult or
> even juvenile type specimens.


For some dinosaur groups (e.g., ceratopsians, hadrosaurs) genera only acquire 
their diagnostic characters with maturity.  So "genera" such as 
_Brachyceratops_ and _Monoclonius_ are regarded as nomina dubia, because they 
represent immature growth stages of other centrosaurines.  This problem crops 
up in other dinosaur groups too - but it isn't usually as pronounced as in 
ceratopsians and hadrosaurs, where most (if not all) species-level diagnostic 
characters are bound up in the cranial morphology.


Interestingly, the type specimen for _Apatosaurus ajax__ is a juvenile.  
_Brontosaurus excelsus_ is based on an adult.  Although the two species are 
currently considered synonomous at genus level, Riggs (1903) thought that the 
_A. ajax_ type specimen lacked species-level diagnostic characters (his exact 
words were: "_Apatosaurus ajax_ is based upon a specimen too young to admit of 
specific determination").  On that basis, _A. ajax_ would be regarded as a 
nomen dubium.  This would render _Brontosaurus_ a valid genus, which would make 
some people extremely happy.


Cheers

Tim