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RE: Ceratopsine phylogeny questions



Tim Williams wrote:
 
> So "genera" such as _Brachyceratops_ and _Monoclonius_ are regarded as nomina 
> dubia
 
Hate to be a party crasher here, but Dodson (1992) showed that _Monoclonius_ 
was a vlid genus. _Brachyceratops_, however, is indeed a jeuvenile of some 
other centrosaurine, probably _Styracosaurus_.
 
Ref.
 
Dodson, Peter (1992). On the status of the ceratopsids _Monoclonius_ and 
_Centrosaurus_. In: Dinosaur systematics: Approaches and Perspectives. 
Cambridge University Press. pp. 231 - 245. ISBN 0521438100.
 

> On that basis, _A. ajax_ would be regarded as a nomen dubium. This would 
> render>_Brontosaurus_ a valid genus,

 
:)  :)  :)  :)  *Happy Happy Joy Joy*  :D  :D  :D  :D 

 
> which would make some people extremely happy.

 
Too late.

 

> Date: Tue, 6 Oct 2009 23:42:55 -0700
> From: tijawi@yahoo.com
> To: dinosaur@usc.edu
> CC: tijawi@yahoo.com
> Subject: Re: Ceratopsine phylogeny questions
> 
> 
> 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
> 
> 
> 
> 
>                                         
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