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RE: Ceratopsine phylogeny questions
Tim Williams wrote:
> So "genera" such as _Brachyceratops_ and _Monoclonius_ are regarded as nomina
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_.
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.
> Date: Tue, 6 Oct 2009 23:42:55 -0700
> From: firstname.lastname@example.org
> To: email@example.com
> CC: firstname.lastname@example.org
> 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.
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