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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.