Re: Sequential Centrosaurines, Batman!

["Tim Williams" <twilliams_alpha@hotmail.com>]

Andy Farke wrote:

>    SAMPSON, S. D., M. J. RYAN, AND D. H. TANKE. 1997. Craniofacial 
> ontogeny in centrosaurine dinosaurs (Ornithischia: Ceratopsidae): taxonomic 
> and behavioral implications. Zoological Journal of the Linnean Society, 
> 121:293-337.

This is the paper I had in mind.  Even _Monoclonius lowei_, despite its 
relatively large size, appears to be based on an immature specimen; and the 
frill characters use to diagnose this species are immature centrosaurine 
characters.  Ceratopids apparently 'bloomed' into adulthood, and the 
characters used to distinguish closely related species do not appear until 
late in ontogeny.  The same thing appears to be true for hadrosaurids.

I know opinion is not unanimous on this point.  Dodson, in his book _The 
Horned Dinosaurs_, was willing to hold on to _Brachyceratops montanensis_ 
and _Monoclonius lowei_ as valid species.

>   Of course, this is not to say that juveniles can't have features 
diagnostic
> to species (e.g., Nanotyrannosaurus/Tyrannosaurus), but just that 
> ceratopsian juveniles often don't.

_Avaceratops_ is based on what appears to be an immature specimen - but the 
distinguishing characters are almost certainly diagnostic for the genus.

Jaime Headden wrote:

>    Skulls referred to *Monoclonius* include the species *M. lowei* and *M. 
> dawsoni,* the former which is distinct from both *Centrosaurus* and 
> *Styracosaurus,* as well as other centrosaurines, based on its frill 
> morphology, and indicate they are possible unique species, even if they may 
> not validate the genus they were placed it.

The type species of _Monoclonius_ is non-diagnostic (and I think originally 
based on a mixture of ceratopid and hadrosaurid material).  As I said above, 
Sampson and co. present a case for regarding the diagnostic characters of 
_M. lowei_ (which is based on a nice skull) as immature centrosaurine 
characters.


Tim

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