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Centrosaurines




On Sun, 29 Oct 1995 RaptorRKC@aol.com wrote:


> Some groups of dinosaurs confuse me.  For example, the Styracosaurs,
> Eucentrosaurs, and Monoclonians.  Their skulls are all startingly similar --
> it appears to me that the horn and spike ornamentation is what keeps them
> separated.  I would like to be more informed on these dinosaurs and their
> status.  (Info would be appreciated -- but please give me the info directly,
> don't point me towards refs!!!)
> 
> 
> 
In giving this answer I am following Lehman's phylogeny of Centrosaurines 
(the short frilled ceratopids) in his chapter in the book "Dinosaur 
Systematics", so you might like to chase it up (I know you specifically 
requested no refs but finding primary literature is a good habit to get 
into). Monoclonius crassus, Eucentrosaurus apertus and Styracosaurus 
albertensis are very similar animals (I know nothing of S. ovatus but I 
assume it is the sister of S. albertensis). However Eucentrosaurus and 
Styracosaurus are more closely related to Pachyrhinosaurus (put another 
way "share a more recent common ancestor with") than either is 
to Monoclonius. Most modern systematists agree (but by no means all - see 
G.O. for dissent)  that classifications of 
animals should strive to consist  monophyletic (that is it should include 
ALL descendants of the common ancestor of the group in question). It is 
this sort of classification that leads to the placement of Aves within 
Dinosauria. Therefore a monophyletic genus that included Monoclonius, 
Eucentrosaurus and Styracosaurus would have to include 
Pachyrhinosaurus as well, wich most would judge is simply too different for 
inclusion with the others. However as Eucentrosaurus and 
Styracosaurus share a more recent common ancestor with each other 
than either does with Pachyrhinosaurus (ie. they are sister genera), they  
could be united ("lumped") into a monophyletic genus. Lehman 
decided not to take this step so that apertus, crassus and 
albertensis could be discussed as sepparate entities. Personally 
I say lump 'em.
It is worth mentioning why the three genera should appear so 
similar while two of them are closer to a very differant genus. 
It is because the characters that unite them are primitive 
(pleisiomorphic, ancestral, whatever) at the level of advanced 
centrosaurines. That is they were inherited from the common 
ancestor of the whole mess. Styracosaurus, Eucentrosaurus and 
Pachyrhinosaurus share derived (apomorphic, advanced) 
characters, such as the possesion of parietal hooks at the 
back of their frills, that suggest they are more closely 
related to each other than they are to Monoclonius.

Hope that was remotely understandable

Adam Yates