Avaceratops and Ceratops (was Re: THE NEW ISH OF JVP)

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

Pete Buchholz wrote:

> Speaking of new cladograms....  Penkalski and Dodson describe what appears 
> to
> be a new skull of _Avaceratops_ and use info from both specimens to 
> generate
> a cladogram that places _Avaceratops_ outside of Chasmosaurinae +
> Centrosaurinae.  They also move the Ceratopidae down to the node joining 
> all
> three :-P instead of ={Chasmosaurus + Centrosaurus} like everyone else has 
> it

This new skull comes from an adult ceratopsid which shows a pair of orbital 
horns.  It's referred to _Avaceratops lammersi_ (whose holotype is an 
incomplete subadult skeleton) mostly on the basis of features in the frill.  
The referral of this second skull to _Avaceratops_ is only provisional, 
though.

Penkalski and Dodson's paper contains a few interesting tidbits.
(1) The orbital horns of this new skull show a striking resemblence to the 
horn cores of the type of _Ceratops montanus_, in their shape and 
orientation (directed outwards at quite a large angle).  Could _Avaceratops_ 
be a juvenile _Ceratops_?  (For those who believe that _Ceratops_ is a 
defunct genus, read on...)

(2) Even if this new skull does not belong to _Avaceratops_ it belongs to a 
closely related genus, probably a basal ceratopsid like _Avaceratops_.  
Maybe _Ceratops_?  (_Ceratops_ is considered a nomen dubium by Penkalski and 
Dodson, but I've heard of good material from the "Milk River" site that may 
pertain to this genus.  Anybody know about this?)

(3) In another paper in the same issue of JVP, Sereno calls the sister-group 
to the Centrosaurinae Ceratopsinae rather than Chasmosaurinae.  Now, Sereno 
is a stickler for nomenclatural correctness (e.g. he abandoned the name 
Titanosauridae in favor of Saltasauridae because the _Titanosaurus_ type 
material is probably non-diagnostic).  Sereno must believe that _Ceratops_ 
must be a good genus to name a higher-level taxon after it (especially since 
almost everyone else, George Olshevsky excepted, calls this clade the 
Chasmosaurinae).  At any rate, there's no good evidence that _Ceratops_ is a 
chasmosaurine (see (1) and (2)).


On a completely separate topic, a while back somebody mentioned that the new 
ornithomimosaur from Thailand does NOT have an arctometatarsal pes.  It 
does.  Buffetaut and Suteethorn's preliminary descriptions and figures spell 
that out.

Tim


References

Penkalski, P and Dodson, P.  1999.  The morphology and systematics of
_Avaceratops_, a primitve horned dinosaur from the Judith River Formation 
(Late Campanian) of Montana, with the description of a second skull.  JVP 
19(4):692-711.

Sereno, P C.  1999.  A rationale for dinosaurian taxonomy.  JVP 
19(4):788-790.

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