Re: _Avaceratops_

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

Allan Edels wrote:

>  Having seen _Avaceratops_ many, many times over the last 15+ years > (and 
> knowing Peter Dodson) - I do know that the classification of
> _Avaceratops_ as a juvenile should more probably be refined as a  
> sub-adult.  Based on the occipital condyle being nearly completed fused, 
> this indicates that the animal was nearly fully mature, and > as such its 
> features are more likely to
> be very close to those of the adult form.  Its size is probably 80% > of 
> full-grown.

_Avaceratops_ just happens to be one of my favorite dinosaurs!  :-)

There is no doubt that _Avaceratops_ is a valid genus.  The shape of the 
squamosal is unique among ceratopsids - squarish, almost "bow-tie" shaped, 
very different to the curved, scythe-like squamosals of chasmosaurines and 
the "stepped" squamosals of centrosaurines.  There are a number of primitive 
features seen in _Avaceratops_ which distinguish it from 
centrosaurines+chasmosaurines, such as the shape of the pedal unguals 
(intermediate between claw-like and hoof-like) and the apparently solid 
frill.

The holotype (ANSP 15800) is a half-complete specimen, about 2.8m long 
according to Penkalski and Dodson (1999).  They suggest the holotype is 
immature (unfused skull elements, striated surface bone texture of skull).  
The argument centers around HOW immature the type specimen is.

A referred skull (MOR 692) is a partial skull found around 125 km NE from 
the holotype.  (Unfortunately the squamosal of MOR 692 is too shattered to 
reconstruct the shape, but the distinct shape of the premaxilla, the solid 
frill, and the presence of certain "bumps" on the squamosal prompted 
Penkalski and Dodson (1999) to provisionally refer MOR 692 to 
_Avaceratops_.)   MOR 692 shows postorbital horn cores (actually rather 
similar to those of the _Ceratops_ type specimen (USNM 2411), found around 
160 km north of the _Avaceratops_ type locality).  MOR 692 is an adult 
specimen (bone texture, fusion) and comes from an animal around 4.2m long 
(Penkalski and Dodson, 1999).

The holotype is then around two-thirds the size of MOR 692.  But (as 
suggested in the post above) the degree of fusion of the occipital condyle 
in the holotype suggests that it should be closer to fully-grown than this 
(?80%).

However, a few things which I think should be taken into consideration.
(1) Ceratopsids had very heavy heads - might the occipital condyle fuse a 
little earlier in ontogeny (compared to, say, other elements of the skull)?
(2) Same-age individuals may vary in size a great deal within any given 
dinosaur species (or populations).  Sampson et al. (1997) regard one very 
large centrosaurine skull (NMC 8790, type of "_Monoclonius lowei_") as 
subadult (probably _Centrosaurus_), even though it represents one of the 
largest known centrosaurine skulls, and larger than the skull of any known 
fully-grown Centrosaurus.  No big deal.  Just good ol' intraspecific 
variation, say Sampson et al.


Tim

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