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Re: _Avaceratops_

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.


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