Re: Triceratops and other Cha...

[Tom Holtz <tholtz@geochange.er.usgs.gov>]

>  Tom, in your answer about the triceratops and chasmosaurii you did not
>mention the relationship that monoclonius has with them.  I think it was Dr.
>Currie from Alberta that theorised at the SVP that monoclonius was merely the
>juvenile form of several different chasmosaurian types, (including
>pachyrhinosaurus, if I remember right).

Monoclonius is believed to be the subadult form of the **Centrosaurinae**,
not the Chasmosaurinae.  Even if it is an adult, it is not a chasmosaurine.

>    How do you feel about this possible relationship?  I believe the theory
>was the result of studying the mass bonebeds and the proportion of species at
>each site, combined with the represented ages of all individuals present, and
>that they found that the juveniles of a certain age appeared to be
>unrepresented in all the sites, yet monoclonius (which does not share
>features of the other species) seems to fit the size of the missing age
>group, and always appears where the age group of chasmosaurs, or styracosaurs
>would be missing.

This is pretty much the idea (except change chasmosaurs to centrosaurs).
The thought is that some previously named centrosaurine genera are actually
growth stages (juveniles and subadults) of other species.  It might be that
both ideas are true (i.e., that Monoclonius is a valid genus, but that
Centrosaurus, Styracosaurus, etc. pass through a Monoclonus-like stage
during their growth).

>   As you can see, I don't have the whole theory straight in my head, so
>further enlightenment would be great.....

To breifly distinguish the two major groups of ceratopsids:  Centrosaurines
are characterized by relatively short and deep (top-to-bottom) snouts,
large nose horns, small brow horns, and relatively short frills.  Typical
centrosaurines are Centrosaurus, Styracosaurus, and the pachyrhinosaurs
(Pachyrhinosaurus and two new genera to be officially named in the near
future).   Chamosaurines have relatively long and shallow (top-to-bottom)
snouts, small nose horns, large brow horns, and relatively long frills.
Typical chasmosaurines are Chasmosaurus, Torosaurus, and the
Triceratops-Diceratops clade (in which the frills become secondarily
shortened).

Hope that's enlightenment enough for now.

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.