Re: Diceratops and Torosaurus (was: Aquatic Dinosaurs)

[Stan Friesen <swf@ElSegundoCA.NCR.COM>]

From: "Tim Williams" <T.Williams@cclru.randwick.unsw.edu.au>
 >  From Stan Friesen
 > 
 > > This produces a fairly interesting chain:
 > >     basal Torosaurus => Diceratops => T. horridus => T. prorsus
 > 
 > Except that Torosaurus has an extraordinarily long frill, and 
 > Diceratops and Triceratops both have short frills.  Is it possible 
 > that Torosaurus and Diceratops both evolved from a common ancestor 
 > which had a medium-length frill - Arrhinoceratops maybe.

It is a fairly complex issue.  Diceratops shares a short frill with
Triceratops and a couple of other skull apomorphies with Torosaurus.

Either there is a reversal in there somewhere, or at least one feature
evolved twice. By raw count (strict parsimony) the choice would be
two origins of a short frill - which would make Diceratops derived
from Torosaurus directly.  However, with just three characters involved
I do not think parsimony is very useful.  Besides I suspect the "two"
Diceratops-Torosaurus shared features may be functionally linked, and
thus constitute only one "effective" character" (they are certainly
similar in nature). This changes the parsimony count to a three-way tie.

Under the circumstances I am more inclined to believe reversal of the
minor Torosaurus features than multiple origins of a short frill in
an otherwise medium-to-long frilled group like the chasmosaurines.

swf@elsegundoca.ncr.com         sarima@ix.netcom.com

The peace of God be with you.