Re: The Enigma of Psittacosaurus

["Steve Brusatte" <dinoland@lycos.com>]

On Fri, 12 Jul 2002 19:32:49  
 Alessandro Marisa wrote:
>And what about the partecipation of the frontal in the supratemporal fossa?
>In P. sinensis, and P. youngi the partecipation of the frontal in the
>formation of supratemporal fossa is present, while in P. mongoliensis, P.
>neimongoliensis, P. meileyingensis the frontal don't partecipate, in
>outgroups as Neoceratopsian the condition is that the partecipation is
>present, this is the case also in the very primitive Neoceratopsian
>Liaoceratops yanzigouensis, and Archaeoceratops oshimai, and also in
>Lesothosaurus diagnosticus (used by me as outgroups), I've used
>Archaeoceratops because the condition in Chaoyangsaurus youngi is unknown,
>unfortunately the condition is unknown also in P. xinjiangensis and P.
>mazongshanensis.

Interesting character.  Thanks for providing it.  I haven't really thought 
about it, mostly because I only have illustrations that show the supratemporal 
fossae of _P. sinensis_ and _P. mongoliensis_.  But, yes, the condition is 
different in both.  A posterior ramus of the frontal (I don't know if a term 
for this already exists, but I'm inventing it here) extends to abut  the 
anterolateral edge of the supratemporal fossa in _P. sinensis_, while the 
frontals stop short of the fossa in _P. mongoliensis_.  In fact, the frontals 
of _P. mongoliensis_ do not have the posterior ramus.  I wonder if some of the 
forms lacking the frontal participation do have this ramus?  If so, this ramus 
might also be a potential synapomorphy.  Or, maybe the ramus is primitive.  Is 
this ramus present in your outgroups.  (BTW, I also use _Hypsilophodon_ as an 
outgroup.  The condition appears to be more similar to _P. mongoliensis_, 
therefore complicating the character...hmm...).

This is another potential synapomorphy that links _P. mongoliensis_, _P. 
meileyingensis_, and _P. neimongoliensis_.  It's very, very difficult to 
perform a cladistic analysis of every _Psittacosaurus_ species, but it is 
becoming clear that these three genera might very well be linked.

In a few days I hope to post my thoughts on the position of _Chaoyangsaurus_.  
I've examined both Makovicky and Sereno's characters, and I have a few 
thoughts.  Basically, I have quite a few qualms with some of Sereno's 
characters.  But, that's for another time.

Steve

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