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Re: Psittacosaurus queries



On Wed, 3 Jul 2002 23:33:31   
 Alessandro Marisa wrote:
>----- Original Message -----
>From: "Steve Brusatte" <dinoland@lycos.com>
>To: <dinosaur@usc.edu>
>Sent: Wednesday, July 03, 2002 4:10 AM
>Subject: Psittacosaurus queries

>After viewing some illustration of the skulls of various Psittacosaurus
>species, I think that the best way is to using the term of P. Dodson  (The
>Horned Dinosaurs), infact in my preliminary analysis of basal Neoceratopsia
>instead of use only Psittacosaurus mongoliensis, I've the intention to use
>all the Psittacosaurus species (with the exception of P. sattayaraki) and
>about the condition of the antorbital fossa I using the follow character:
>Antorbital fossa:
>present (0):
>present as a shallow depression or foramen (1):
>absent (2):
>
>> I am fairly sure that _Psittacosaurus_ does not possess a true antorbital
>fossa (or fenestra), and that this is >diagnostic of the genus.  But, am I
>wrong here?
>
>Yes, the presence of a shallow depression or a foramen instead of a true
>antorbital fossa is diagnostic of the genus Psittacosaurus, but this is only
>my point of view.

Interesting ideas.  In _The Dinosauria_, Sereno lists both the absence of an 
antorbital fossa and the absence of an antorbital fenestra as diagnostic of the 
Psittacosauridae.  AFAIK, only _Psittacosaurus mongoliensis_ possesses the 
enigmatic depression on the maxilla, but Sereno wrote that "several" species 
possess it.  In fact, this depression is listed as a separate diagnostic 
character for _P. mongoliensis_, but Sereno's statement has me confused.  What 
other species might possess this feature?

Saying that _Psittacosaurus_ can be diagnosed by the presence of a shallow 
depression or a foramen might be pushing it.  However, your idea that this 
maxillary depression does represent a modified antorbital fossa is intriguing.  
An ordered character state such as the one you mention above might show the 
presence of a shallow antorbital fossa/depression or the absence of said 
depression as a Psittacosaurid autapomorphy, and also place _P. mongoliensis_ 
as a basal form.  Interesting...much in line with what Pete Buchholz suggested 
a few years ago onlist (_P. mongoliensis_ as a basal species).

>>And, if scans are impossible or difficult to send, if someone would list
>any diagnostic characters or other >interesting osteological features
>(especially those regarding the premaxilla-maxilla-lacrimal-jugal joint,
>maxillary >secondary depression, lacrimal foramen, and dentary) it would
>also be appreciated.
>
>In my analysis I use also the premaxilla-maxilla-lacrimal-jugal that
>converge to a point anteroventral to orbit as a diagnostic features of the
>genus Psittacosaurus, I use also this characters to separate some species of
>Psittacosaurus, 

This converging joint certainly is a diagnostic feature of the 
Psittacosauridae, and there certainly are at least two major forms of it (which 
I have also recognized, and I'm glad that you've done the same thing 
independently).  The major problem with this character, though, is polarity.  
None of the psittacosaurid outgroups possess it, so it is difficult to 
determine what is primitive and what is derived.  However, it still might be a 
character that, if other characters agree, could separate a new genus.

>infact the premaxilla-jugal contact is separate by the
>narrow maxilla-lacrimal contact in P. mongoliensis, P. neimongoliensis, and
>P. meileyingensis, while in P. sinensis, P. youngi, and perhaps in  P.
>xinjiangensis the premaxilla-jugal contact separate the maxilla-lacrimal
>contact. 

Right.  What data do you have for _P. xinjiangensis_?  I'm unfamilar with the 
sutural form in this species.

>Moreover, in all the Psittacosaurus species the rostral bone
>contact the slender rostroventral processes of the nasal, a features absent
>in all the other Neoceratopsia, perhaps this is another diagnostic features
>of the genus.

It most certainly is.  This is a list of diagnostic features of the 
Psittacosauridae which I have compiled from the literature, mostly from 
Sereno's chapter in _The Dinosauria_:

1) Short preorbital skull segment (less than 40% of skull length)
2) external naris positioned very high on the snout, with the ventral margin of 
the external naris lying dorsal to the ventral margin of the orbit
3) nasal extending rostroventrally below the external naris and esbablishing 
contact with the rostral via the rostroventral process
4) caudolateral premaxillary process broad, separating the maxilla from the 
external naris
5) premaxilla, maxilla, lacrimal, and jugal convering to a point
6) antorbital fenestra absent
7) antorbital fossa absent (maybe not so clear now...)
8) unossified gap in the wall of the lacrimal canal (possibly not seen in all 
species)
9) eminence on the rim of the buccal emargination of the maxilla near the 
junction with the jugal
10) elongate pterygoid mandibular ramus (I don't know too much about this 
character)
11) dentary crown with bulbous primary ridge
12) manual digit IV with only one simplified phalanx
13) manual digit V absent

There might be a few more, but these are the basics.  The postcrania, being 
extremely primitive, offers little information of diagnostic value.

Me:
> And, yes, pachycephalosaurs possess the AOF, as do basal neoceratopsians.
>I assume that _Chaoyangsaurus_ and _Liaoceratops_ have this feature?

>Yes Liaoceratops posses the AOF as do Archaeoceratops and Asiaceratops,
 >unfortunately the condition in Chaoyangsaurus is unknown.

Hmmm...I really wish that the feature was preserved in _Chaoyangsaurus_!!  Now 
the character is much more difficult to evaluate, although the absence or 
reduction of the AOF is likely a derived character.  The degree of reduction 
might even be a potential synapomorphy that links various _Psittacosaurus_ 
species.  What is the max/premax/lac/jugal area like in _Chaoyangsaurus_?  Is 
it preserved?  I really wish I had that paper...  

Also, what is the current consensus on the position of _Chaoyangsaurus_?  
Sereno places it as a neoceratopsian based on something like eight or nine 
characters, but a few of those are based on the teeth.  Because teeth number, 
position, morphology, etc. are so variable in _Psittacosaurus_, these 
characters are weakened.  However, Makovicky listed something like three 
characters that placed _Chaoyangsaurus_ outside the 
Neoceratopsian-_Psittacosaurus_ clade.  One of these regards the jugal horns, 
which due to some possible convergence might not be accurate.  Where was this 
genus placed in the _Liaoceratops_ paper??

Steve

---
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