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Re: The Enigma of Psittacosaurus



On Tue, 9 Jul 2002 00:34:32   
 Mickey Mortimer wrote:
>Alessandro Marisa wrote-
>
>> The high number of denticles could be a character that linking P.
>xinjiangensis and P.
>> mazongshanensis.
>
>That was suggested by Xu (1997), who also grouped the two together based on
>"slender secondary ridges of tooth crowns".  I don't see any difference in
>secondary ridge width myself, but it would be correlated to denticle number
>in any case.  Brinkman et al. (2001), in their description of new P.
>xinjiangensis material, found that some P. xinjiangensis teeth have 9-11
>denticles, like other psittacosaurs.  Whether this is due to individual
>variation, or variation within the tooth row, is uncertain.  Either way, it
>makes the character "high number of denticles" problematic.

More evidence that shows Sereno and Coombs were correct in saying that 
characters relating to tooth morphology and, especially, tooth count are not 
wise to use in _Psittacosaurus_.

>Sereno has a habit of making taxa nomina dubia without much published
>analysis, though plenty of others nowadays do so too.  He did write his
>dissertation on Psittacosaurus, but still....

That's a bad habit to get into...but I suppose, as may be in Sereno's case, 
when you get to know a particular taxon so well, it is easier to dismiss 
potential new species of that taxon based on little comparison or study.  I 
mean, I know little about psittacosaurs so I am trying to be very careful in 
any conclusions I draw.  But, if I were the "world's expert" on them, I might 
not be as thorough simply because I already had such a wide knowledge.  At 
least I hope that's the case. :-)

>>    1. skull rectangular with long antorbital region (0); intermediate (1);
>> >rounded with short antorbital region (2).
>>
>> A potential valid character.  There certainly seems to be significant
>differences between both the shape of the skull and length of the snout in
>different species.  _P. meileyingensis_ has a rounded skull with a snout
>that makes up 27% of the skull length, and other species do show a
>rectangular and intermediate dichotomy, IMHO.
>
>Needs to be quantified.

Certainly does...but another tough thing to do via the literature.  If I get my 
hands on the description of every psittacosaur species, it might be easier.

>> >2. antorbital fossa very shallow or absent.
>>
>> Maybe a better way of doing this would be an ordered character, as
>Alessandro Marisa mentioned a few days ago.  Something like:
>> Antorbital fossa present (0), present as a shallow or "secondary"
>depression a la Sereno (1), or absent (2).  With outgroups that all possess
>antorbital fossae, its absence would definitely be a derived character.
>This is one of the few characters, IMHO, that doesn't suffer from an unknown
>polarity.
>
>True, but how shallow does it need to be to be 1 and not 0?  And could you
>tell from an illustration or photo?

The downfalls of trying to do any character analysis by relying solely on the 
literature...  The fact is that, as many on the list have asserted in the past, 
it is crucial to see the actual specimens in order to conduct a thorough study. 
 My literature analysis is only preliminary.

As for how shallow it has to be to be coded a 1 and not a 0, I (or anyone for 
that matter) would have to examine the skull of every species first and then 
draw up a conclusion based on the relative shallowness of the depression in 
each species.  That is the best way, I would assume.  It's still very much open 
to human interpretation...which shouldn't necessarily be the case in a good 
cladistic analysis.  

>> >3. ten or less maxillary and dentary teeth.
>>
>> Bad character, in my opinion.  The work of Coombs and Sereno has shown
>that tooth count is extremely variable throughout ontogeny.  It might have
>been best if they left this out, as there appears to be no set pattern in
>psittacosaur tooth count.
>
>The pattern seems to be P. mongoliensis has more than most other species, so
>is most basal.  And if you only used adult specimens, ontogenetic
>differences would not matter.

Ah, I should have been more clearer.  Not only are there strong ontogenic 
differences, but apparently the number of teeth varies between individuals of 
the same species.  With ontogenic and individual variation coupled together, 
characters relying on the number of teeth are not the best options.  What 
Mickey says might make _P. mongoliensis_ more basal, but tooth counts likely 
would have little significance in, say, trying to discern the relationship 
between _P. sinensis_ and _P. meileyingensis_.

>> >5. postorbital region broad
>>
>> Hmmm...another possible bad character.  The breadth (is that the word??)
>of the postorbital region might be functionally correlated with the size of
>the snout.  More work probably needs to be done on this.  Also, the paucity
>of material in some species makes this somewhat difficult to document.
>
>Good observation.

Well, I'll credit HP Headden for opening up my eyes regarding this.  There is 
at least a potential correlate between snout and postorbital breadth.

>> >6. horizontal ridge strongly developed on postorbital.
>>
>> Don't know enough about this character, but, based on illustrations, the
>ridge appears to be more strongly developed on _P. mongoliensis_ and _P.
>youngi_ than on _P. sinensis_ and _P. meileyingensis_.  But, this could be
>an artifact of either incomplete material or poor illustration...or both.
>
>Victim to the same problems as the antorbital fossa depth.

Certainly.  I would need to see original material to even start examining this 
character better.

>> >11. ventrolateral dentary ridge.
>>
>> Too hard to tell from illustrations.
>
>That's the "dentary flange" you refer to below.  Easy enough to see.

Okay, in that case, yes, it is very easy to see in illustrations.  The fact 
that Sereno and those onlist refer to it as a "flange" threw me off.  I don't 
know if "ridge" is the best word for it, though. :-)

>> And, for one final point to end this unbelievably long post, awhile back
>it was mentioned on list that
>> _Hypsilophodon_, _Iguanodon_, and _Stegoceras_ have coronoids, along with
>the neoceratopsians > _Protoceratops_ and _Montanaceratops_, while
>_Chaoyangsaurus_ and _Psittacosaurus_ lack them.  > How much potential does
>this have as a possible psittacosaurid + _Chaoyangsaurus_
>> synapomorphy.
>
>Speaking as someone who's seen oviraptorids coded as lacking coronoids until
>one was found in Citipati just last year, I think such a character is prone
>to preservational errors.

Thank you.  With that being said, what other synapomorphies might possibly link 
_Psittacosaurus_ and _Chaoyangsaurus_?

Alessandro wrote:

> I think that the usage of laterally projecting jugal horn is a generalized
   character for Psittacosaurus species, infact within Psittacosaurus the jugal
   horn is highly varibale, in most species the jugal horns point backward with
   the skull in dorsal view, nevertheless P. sinensis and P. xinjiangensis have
   jugal horn that point almost forward and the rostral edge is nearly
   perpendicular to the skull mid-line. P. meileyingensis has small jugal horn,
   while P. neimongoliensis and P. youngi have larger jugal horn directed
   backward. P. ordosensis has fairly large jugal horns also directed backward,
>P. mongoliensis has long jugal horns directed laterally.

Right.  I would say the presence of said jugal horn is a generalized character 
for _Psittacosaurus_.  It's the position and development of that horn that 
makes it one of the few, IMHO, solid potential synapomorphies that can be used 
to discern interrelationships.  _P. sinensis_ and _P. xinjiangensis_ do have 
the extended jugal horns that extend far to the side.  As far as I can tell 
from illustrations, _P. meileyingensis_ does have a small horn.  _P. youngi_ 
also appears to have a somewhat large jugal horn.  The horns of _P. 
mongoliensis_ also point to the side, although its lateral point is directed 
somewhat backward (more so than in _P. sinensis_).

Steve

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