[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

More on Psittacosaurus



On Fri, 5 Jul 2002 13:52:39   
 Alessandro Marisa wrote:
>----- Original Message -----
>From: "Steve Brusatte" <dinoland@lycos.com>
>To: <dinosaur@usc.edu>
>Sent: Thursday, July 04, 2002 9:15 PM
>Subject: Re: ANKYLOSAURS, STEGOSAURS, AND PSITTACOSAURS

>> 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?
>
>Well, the condition of the absence and/or presence of an antorbital
>fossa/depression in other Psittacosaurus species is as follows: the
>depression seem to be present in P. mongoliensis, P. meileyingensis, P.
>xinjiangensis, P. sinensis, and P. youngi, but are totally absent in P.
>neimongoliensis and P. ordosensis. It is interesting to note that this
>depression is well defined only in P. mongoliensis, while in P.
>meileyingensis and P. xinjiangensis is shallow, and in P. sinensis and P.
>youngi is extremely weak, 

Interesting.  From the illustrations I have seen of _P. sinensis_ and _P. 
youngi_ I cannot discern any depression, but that makes since if it is very 
weak.  

>the condition present in P. meileyingensis and P.
>xinjiangensis seem to be intermediate between that in P. mongoliensis and
>that in P. sinensis/P.youngi. If we accept the fact that this depression
>represent a modified antorbital fossa we have a line that start with the
>presence of an antorbital fossa (the condition in Neoceratopia) and stopped
>to the totally absence of it, and if we accept the suggestion of HP Pete
>Buchhoz that P. mongoliensis is the basal form we can conclude that the
>presence of an antorbital fossa is the primitive condition within the
>Ceratopsia

I like this idea, but I haven't seen enough skeletal material yet to claim it 
is completely correct. However, I do find it odd that psittacosaurs completely 
lost their antorbital fossa and at the same time developed this "neomorphic 
depression," while other neoceratopsians, pachycephalosaurs, and more basal 
outgroups do possess said fossa.  Perhaps Sereno is wrong, and maybe this 
secondary depression does indeed represent a modified antorbital fossa. It 
seems logical, but who knows?

>> 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)
>
>I use this in my Neoceratopsian analysis, but for relationships of
>Psittacosaurus species I use this one:
>1a) Skull profile rectangular with a long preorbital region; intermediate;
>rounded with short preorbital region.

I'll have to look at this better.  _P. meileyingensis_ certainly has a very 
well rounded skull.  How do you code the other species?

>And also:
>14) lateral margin of prefrontal not upturned; strongly upturned
>15) postorbital region narrow; broad

Seems to relate to the snout length, but might be an entirely separate 
character.  It might be best to use only one of these (preorbital length or 
postorbital length), though.

>> There might be a few more, but these are the basics.  The postcrania,
>>being extremely primitive, offers little information of diagnostic value.
>
>In regard to the postcranial you can added:
>19) length of metatarsal I, 60% that of metatarsal III; 70% that of
>metatarsal III.
>20) tibia shorter than the skull; equal/longer than the skull.
>21) Ischium shorter than the femur; longer than the femur
>22) Distal end of ischium distally flattened absent; present.
>Some of this postcranial characters are from personal comment to HP Rutger
>Jansma.

Hmmm...it's interesting that I've seen very few postcranial diagnostic 
characters mentioned in print.  Perhaps some of these are plesiomorphies...or 
maybe you're onto something.  I'll have to look at these better, also.

>> That is if we recognize _P. sattayarki_ as valid.  Sereno seems to think
>>not.  I have no opinion yet, as I have not yet read its description.
>>However, if indeed the type specimen comes from two different individuals,
>>and if indeed the dentary it fragmented, broken, and worn, then it is very
>>difficult to place this fossil in its own species.
>
>The main point and much probably the much important in the resolution of the
>validity of  P. sattayaraki is to understand if the material assigned to P.
>sattayaraki belong to a fully-grown or juvenile individual. A study in
>the ontogenetic changes in Psittacosaurus have been documented by Coombs
>(1982) on the basis of juvenile specimen (AMNH 6535 and AMNH 6536) of
>Psittacosaurus mongoliensis, but unfortunately no juvenile features are
>listed for the dentary, which not seem to have very notable morphological
>changes during growth. The dentary assigned to P. sattayaraki is twice the
>size of that of P. mongoliensis (AMNH 6536) and three times the size of AMNH
>6535, it's 2/3 the size of the type of P. mongoliensis and about the same
>size of the type of P. youngi and P. meileyingensis; this seem to prove that
>the type-dentary of P. sattayaraki do not belong to a particularly young
>individual. 

I would agree, but I'm not an expert and I haven't had the chance to examine 
_P. sattayaraki_ in detail yet.

>Interesting is the fact that in the very small skull of P.
>mongoliensis from Mongolia the dentary and predentary are fused together,
>which is not the condition in P. sattayaraki, because the predentary is
>separated from the dentary at the level of the unfused sutural region. 

Could this related to the supposed fragmentary nature of the dentary?  Sereno 
also mentioned possible erosive wear that might have damaged its articular 
surface with the predentary.

>The
>number of dentary teeth may also has some relevance in the question of the
>individual age of P. sattayaraki because the number increased during growth
>in Psittacosaurus as pointed out by Sereno (1990a 1990b). 

It most certainly did...but tooth number seems so variable that any character 
using the position or number of teeth is not well founded.  But, in _P. 
sattayaraki the number might very well elucidate the specimen's age.

>In this view appears that P. sattayaraki differs from the other
>Psittacosaurus species by at least one or two characters of the dentary,
>that much probably seem justified its separation at the specie-level. And in
>view of the fact that the dentary flange is absent in Neoceratopsia...

Maybe...but a flange seems to be present in _Liaoceratops_, but not in 
_Chaoyangsaurus_.  If _Chaoyangsaurus_ is indeed basal to the 
psittacosaurid-neoceratopsian split, then said flange might be a synapomorphy 
of Psittacosauridae + Neoceratopsia.  The absence of a flange in certain 
_Psittacosaurus_ species then might simply be a reversal to the more primitive 
state.  So, the mere presence of a flange might not be a synapomorphy of 
certain possible  _Psittacosaurus_ clades, but perhaps the extent of 
development of the flange is.

>much
>probably the presence and the great development of this flange is a derived
>condition,

Well, maybe not the presence, but certainly the great development seen in _P. 
mongoliensis_ and _P. meileyingensis_ is a potential synapomorphy.

>in view of this P. sattayaraki is more primitive than P.
>meyleingensis and P. mongoliensis, because has a less developed flange, but
>is more derived than P. sinensis, P. youngi and P. xinjiangensis because
>this three species do not have the flange.

Maybe...but as stated above, _P. sinensis_, _P. youngi_, and _P. xinjiangensis_ 
 might have simply reversed to a more primitive state.  _P. mongoliensis_ and 
_P. meileyingensis_ might be united by the well developed flange, which seems 
to be much more developed than in _Liaoceratops_.  In this sense, this does not 
contradict the possible placement of _P. mongoliensis_ and _P. meileyingensis_ 
as basal forms, as you might have been implying.

Also, one final note, I know I asked this a few days ago, but I'm still a bit 
confused over the current consensus: is _Chaoyangsaurus_ more likely a basal 
neoceratopsian (a la Sereno), or basal to the neoceratopsian-psittacosaurid 
split (a la Makovicky).  Or, might it be a psittacosaurid?  To me it seems more 
basal to the neoceratopsian-psittacosaurid split, as I am not too fond of a few 
of the characters Sereno used to place it as a neoceratopsian (mostly related 
to teeth).  But, I'm curious as to the list's opinion. :-)

Steve

---
***************************************************************
Steve Brusatte-DINO LAND PALEONTOLOGY
SITE: http://www.geocities.com/stegob
ONLINE CLUB: http://clubs.yahoo.com/clubs/thedinolanddinosaurdigsite
WEBRING: http://www.geocities.com/stegob/dlwr.html
INTERNATIONAL LANGUAGE SITE: http://www.geocities.com/stegob/international.html
****************************************************************


_____________________________________________________
Supercharge your e-mail with a 25MB Inbox, POP3 Access, No Ads
and NoTaglines --> LYCOS MAIL PLUS.
http://www.mail.lycos.com/brandPage.shtml?pageId=plus