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Re: ANKYLOSAURS, STEGOSAURS, AND PSITTACOSAURS



On Wed, 3 Jul 2002 00:57:48   
 Tetanurae wrote:
>Steve Brusatte wrote:
><<I'm currently trying to use the literature to discern any possible 
>characters that might be able to divide _Psittacosaurus_ into two or more 
>genera, as Jaime Headden and Pete Buchholz have both discussed onlist before.>
>>
>What's a genus?

Damned if I know...but it still seems as if we are using them.

><<First, there seems to be some confusion among different authors (okay, 
>basically among the exact same author: Paul Sereno) about the presence of 
>antorbital fossae in _Psittacosaurus_.>>
>
>You are correct.  Sereno illustrates the AOF as being present and also not 
>being present in P mongoliensis, and no photos I've seen show the area with 
>enough clarity to really discern if it's there or not.  What is interesting 
>is that no other Psittacosaurus species have an AOF present, and that basal 
>neoceratopians retain it.

Sereno seems to rectify himself in _The Age of Dinosaurs in Russia and 
Mongolia_.  Apparently his earlier description of the maxillary depression of 
_Psittacosaurus mongoliensis_ as an antorbital fossa was incorrect, and it is 
instead a neomorphic secondary depression unrelated to the sinus system.  

And, yes, pachycephalosaurs possess the AOF, as do basal neoceratopsians.  I 
assume that _Chaoyangsaurus_ and _Liaoceratops_ have this feature?  Its closure 
seems to be a synapomorphy that unites every known psittacosaurid with no 
exception.

>Psittacosaurus mongoliensis seems to share one character with P 
>meileyengensis, namely the deep flange on the dentary.  This is apparently 
>also present in Liaceratops, so it may in fact be plesiomorphic for the 
>clade.

This flange is best developed in _P. meileyengensis_, but is definitely also 
present in _P. mongoliensis_.  What's interesting is that _P. sinensis_ also 
appears to have a very small flange (personal observation, based on 
illustrations), but this could be an artifact of poor illustration.  _P. 
youngi_ has a very flat ventral portion of the dentary, which is one feature 
that, although not necessarily diagnostic, might separate it from _P. 
sinensis_.  _P. mongoliensis_ and _P. meileyengensis_ also share a unique 
articulation between the lacrimal, jugal, premaxilla, and maxilla, but the 
polarity of this character is very difficult to evaulate.

By the way, didn't Makovicky mention possible psittacosaurid parallelisms in 
_Liaoceratops_?  Perhaps this could account for its ventral dentary flange, and 
may also mean that this flange is truly a synapomorphy of _P. mongoliensis_ and 
_P. meileyengensis_.

><<I had once read (maybe onlist) that seven species were recognized.  Sereno 
>sees these as valid: _P. mongoliensis_, _P. sinensis_, _P. neimongoliensis_, 
>_P. ordosensis_, _P. xinjiangensis_, and _P. meileyingensis_.  Am I missing 
>any valid species?  Of coures, some of the others (_P. osborni_, _P. tingi_, 
>and _P. guyangensis_ ) might be valid, but likely not.>>
>
>P youngi is probably valid.  Sereno's referal of this species to P sinensis 
>is a little surprising as P youngi doesn't have the extremely large jugal 
>horns that point horizontally and are perpendicular to the sagital plane of 
>the skull, the two characters that make P sinensis so unique.  

Well, I think, for some reason or another, Sereno implied the presence of said 
jugal horn in _P. youngi_.  I'll have to continue combing the literature to 
find out why.  Perhaps there is some unpublished or unprepared material that 
Sereno saw during his visit to China in the mid-80's??  I believe this jugal 
horn is also present in _P. xinjiangensis_, which might imply a distinct clade.

>P youngi also 
>has a very deep rostral dentary and a square-shaped lower jaw tip, instead of 
>the gradually tapering rostral dentary and triangular lower jaw tip seen in P 
>sinensis and almost all other Psittacosaurus specimens.

The lower jaw tip of _P. youngi_ is, IMHO, only slightly different than that of 
_P. sinensis_, but it does appear to be more square.  _P. youngi_ also appears 
to have a dentary that contributes more significantly to the lower jaw than _P. 
sinensis_ and other species, although this character might be superficial.

>The only species that shows a similar mandibular morphology is P 
>neimongoliensis.  P youngi may in fact be a senior synonym of P 
>neimongoliensis, but more work needs to be done to discern this for sure.

Sereno, in _The Age of Dinosaurs in Russia and Mongolia_, states that _P. 
neimongoliensis_ appears to be very close to _P. mongoliensis_, but I haven't 
examined _P. neimongoliensis_ in enough detail to assess this claim yet.

Any additional insights appreciated!  Feel free to jump in (especially Jaime, 
who I know was interested in this very subject a few months ago).

Steve

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Steve Brusatte-DINO LAND PALEONTOLOGY
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