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Re: ANKYLOSAURS, STEGOSAURS, AND PSITTACOSAURS
On Wed, 3 Jul 2002 00:57:48
>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
>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
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
><<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 Brusatte-DINO LAND PALEONTOLOGY
ONLINE CLUB: http://clubs.yahoo.com/clubs/thedinolanddinosaurdigsite
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