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Bruce Danz wrote:
<< I have noticed wide variation in the degree of jugal protrusion in
 illustrations of Psittacosaurus skulls.  Does anyone have any
 information as to whether this reflects species differences, individual
 maturity, sexual dimorphism or some other trait? >>

Jugal horn variation is indeed one of the most important factors in diagnosing
the various psittacosaurid species.  

In most species of Psittacosaurus, the horns point somewhat backwards when the
skull is viewed in dorsal aspect.  Two species however differ in this respect:
P. sinensis and P. xinjiangensis have jugal horns that are swept somewhat
forward with the rostral edge nearly perpendicular to the skull's midline.

P. sinensis and P xinjiangensis differ in the fact that the horns point
horizontally in P sinensis and point downward in P xinjiangensis.

P meileyengensis has fairly small horns and is distinguished from other
psittacosaurs by the dentary flange (also seen in P mongoliensis and P
sattayaraki) and the very tall profile of the skull.  George Olshevsky informs
me that P meileyengensis also has a single premaxillary tooth in each premax
which is an autapomorphy of this species.

P neimongoliensis is very similar to P youngi because they both have very deep
distal dentaries and tall predentaries.  The jugal horns on both species are
fairly large and backswept.  P neimongoliensis differs from all otther
psittacosaurs in the fact that the maxilla invades the lacrimal and makes the
lacrimal look like an upsidedown V.

P ordosensis has fairly large backswept jugal horns, but since it is missing
most of its face, not much else can be learnt.

P mongoliensis has fairly long jugal horns as well, but unlike any other
psittacosaur species retains a prominent anteorbital fossa and fenestra and
has a relatively longer head than in any otther species.  Like P
meileyengensis and P sattayaraki, there is a prominent flange on the dentary.

Peter Buchholz

Buffetaut, E and Suteethorn.  1992.  A new species of the ornithischian
dinosaur _Psittacosaurus_ from the Early Cretaceous of Thailand.  Paleontology

Osborn, H F.  1923.  Two Lower Cretaceous dinosaurs of Mongolia.  American
Museum Novitates 95:1-10.

Osborn, H F.  1924.  _Psittacosaurus_ and _Protiguanodon_: two Lower
Cretaceous iguanodonts from Mongolia.  American Museum Novitates 127:1-16.

Russel, D A and Zhao, X-J.  1996.  New psittacosaur occurences in Inner
Mongolia.  Canadian Journal of Earth Sciences.  33:637-648.

Sereno, P C.  1990a.  New data on parrot-beaked dinosaurs (_Psittacosaurus_).
Pp 203-210 in: Carpenter, K and Currie, P J (eds).  Dinosaur Systematics:
Approaches and Perspectives.  Cambridge University Press.

Sereno, P C.  1990b.  Psittacosauridae.  Pp. 579-592 in: Weishampel, D B,
Dodson, P and Osmolska, H (eds).  The Dinosauria.  University of California
Press (Berkeley).

Sereno, P C and Chao Sh.  1988.  _Psittacosaurus xinjiangensis_ (Ornithischia:
Ceratopsia), a new psittacosaur from the Lower Cretaceous of Northwestern
China.  Journal of Vertebrate Paleontology 8(4):353-365.

Sereno, P C, Chao Sh, Cheng, Zh, and Rao Ch.  1988.  _Psittacosaurus
meileyengensis_ (Ornithischia: Ceratopsia), a new psittacosaur from the Lower
Cretaceous of Northeastern China.  Journal of Vertebrate Paleontology