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BASAL ORNITHISCHIAN SYSTEMATICS
Tony Thulborn wrote:
Actually, upon examination of the figures in Thulborn 1972 and Sereno 1991
(which illustrate the ischium the same [correct] way), the process which is
assumed to be the obtuartor in Thulborn 1972, actually bares little, if any
resemblance to the obturator processes in any true ornithischians, even very
basal ones such as _Agilisaurus_.
In basal true ornithopods such as _Thescelosaurus_ (Galton 1974b), _Othnielia_
(Galton and Jensen 1973), _Hypsilophodon_ (Galton 1974a), "_Yandusaurus"
multidens_ (He and Cai 1984), and _Agilisaurus_ (Peng, 1990, 1992), the
obturator process is a rectangular flange that is perpendicular and ventral to
the main shaft of the ischium.
However, in RUB 17 (the holotype of _Lesothosaurus diagnosticus_, but we'll
get to that later....), the shaft of the ischium bends slightly dorsal when it
meets the shaft fo the pubis and there is a simple corner there which has been
called the obturator process. Although in the right place to be the obturator
process, its homology must be put into serious question. Strangely enough, a
very similar process exists on the British Stegosaur _Lexovisaurus
durobrivensis_ (check out Galton 1985, figure 14 W).
<<Incidentally, I'd maintain that the critter's name is Fabrosaurus, not
Lesothosaurus... but not everyone would agree (another long story, which
you'll find in Geobios).>>
On this point, I'd have to disgaree on philosophical grounds. Although Galton
is almost certainly incorrect in maintaining that _Fabrosaurus_ is distinct
from _Lesothosaurus_ (as explained by Thulborn 1992 and Sereno 1991), that
does not necisarily mean that the material reffered to _Lesothosaurus_ has to
be called _Fabrosaurus_. In the type of _Fabrosaurus_ (a partial jaw with
teeth) there are no autapomorphies and every feature is symplesiomorphic for
the Ornithischia as a whole, where as RUB 17 (the type of _Lesothosaurus_)
does have several autapomorphies. Therefore, I would refer to _Fabrosaurus_
as Ornithischia _incertae sedis_, and _Lesothosaurus_ as the immediate
outgroup of the node Genasauria.
One more thing though... I would ask that we please please please continue
the discussion of mythology in private. This discussion has drifted far out of
the realm of dinosaurs....
-Galton, P M. 1974a. The ornithischian dinosaur _Hypsilophodon_ from the
Wealden of the Isle of Wight. Bulletin of the British Museum (Natural
History): Geology 25:1-152.
-Galton, P M. 1974b. Notes on _Thescelosaurus_, a conservative ornithopod
dinosaur from the Upper Cretaceous of North America, with comments on
ornithopod classification. Journal of Paleontology 48(5):1048-1067.
-Galton, P M. 1985. British plated dinosaurs (Ornithischia, Stegosauridae).
Journal of Vertebrate Paleontology 5(3):211-254.
-Galton, P M and Jensen, J A. 1973. Skeleton of a hypsilophodontid dinosaur
(_Nanosaurus (?) rex_) from the Upper Jurassic of Utah. Brigham Young
University Geological Studies 20:137-157.
-He X-L and Cai K-J. 1984. The Middle Jurassic dinosaurian fauna from
Dashanpu, Zigong, Sichuan. Vol 1: The ornithopod dinosaurs. Sichuan
Scientific and Technological Publishing House. 71 pp.
-Peng G-Zh. 1990. [A New Species of Ornithopod Type from Zigong, Sichuan].
Journal of Zigong (issue number unknown): 19-30. (In Chinese)
-Peng G-Zh. 1992. [Jurassic Ornithopod _Agilisaurus louderbacki_
(Ornithopoda: Fabrosauridae) from Zigong, Sichuan, China]. Vertebrata
PalAsiatica 30(1): 39-53. (In Chinese with English summery)
-Sereno, P C. 1991. _Lesothosaurus_, "fabrosaurids," and the early evolution
of the Ornithischia. Journal of Vertebrate Paleontology 11(2):168-197.
-Thulborn, R A. 1972. The post-cranial skeleton of the Triassic
ornithischian dinosaur _Fabrosaurus australis_. Paleontology 15(1):29-60.
-Thulborn, R A. 1992. Taxonomic characters of _Fabrosaurus australis_, an
ornithischian dinosaur from the Lower Jurassic of Southern Africa. Geobios