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The JVP paper on pachycephalosaurs by Sullivan (2003) is available in PDF
format (as I recall, a glitch held it back from being downloaded when it
first went up on the JVP website).
REVISION OF THE DINOSAUR _STEGOCERAS_ LAMBE (ORNITHISCHIA,
PACHYCEPHALOSAURIDAE). Journal of Vertebrate Paleontology: Vol. 23, No. 1,
Sullivan completes his earlier (2000) revision of the genus _Stegoceras_.
This genus is limited to one species, _S. validum_ (once called _Troodon
validum_ - another nomenclatural saga lies behind that). _S. validum_
(sensu stricto) is "a primitive, incipiently-domed pachycephalosaur that is
characterized by a well-developed squamosal shelf and open supratemporal
fossae. It is the sister taxon to the fully-domed Pachycephalosaurinae."
_Ornatotholus browni_ appears to represent an earlier growth stage of _S.
validum_,as earlier proposed by Goodwin et al. (1998).
_Stegoceras_ bridges the phenetic gap between the "flat-topped" basal
pachycephalosaurs ("Homalocephalidae") and the dome-headed
pachycephalosaurines. However, Sullivan is careful to avoid the conclusion
that the known taxa constitute a linear progression.
Sullivan (2003) overhauls the classification of the pachycephalosaurs, and
recognizes 11 genera as valid: _Stenopelix_, _Wannanosaurus_, _Goyocephale_,
_Homalocephale_, _Stegoceras_ (sensu stricto), _Prenocephale_, _Tylocephale_
(provisional; may be referrable to _Prenocephale), _Stygimoloch_,
_Pachycephalosaurus_, and two new genera _Colepiocephale_ ("knucke-head")
and _Hanssuesia_, both based on species formerly referred to _Stegoceras_.
Sullivan's inclusion of _Stenopelix_ in the Pachycephalosauria is
provisional. Sullivan repeats his early interpretation (2000) that
_Yaverlandia_ is not a pachycephalosaur - though what exactly kind of
creature the skull cap comes from is unclear (ankylosaur?, abelisaur?).
_Majungatholus_ is of course an abelisaurid.
_Ornatotholus_, _Stenotholus_, and _Sphaerotholus_, are regarded as junior
synonyms of _Stegoceras_, _Stygimoloch_ and _Prenocephale_, respectively.
This last genus is the only pachycephalosaurian genus to contain both Asian
and North American species. (I think a case could be made for resurrecting
_Sphaerotholus_, based on _S. goodwini_. It all depends on how sensitive
your "genericometer" is.)
_Micropachycephalosaurus_ and _Gravitholus_ are considered _nomina dubia_.
_Tylosteus_ (referred by Galton and Sues  to _Pachycephalosaurus)
probably also is as well.
One weird thing to come out of this paper: according to Sullivan (2003)'s
classification scheme, no name that ends in "-tholus" is considered a valid
pachycephalosaurian genus. _Ornatotholus_, _Stenotholus_, and
_Sphaerotholus_ are sunk into other genera; _Gravitholus_ is toast; and
_Majungatholus_ is a theropod. By contrast, every genus to end in
"-cephale" is considered valid by Sullivan: _Goyocephale_, _Homalocephale_,
_Prenocephale_, _Tylocephale_, and _Colepiocephale_. What are the odds?
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