Stegoceras

["Tim Williams" <twilliams_alpha@hotmail.com>]

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, 
pp. 181?207.

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 [1983] 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?



Tim

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