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Stegoceras



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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