Re: Hongshanosaurus houi, new psittacosaur

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

Dinogeorge@aol.com wrote:

> Frankly, I think the overall relationship of heterodontosaurs with 
> marginocephalians is pretty obvious and doesn't really need a phylogenetic 
> analysis.

Nevertheless, some of us diehard skeptics would like to see a phylogenetic 
analysis before we're convinced that heterodontosaurids truly belong in the 
Marginocephalia.

If nothing else, an analysis would examine competing scenarios - such as 
whether the characters shared by heterodontosaurs and marginocephalians are 
(1) primitive (symplesiomorphous) for the Genasauria, and lost independently 
by ornithopods and thyreophorans; or (2) independently acquired in 
heterodontosaurids and marginocephalians.

As a cautionary tale, fifteen years ago a close relationship between 
segnosaurs and sauropodomorphs seemed obvious to many people.  At the time, 
I was certainly convinced that segnosaurs were sauropodomorphs - it seemed 
"obvious" to me too.  But further material and detailed anatomical studies 
followed by phylogenetic analyses revealed that segnosaurs 
(therizinosauroids) were most likely card-carrying members of the Theropoda. 
 Evolution is full of surprises, and the heterodontosaur-marginocephalian 
link is not yet a "done deal".

> Right now we don't have anything marginocephalian temporally between 
> heteros and the rest to be able > to say more than that heteros are basal 
> marginocephalians.

If _Echinodon_ is a heterodontosaurid, then this taxon extends the range of 
this group into the Early Cretaceous.  The temporal range of 
heterodontosaurids therefore overlaps with the temporal range of undoubted 
marginocephalians.  But I agree that, aside from heterodontosaurids (if 
indeed they are marginocephalians) and _Chaoyangsaurus_, the 
pre-Late-Jurassic marginocephalian fossil record is so far non-existent.

Heterodontosaurids lack the parietosquamosal shelf seen in ceratopsians and 
pachies (the reason behind the name "Marginocephalia"), and their hindlimbs 
are surprisingly bird-like in the way the distal elements are coossified.  
In contrast to the derived headwear, and with the exception of the pelvis, 
the postcrania of pachycephalosaurs and early ceratopsians is fairly 
unspecialized by ornithischian standards.  Thus, if we found a partial 
postcranium of a Triassic or Jurassic marginocephalian, would we necessarily 
recognize it as such?

[ BTW, what is the definition of Marginocephalia?  (Mike, are you out there? 
 ;-)  )   Could the current definition accommodate heterodontosaurids; or 
would a new name have to be coined for the 
heterodontosaurid-ceratopsian-pachycephalosaurian clade? ]



Tim

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