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Re: Hongshanosaurus houi, new psittacosaur

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? ]


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