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Re: Hongshanosaurus houi, new psittacosaur
Frankly, I think the overall relationship of heterodontosaurs with
marginocephalians is pretty obvious and doesn't really need a phylogenetic
Nevertheless, some of us diehard skeptics would like to see a phylogenetic
analysis before we're convinced that heterodontosaurids truly belong in the
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
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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