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HETERODONTOSAURIDS AND BASAL CERAPOD PHYLOGENY
Amazing!! Something near and dear to my heart pops up on the old theropod
Jack Conrad wrote:
<< Just looking at the old cerapod phylogeny and shaking my head about the
placement of heterodontosaurids. If you look at the jugal boss and
caniniform teeth, don't you see basal pachycephalosaur? Now if _Lanasaurus_
is a valid taxon and it does lack the caniniform teeth, wouldn't this be a
good candidate for sister-group to Marginocephalia, perhaps something like:>>
Then Tim Williams added:
<<_Orodromeus_ and _Zephyrosaurus_ also show a jugal boss.
Orodromeus and Zephyrosaurus (and others in fact) do indeed have jugal
bosses, but these are formed in an entirely different fashion than the ones
in Heterodontosaurs, and Marginocephalians SS.
The bosses in the marginocephalians are formed by a ridge descending the
postorbital ramus of the jugal and come to a point that makes the skull
significantly wider. In zephyrosaurs however, the boss looks somewhat like
an altoid glued on to the side of the face, ie small and round and without
the ridges. The boss is also caudal to the convergence of the three jugal
Weishampel and Heinrich 1992 insist that these two structures are homologous
(rather than convergent) and ascert that the condition in zephyrosaurs is a
simple reversal to the basal condition seen in cerapods, since
marginocephalians have the boss and he basal ornithopod heterodontosaurids do
too. They make no argument as to why heterodontosaurids ARE ornithopods
however (and I have yet to find one published ANYWHERE by ANYONE).
<<These are definitely ornithopods - of the family Hypsilophodontidae
I believe this has been broken up into more than one clade by some
Sorry to pick a nit and make an example of someone but... since neither one
of these animals is Lambeosaurus or Triceratops, its status as an ornithopod
can never DEFINATELY be known. In fact, some might doubt if they are in fact
ornithopods, because they are mightily primitive. Some of the animals that
most people all know and love as the homogenous "hypsies" are really not that
ordinary and end up clading all over the ornithischian tree... not just the
<< The only reason I include ceratopsians in a group with
"heterodontosaurids"/pachycephalosaurs exclusive of _Lanasaurus_ is because
of the caniniform teeth in _Protoceratops_, which could be because of
The premaxillary teeth of all ceratopians and the premaxillae in general have
gone through such tremendous transformation, that it is really hard to get
any data out of them that wouldn't be put into substantial doubt.
Also, I think the heterodontosaurid you are referring to (lacking the tusks)
is in fact Abrictosaurus and not Lanasaurus. Lanasaurus is known from a
single partial maxilla with a characteristic tooth replacement pattern,a dn
it can't be known about premax or dentary tusks since those parts of he face
are just not known.
Lanasaurus however does show what looks like a substantial arched max-premax
diastema, which is known in all heterodontosaurids (including Abrictosaurus)
<< I understand that the idea of heterodontosaurids as basal members of
Ornithopoda is beginning to lose favor, but have not seen a recent published
phylogeny where it was not considered so.>>
Well it depends on who you ask. The only person who seems to have published
a comprehensive phylogeny of the Ornithischia in the past 10 years is Paul
Sereno. All due respect to Dr. Sereno, but people are at times wrong, and it
is best to have more than one person publishing on a major group. Look at
what's happening in theropods, that sort of thing should be going on in
ornithischians only ten times more, rather than one tenth.
<<Things like _Hypsilophodon_ seem
more primitive in the brevity of the posteroventral projection of the
<<retention of more than 3 premaxillary teeth,>>
Well... Ornithischians ancestorally had 6 premaxillary teeth, thyreophyrans
gained a seventh, and ornithischians and non-node cerapods on the cerapod
branch have five teeth. In my opinion, three premaxillary teeth in
heterodontosaurs and marginocephalians is a synapomorphy of the group.
<< more primitive "cheek" teeth,>>
Compared to what?
<< and morphology of the quadratojugal,>>
The quadratojugal is weird and different in every single ornithischian taxon
<<yet have been considered
more advanced ornithopods than _Heterodontosaurus_. Doesn't removal of the
latter from Ornithopoda seem most likely to be right?>>
Like I said before, "hypsies" aren't the homogenous group that they once
seemed, more to come in the next 10-15 years, but some specifically from me
at SVP 2000 I hope.