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RE: Heterodontosaurs



Well, a few months ago I have done some serious
investigating in this area, the possible relationship
of the Heterodontosauridae with the Marginocephalia.
Reading a lot of articles, comparing pieces of bone,
sometimes thinking he had found something, which later
turned out to be already present in Lesothosaurus or
couldn't find a good description of a corresponding
bone in the sistergroups. But enough whining, I
actually came up with nine, yes nine, characters which
may unit heterodontosaurs with the Marginocephalia.
Some of their are a little ambiguous or little
supported, but yeah one has to start somewhere. I
while list them below and if you have an opinion about
any of these, please react.

-The first one is the presacral count which is 21 for
Heterodontosaurus and most members of the Ceratopsia,
although the Ceratopsidae have gained one, with a
total of 22. Members of the outgroups (Ornithopoda,
Thyreophora and Lesothosaurus) have at least 24
presacrals.

-The next is an often cited one, namely the
premaxillary teeth, which is three in heterodontosaurs
(ok one Abrictosaurus skull has two, but this may be a
skull of a young or sexual related) and
Pachycephalosauria. Most ceratopsians have no
premaxillary teeth, but in Chaoyangsaurus and
Protoceratops two teeth have retained. The earliest
members of the outgroup had 5, 6 or even 7
premaxillary teeth.

-Another famous one, the large caniniform first
dentary tooth which fitted into the maxillary
diastema. Present in heterodontosaurs and
pachycephalosaurs, but absent in the Ceratopsia and in
all members of the outgroup.

-Another teeth related character is that the
premaxillary teeth become progressively longer from
front to back, which can be seen in heterodontosaurs,
pachycephalosaurs, Protoceratops and Breviceratops. In
specimens of the outgroup all the premaxillary teeth
are subequal.

-This one is a little bit harder, lateral predentary
process. When you look at the predentary of
Heterodontosaurus it is vertical at the end regarded
from the side and when you compare this with
Hypsilophodon, you see that it makes a curve down and
makes a curve again. This vertical line can also be
seen in Psittacosaurus, Protoceratops, Leptoceratops,
Bagaceratops and probably Stegoceras (although the
bone hasn't been preserved). The same shape, at least
superficially, as in Hypsilophodon can be seen in all
members of the Ceratopsidae.

-Another obvious one, the presence of a lateral
flange/boss (or whatever you want to call it) on the
jugal, which can be seen on the skull of
Heterodontosaurus, the Pachycephalosauria and all the
Ceratopsia, including Chaoyangsaurus. The same, or at
least the same looking, structure is also present in
Orodromeus and Zephyrosaurus, but this could easily be
explained as a convergence.

--Again the teeth, distribution of enamel on
mandibular teeth. Which is primarily on one side in
Heterodontosaurus, Psittacosaurus, Neoceratopsians and
Pachycephalosauria, but according to Xu Xing (pers.
comm.) on both sides evenly in Chaoyangsaurus. And
also evenly enameled on both sides in the outgroup,
except for the more advanced members of the
Ornithopoda.

-Lets move a little bit lower and look at the scapula.
Where we can see that the scapular shaft is narrow in
comparison with the total scapular length in
Heterodontosaurus, Stegoceras and the early
Ceratopsia. Although it is wide in the Ceratopsidae,
most probably due to their pronounced quadrupedality.

-The last one is the most ambiguous one, namely the
preorbital length relative to total skull length,
measured from the tip of the premaxilla (or rostral)
to the end of the lower part of the quadrate (so the
frill in the Ceratopsia hasn't been accounted). Well
the preorbital length is rather short in the
Heterodontosaurus and members of the Marginocephalia
(less than 44%) and rather large in the outgroup
(>44%).

Well that's all, any comment, additions or any other
interesting thing is always welcome. Hope I haven't
bored any of you with this rather long posting, but
maybe we could finaly get a clearer picture of the
relationship of the heterodontosaurs. I know their are
some people who will support my opinion, but yeah
others wont, but that's what this mailing list is all
about.


Cheers,

Nick Snels
More info about Pachycephalosauria at:
http://www.geocities.com/nick.snels/

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