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HETERODONTOSAURIDS AND BASAL CERAPOD PHYLOGENY



Amazing!! Something near and dear to my heart pops up on the old theropod 
mailing list

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 
rami.

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).

Tim Williams:
<<These are definitely ornithopods - of the family Hypsilophodontidae 
(although 
I believe this has been broken up into more than one clade by some 
authors).>>

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 
ornithopod tree.

Jack Conrad:
<< 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
 convergence.>>
 
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) 
and pachycephalosaurs.

<< 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
 premaxilla,>>

What's that?

 <<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.
 
Peter Buchholz
Tetanurae@aolc.om