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Re: HETERODONTOSAURIDS AND BASAL CERAPOD PHYLOGENY



--On Tuesday, May 04, 1999, 12:03 AM +0000 Tetanurae@aol.com wrote: 

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

Glad to oblige!
  
> They make no argument as to why heterodontosaurids ARE ornithopods 
> however (and I have yet to find one published ANYWHERE by ANYONE).

Something I find disturbing as well.  
 
> 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.

Sereno writes:

"Within the heterodontosaur radiation, the characteristic caniniform teeth
of the posterior premaxilla and anterior dentary appear to be restricted to
a subgroup of heterodontosaurs more advanced than _Abrictosaurus_" (1986:
248).

So you may be right.  However, I have often seen _Abrictosaurus_ referred to
as a possible _Lycorhinus_ or _Heterodontosaurus_.  Less frequently, I have
seen _Lanasaurus_ referred to these other genera.  Whether or not htis is
the case is questionable, I just picked  one of the two.  
 
> Lanasaurus however does show what looks like a substantial arched
max-premax 
> diastema, which is known in all heterodontosaurids (including
Abrictosaurus) 
> and pachycephalosaurs.

This is true.  
 
> 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.

Very true.  Ornithischians attract less media attention though, and this may
be part of the problem.  
 
>  <<Things like _Hypsilophodon_ seem
>  more primitive in the brevity of the posteroventral projection of the
>  premaxilla,>>
> 
> What's that?

Poorly worded.  Perhaps, lateral process?  Subnarial, posterior ramus?  That
little piece of the PM that goes behind the nares.  

>  <<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.

Exactly, this is why it would be difficult to derive ornithopods from
heterodontosaurids.  

> << more primitive "cheek" teeth,>>
> 
> Compared to what?

Comparing basal ornithopods (i.e. _Hypsilophodon_) with heterodontosaurids. 
_Heterodontosaurus_ has closely appressed, edge-to-edge "cheek" teeth
whereas _Hyspislophodon_ has more _Lesothosaurus_ style ones.  

> << and morphology of the quadratojugal,>>
> 
> The quadratojugal is weird and different in every single ornithischian
taxon

Indeed it is.  However, basal ornithopods have very little overlap of the QJ
over the quadrate while said overlap is extensive in heterodontosaurids and
marginocephalians.  

Thanks for your many thoughtful and thought-provoking comments, Peter.  I'd
like to hear more.  

Jack