HYPSILOPHODONT MONOPHYLY AND THE ORIGIN OF BIRDS

[Tetanurae@aol.com]

Well, maybe not....  But Gauthier's title has a nice ring to it.

In truth, only one paper has ever enumerated characters and demonstrated that 
a monophyletic Hypsilophodontidae exists.  I will talk more about those 
characters, but in the meantime, I'd urge everyone to get a copy of:

Weishampel, D B, and R E Heinrich.  1992.  Systematics of Hypsilophodontidae 
and basal Iguanodontia (Dinosauria: Ornithopoda).  Historical Biology 
6(159-184).

In that paper Weishampel and Heinrich come up with a tree topology like this:

Heterodontosauridae (Iguanodontia (Thescelosaurus ((Yandusaurus + Othnielia) 
((Orodromeus + Zephyrosaurus) + (Hypsilophodon + Parksosaurus)))))

Aside from Darren's (true) characters of "small" and "bipedal" they actually 
find some codible characters.  Among them are:

1) absence of ridges that culminate in marginal denticles

2) presence of a cingulum

3) ossification of sternal segments of the cranial dorsal ribs

4) rod-shaped prepubis

and for the clade that excludes Thescelosaurus

5) paired frontals wider than long

6) long axis and base of the braincase at approximately 35 degree angle.

I'm not going to argue for or against hypsilophodont monophyly right now, but 
simply state that in my opinion these characters are terribly weak and at 
best show that a lot more work needs to be done to resolve this stuff.

Character 1) for instance is reversed in Hypsilophodon, Thescelosaurus, and 
Parksosaurus.

Character 3) is only truly present in Othnielia as far as I can tell, but it 
is normally poorly preserved, so I don't know.

Character 4) is variable with ontogeny as Darren pointed out.  Younger 
iguanodontians have prepubes that belong in "hypsilophodonts" but later morph 
into normal iguanodont prepubes.

Character 5) is primitive for the Dinosauria as a whole and is coded 
incorrectly.  More on this later..

Character 6) is hard to code in the first place because various authors have 
various means of deciding which things they are talking about, but is only 
present in Hypsilophodon and Zephyrosaurus.  Othnielia doesn't have a head, 
nothing from the various "Yandusaurus" specimens has had a braincase 
illustrated, much less described, Orodromeus is horrible described and 
Parksosaurus I don't think has that area described.

So we see, the only character that supports a monophyletic Hypsilophodontidae 
is a cingulum on the dentary teeth.  It isn't described in detail, nor 
illustrated.

I would also like to point out that the authors place Heterodontosauridae as 
the outgroup and fail to use some very, very well known basal ornithischians 
like Lesothosaurus as outgroups when they could see that paired frontals 
longer than wide is a primitive character and not a derived one....

Also, the authors seem to sink three very distinct species into one in regard 
to Yandusaurus hongheensis, lumping "Yandusaurus" multidens and Agilisaurus 
louderbacki all into the same species with Y hongheensis....  In my opinion 
it's like sinking Camptosaurus and Parasaurolophus into Iguanodon...

Discussion?  Anyone have opinions?  Science anyone?  Lively debate not about 
word formation?

Pete Buchholz
Tetanurae@aol.com

"Professional wrestling and politics-- they go together about as good as 
cookies and ass."
-A gem from RNC coverage on Comedy Central