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HYPSILOPHODONT MONOPHYLY AND THE ORIGIN OF BIRDS
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
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
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
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
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