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PROBLEMS WITH WEISHAMPEL _ET_ HEINRICH'S HYPSILOPHODONT PAPER



And now for something with substance....

I finally recieved Weishampel and Heinrich's recent Hypsilophodont phylogeny
(thanks Mickey!):

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

And after going through it quite a few times, I am utterly amazed at the weak
support for most of their conclusions of systematics within the
Hypsilophodontidae.  Their analysis supports a monophyletic
Hypsilophodontidae _sensu_ Sternberg with the following ingroup
relationships:

((((ZEP + ORO) + (HYP + PAR)) + (AGI + OTH)) + THE) + IGU

ZEP = Zephyrosaurus; ORO = Orodromeus; HYP = Hypsilophodon; PAR =
Parksosaurus; AGI = Agilisaurus; OTH = Othnielia; THE = Thescelosaurus; and
IGU = Iguanodontia.

[Weishampel and Heinrich apperently synonymize _Yandusaurus hongheensis_ with
_Yandusaurus multidens_.  This is not correct since the two differ
significantly in their tooth morphology.  Peng advocates transfering this
species (_multidens_) to the genus _Agilisaurus_.  I believe this is a step
in the right direction since it is far more similar to _Agilisaurus_ than it
is to _Yandusaurus_, but it still differs from _Agilisaurus_ quite a bit.  I
would advocate the formation of a new genus to hold the species _multidens_.
 For my part, I'd like "Hecaicursor" which honors the original describers, He
Xinlu and Cai Kaiji + _cursor_ - runner.  Should be pronounced like the
English words "Heck-Eye-Curser."  In my critique I do use the name
_Agilisaurus_ for the species _multidens_ however.]

OK, lets get with the business....  A single synapomorphy unites the clade
_Zephyrosaurus_ + _Orodromeus_ and that is an enlarged Jugal boss.  In the
paper it is considered primative fro the Cerapopda since it is present in
both Marginocephalians and Heterodontosaurs, and its appearance ion this
clade is considered to be a reversal.  I think this is fine, from here on
out, I'm going to refer to these guys as Zephyrosaurs.

For the clade _Hypsilophodon_ + _Parksosaurus_ the single character Jugal
excluded from caudal margin of Anteorbital Fenestra supports the clade.  I
think this is just fine and will refer to these guys as Hypsilophodonts
Proper from here on out.

For the clade uniting Zephyrosaurs and Hypsilophodonts proper, the single
character distally positioned Obturator Process of the Ischium is used.  I
think that this is a rather weak and ambiguous character.  How distal is
distal?  Are there even pelvic elements known for _Zephyrosaurus_?

The characters linking _Othnielia_ and _Agilisaurus_ ( and
presumably_Drinker_...) are a little more numorous.  First off is a Reduced
Quadratojugal.  I don't know about you, but the last time I checked,
_Othnielia_ didn't have a head, so how could this be used as a character
uniting _Othnielia_ to anything!?  The morphology of the quadratojugals
_Agilisaurus louderbacki_ and _A. multidens_ also vary quite a bit, so I
doubt this character is very useful.  The next two characters seem to hold up
a bit better though: dorsally curved distal pubis and dorsally curved distal
ischium (the latter is found in _Parksosaurus_ as well).  I think that those
two are enough to unite the clade _Agilisaurs_ + _Othnielia_, and from here
on out, these two (or four including _Drinker_ and a seperate "Hecaicursor")
will be referred to as Othnielians.

The Othnielians are united with the Zephyrosaur + Hypsilophodont proper clade
by the following characters which I think are rather ambiguous.  First of, is
that in dorsal aspect the paired frontals are longer than they are wide.
 Correct me if I'm wrong, but isn't that plesiomorphic for the Dinosauria as
a whole?  The second is that the angle between the skull base and the long
axis of the braincase is approximately 35 degrees.  I don't know too much
about the subject of braincases, but I am a little apprehensive of this
character since I know that (again) _Othnielia_ doesn't have a head, and
there is nothing (to my knowlege) published about either species of
_Agilisaurus_' braincase.  I am also unsure as to whether or not
_Zephyrosaurus_ has a braincase preserved.

These six taxa are united with _Thescelosaurus_ to form a monophyletic
Hypsilophodontidae _sensu_ Sternberg using the following two characters: a
gracile pre-pubic process and a cingulum on the dentary teeth.  The first
could easily be plesiomorphic for the Euornithopoda or Ceropoda, or due to
small size since it is also seen in smaller Iguanodontians and, I believe,
small, basal marginocephalians as well.  Its complete absence in
Heterodontosaurids could be synapomorphic of that family.  The tooth
character I am unsure of because I really don't know what a cingulum is and
it is not illustrated in the paper at all (darn them!).  Anyone care to
enlighten me?

For my part, I see most of the characters supporting a united
Hypsilophodontidae _sensu_ Sternberg to be either weak, ambiguous, or
outright plesiomorphies.  I would have liked to have seen some more dental
characters like those hinted at by Bakker _et al_ in the _Drinker_ paper.
 For instance, a strong primary ridge on the labial (I think) surface of the
maxillary teeth (present in Hypsilophodonts proper and Iguanodontians, but
not any others), a strong primary ridge on the labial (I think) surface of
the dentary teeth (strong in Iguanodontians, weak in Hypsilophodonts proper
and absent in the others), and a reduction in the number of denticles
(present in Hypsilophodonts proper and Iguanodontians).

If this were to be analysed, I believe that their monophyletic
Hypsilophodontidae _sensu_ Sternberg would fall to pieces, with the clade
Euronithopoda comprising of Hypsilophodonts proper and Iguanodontians, and an
unnamed clade nested within the Ornithopoda becoming an unresolved polytomy
comprising of Zephyrosaurs, Othnielians, _Thescelosaurus_, and Euornithopoda.
 For right now, I am also considering Cerapoda to be an unresolved trichotomy
between Marginocephalia, Ornithopoda, and Heterodontosauridae.

Of course I have no analysis of my own to back this up, but as soon as I get
Phylipp to work, I will show you the results, but for now, I believe that
most of the evidence is in favor of a paraphyletic Hypsilophodontidae _sensu_
Sternberg, and a much more restricted Hypsilophodontidae consisting of
_Hypsilophodon_, _Parksosaurus_, and probably _Yandusaurus_ (the REAL
_Yandusaurus_) and _Atlascopcosaurus_, and a number of basal ornithopod
clades comprising of animals commonly considered to be Hypsilophodontids.

Anyways.... who would have thought Ornithischian systematics could be so fun?
 HA!  This is just getting even with all the messes you theropod guys make
:-) .  Anyways, does anyone have any comments on my critique?  And does
anyone know what a cingulum is?

Peter Buchholz
Tetanurae@aol.com

"huhd-sin-el-PI-dee-uh"  I can say it just fine Darren.