[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

MUSINGS ON BASAL ORNITHISCHIANS



I appologize for not responding to this message in a more timely fashion.  I
have been quite ill for the past two weeks with hepatitis, and am slowly
recovering my natural, not yellow, skin color.

Additionally, I would like to add that I never intended to be crass or
confrontational in my post, I was simply stating my opinion on whether or not
_Lesothosaurus_ has an obturator process, and whether or not RUB 17 should be
called _Lesothosaurus_ or _Fabrosaurus_.  Again, I have not had the advantage
of viewing the specimens, and am simply stating my opinion on what I could
gather from the literature.

Tony Thulborn wrote (quoting me):
>(1)  "... the process which is assumed to be the obtuartor in Thulborn 1972,
>actually bares little, if any resemblance to the obturator processes in any
>true ornithischians [later corrected to 'ornithopods']..."
>     Exaggeration.  So far as I recall, no one who's examined this feature
>has doubted that it is the obturator process.  You agree that this feature
>is "...in the right place to be the obturator process..." (below).  So the
>supposed lack of resemblance must be in size and/or shape.

I have given this a lot of thought, and in fact, the process may well be the
obturator process.  The obturator process in fact appears to be primitive for
the Dinosauria as a whole (visible in _Herrerasaurus_, _Eoraptor_ and
_Anchisaurus_ among others), and indeed, it is more likely that RUB 17 had
one, though, again, different in form from those in basal, true ornithopods
like _Agilisaurus_.

>(2)  "In basal true ornithopods ... the obturator process is a rectangular
>flange that is perpendicular and ventral to the main shaft of the ischium."
>     Yes and No.  The obturator process is (usually) a flange, and it does
>extend anteroventrally from the shaft of the ischium.  But it isn't
>necessarily "rectangular" in ornithopods.  It can be triangular, spatulate,
>trapezoidal, even spoon-shaped or slightly hooked.

Indeed, there is great variation in the shape of the obturator process in
ornithopods as a whole, but "hypsilophodontid" grade ornithopods
(_Agilisaurus_, _Othnielia_, _Thescelosaurus_, _Hypsilophodon_ etc) seem to
show very little variation in the shape of the process, being roughly
rectangular, and perpendicular and ventral to the long axis of the ischium.
That is all I was saying.

>     Many ornithischians have a bend or curve in the shaft of the ischium.
>And the "simple corner" isn't quite so simple.  In the left ischium of the
>type specimen the obturator process is broken;  a sizeable wedge-shaped
>fragment (at least) is missing, and behind that a ragged edge extends down
>the lower edge of the ischium.  When illustrating the specimen (1972, fig.) I
>refrained from restoring the obturator process with a dotted line -
>because, given enough time, dotted lines  evolve into solid lines and become
>embedded in the literature as 'fact'.  [Despite my precaution, the ischium
>of _Fabrosaurus_ did nevertheless evolve into questionably 'factual' form -
>see Weishampel, Dodoson & Osmolska (1990), "The Dinosauria", fig. 19.4, p.
>421.]   In short, there is definitely a respectable obturator process of
>some sort, and it's in the appropriate anatomical position.  But it is so
>battered that I was unwilling to stick out my neck and restore its original
>shape.

One question I would like to ask.  Do you propose that the process was indeed
more similar to those in basal ornithischians (rectangular, etc), or more
triangular as has been illustrated by yourself and Sereno?

>Somehow, Peter, you seem to have assumed (a) that you DO know its
>original shape [= a speculation], and (b) that such a shape is so weird that
>the identity of the obturator process must be open to "serious question" [=
>an exaggeration].

I do not pretend to know the original shape.  I had just assumed (perhaps
incorrectly) that since Paul Sereno as well as yourself had illustrated the
ischium nearly identically, that that was close to the "right" way that RUB
17's ischium looked.

>(4)  "On this point [ = that the critter's name is Fabrosaurus, not
>Lesothosaurus] , I'd have to disgaree on philosophical grounds.  Although
>Galton is almost certainly incorrect in maintaining that _Fabrosaurus_ is
>distinct from _Lesothosaurus_  ... that does not necisarily mean that the
>material reffered to _Lesothosaurus_ has to be called _Fabrosaurus_."
>     True, and I never implied otherwise.  I trust you're not setting up a
>straw man.

Indeed I hope not.  It is for the same reasons I don't use _Fabrosaurus_, that
I don't use _Troodon_.  Gow and Currie respectively are certainly correct that
the nearly complete material of _Lesothosaurus_ and _Stenonychosaurus_
certainly look a lot like the scrap types of _Fabrosaurus_ and _Troodon_.
That however, does not mean that they *are* the same thing.  There are two
"fabrosaurs" from the Upper Elliott Formation of southern Africa, either one
of which the type of _Fabrosaurus_ could presumably have come from.  So far,
only one saurornithoidid is known from the Judith River/Two Medicine
Formations, but there could very easily be another.  Indeed, the very similar
Nemegt Formation of Mongolia has at least three saurornithoidids.
 
Peter Buchholz
Tetanurae@aol.com

Are you ok dad?
Yes, just bones and organs.