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Basal Ornithischian Systematics (LONG & BORING)



In response to comments of Peter Buchholz, on "Basal Ornithischian
Systematics" (= is there an obturator process in fabrosaurids?  Is
Fabrosaurus a determinate taxon?  Is Lesothosaurus synonymous with
Fabrosaurus?):

(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.

(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.

(3)  "However, in ... the holotype of _Lesothosaurus diagnosticus_ ... the
shaft of the ischium bends slightly dorsal when it meets the shaft fo the
pubis and there is a simple corner there which has been called the obturator
process.  Although in the right place to be the obturator process, its
homology must be put into serious question."
     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.  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].
     Even if your speculation proved correct, and the process were "a simple
corner", this wouldn't betray anything of great significance.  In fact, you
go on to say that "... a very similar process exists on the British
Stegosaur _Lexovisaurus durobrivensis_ ...".   And, with a bit more digging
around among the ornithischians in general, and ornithopods in particular,
you'd find even more examples of "a very similar process" (e.g. in some
examples of _Dryosaurus_).  You'd also find that there is immense variation
- and even intraspecific variation - in size and shape of the obturator process.

(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.
 
(5)  "In the type of _Fabrosaurus_ (a partial jaw with teeth) there are no
autapomorphies and every feature is symplesiomorphic for the Ornithischia as
a whole..."
     An ex cathedra statement of opinion, not of fact.  It excludes the
possibility that the teeth of the type specimen have unique distinguishing
characters.
     Let me offer the alternative opinion, paraphrased from paper in Geobios
(1992).  The teeth in the holotype of _Fabrosaurus australis_ are
morphologically unique among ornithischians.  The suggestion that teeth
similar or identical to those of _F. australis_ were widespread among early
ornithischians (originating with Charig & Crompton 1974) is merely a
off-the-cuff suspicion that has never been substantiated.  That suggestion
is unrealistic, as it fails to acknowledge the diversity and structural
complexity of dinosaurian tooth structures.  In some instances dental
characters alone, like any other morphological characters, may be
insufficient to discriminate between closely related species - e.g. among
ankylosaurs.  But in other instances dental characters can certainly be used
to distinguish genera (e.g. some theropods) and higher taxa (e.g.
hadrosaurian families).  Each case must be judged on its individual merits,
and not dismissed with the sweeping assumption that dinosaur teeth are
worthless or inadequate for taxonomic purposes.  In this context it may be
salutary to recall that one of the most famous dinosaurian genera,
_Iguanodon_, was founded on isolated teeth yet is universally accepted as a
determinate taxon.  Given this precedent, it is reasonable to regard the
genus _Fabrosaurus_ as being adequately defined by its unique dental
morphology, exemplified in the type specimen.
     OK, that's my opinion, and you choose to disagree (on philosophical
grounds).  But your disagreement is inconsequential if you merely shrug
aside my opinion by asserting that there is no evidence to support it (=
"there are no autapomorphies and every feature is symplesiomorphic").   I
have published the factual evidence to substantiate my opinion; if you
choose to disagree, then please explain exactly where that factual evidence
is wrong - rather than claiming that it doesn't exist.
     Using a cladistic analogy: I've offered a hypothesis [Lesothosaurus =
Fabrosaurus], and you say it's wrong.  But you haven't tried to falsify that
hypothesis.  Instead you've offered the ex cathedra pronouncement that
there's no evidence for it, full stop.  Well, if you don't test and falsify
the hypothesis, it remains viable.  (Those are my philosophical grounds!)
Conversely, I have tested and (to my satisfaction) falsified two competing
hypotheses (that F. australis is an indeterminate ornithschian, and that L.
diagnosticus is demonstrably different from F. australis).

(6)  "... RUB 17 (the type of _Lesothosaurus_) does have several
autapomorphies..."
     Yes.  It has them IN ADDITION to sharing unique dental characters with
the type of _Fabrosaurus australis_.  And for that reason I regard
_Lesothosaurus diagnosticus_ Galton, 1978, as a subjective junior synonym of
_Fabrosaurus australis_ Ginsburg, 1964.

Apologies for clogging your mail-box,

Tony Thulborn