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Re: Dinosaur Web Pages' Re-Opening




On Thu, 4 Sep 1997, Jonathan R. Wagner wrote:

> 
> >I have yet to see any cladistic analysis
> >that demonstrates beyond reasonable doubt that any terrestrial vertebrate has
> >>regained< a functional digit that was previously lost.
>         So digits are "special"? George, you have proven positively
> intractable on this topic, despite numerous attempts by professionals,
> semi-professionals, and myself to demonstrate the fallacy of your arguments.
> I will not repeat the arguments here.

I am not a professional, but I would like to mention that it does seem
that once a digit is gone, it's gone.  Look at all mammals, birds,
reptiles, and amphibians.  Also, see Gould's wonderful report "Eight (or
fewer) little piggies." (Nature magazine, not sure what month or year
right now.)  As the autopodia become more and more specialized it has been
seen time and time again that tetrapods lose digits, never gain them (the 
closest possible thing is the false thumb of the panda, which is not
really a thumb or digit, just a specialized carpal). You are right that
this does not prove that it absolutely cannot happen, only that the
potential to do so has never been shown in any group of tetrapods ever.  
 
> 
> ><< >(3) Heterodontosaurids are not ornithopods [...]
> > and there is [not] an obturator process on the ischium. 
> >        It is possible that many of common conceptions concerning the
> > diagnostic utility of this element are in error. [...] >>
> >In Ornithischia, the obturator process developed well along in the group's
> >evolution. [...] it is a synapomorphy of Ornithopoda.
>         This according to orthodox analysis. I would be interested to see
> what happens if the definition of this character is clarified and the coding
> of this character is explored (using _Lesothosaurus_ and _Eoraptor_ as
> outgroups, perhaps).

_Lesothosaurus_ did not possess an obturator process either.  Is it
possible that _Lesothosaurus_ gave rise to thyreophorans (via
_Scutellosaurus_), marginocephalans (via heterodontosaurids) AND
ornithopods (via hypsilophodonts)?  If you will notice, the hypsilophodont
tooth is less specialized than that found in heterodontosaurids, the hands
of hypsies are small as is the case in what we know of the hand of
_Lesothosaurus_.  The premaxillary is of similar form between the latter
two as well.  

> 
> ><<>The feet
 
>         In any case, your implicit criticism is valid. I should be listing
> characters to support a counter hypothesis, not characters for several
> different hypotheses. Touche.  :)

I'm not rooting for either of you in this (I'm still trying to figure it
out myself), but I like to see someone admit when the other has a point.
Congrats on being a reasonable person (something paleontology needs more
of).  

>         In case you haven't been keeping up on current events, phylogenetic
> reconstruction is based on "lumps and bumps", and the sum total of their
> affinities. If we waited around for "robust character suites", we could
> hardly expect to resolve most of the controversies in paleobiology.

>         And they have not been lightly dismissed. The theories of
> knowledgeable and experienced people such as yourself, Gregory Paul, and
> other dinosaur workers were considered heavily. However, recently, new data
> have caused them to be dissmissed, not lightly, but with a heavy heart and a
> heartfelt "well done" for doing the best that could be done with the
> material then available.

VERY well said.  

By the way, my little hypothesis above (about _Lesothosaurus_ and all)
includes one very questionable point when I suggest that the
heterodontosaurids gave rise to the marginocephalans.  PLEASE do not slay
me on the fact that I infer tooth structure similarities between the two
(when I say I infer this, I mean by my later remark on hyspies and
_Lesothosaurus_).
I know that the pachycephalosaurs had more primitive teeth in their jaw
than the heterodontosaurids, my point was that it seems unlikely that
heterodontosaurids would develop derived teeth from the primitive
_Lesothosaurus_ form, then the hypsies would lose these specializations,
and then they would re-develop in later ornithopods.  Of course by
suggesting heterodontosaurids could have given rise to pachycephalosaurs
and ceratopsians I am committing the same sin in a different area, but
focus, for now, on the original point (slay me later when I offer my
explanation for the marginocephalan dilemma :)

Jack