Re: Sexual Dimorphism in _Coelophysis_

["Jerry D. Harris" <dinogami@hotmail.com>]

> I believe
> there is a pretty strong case for it in _Coelophysis_ _bauri_, using neck
> vs. torso length, limb ratios and skull lengths. Perhaps this type of data
> could be applied to other closely related theropods (which is actually work
> that I am in the process of doing right now).

  Not that I necessarily agree or disagree on sexual dimorphism in 
_Coelophysis_, but I'm not positive that that particular population is the 
best suited for this kind of analysis.  After all, I've worked with many 
fossils of _Coelophysis_, and they remain some of the most 
dimensionally-altered (diagenetically) fossils I've ever seen (as are most 
Chinle specimens -- I've even seen one phytosaur skull so twisted that the 
caudal half of the skull is preserved in dorsal view while the _articulated_ 
rostral portion is in left lateral view!)  As otherwise beautifully 
preserved as most (but not all!) of the Ghost Ranch stuff is, it's still all 
heavily squished, and whatever the prevalent vector of diagenetic 
displacement was, it would act differently on bones oriented at different 
angles to that vector.  I've never seen anyone attempt to quantify this 
degree of diagenetic alteration and correct for it when making measurements 
of the fossils thus affected -- it would certainly be an interesting 
experiment!  As things stand, people simply use their own backgrounds and 
intuition in mentally (and for drawings) un-alter the fossils.  As we all 
know, this can easily lead to all kinds of disagreements about the true 
nature of a squished fossil -- see, for example, the multiple and not 
necessarily complementary reconstructions over the last 100+ years of the 
skull of _Archaeopteryx_ (for which studies on the Solnhofen sediment have 
shown an ~80% diagenetic compaction, but not, as I understand it, much 
lateral motion).

  I'm not saying that the amount of displacement of Ghost Ranch bones is 
huge, but it is certainly noticeable on many, many specimens, and it's 
conceivable that the difference between statistically significant 
measurement differences and statistical noise is there within the diagenetic 
history of the fossils...

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Jerry D. Harris
Dept of Earth & Environmental Science
University of Pennsylvania
240 S 33rd St
Philadelphia PA  19104-6316
Phone: (215) 898-5630
Fax: (215) 898-0964
E-mail: jdharris@sas.upenn.edu
and     dinogami@hotmail.com
http://www.sas.upenn.edu/~jdharris

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