Re: T. Rex sex(es)

[Mikiel <Mikiel@ASU.Edu>]

On Tue, 9 May 1995, James Shields wrote:

> >What is the latest research on T. Rex? Do the robust and gracille versions 
> >represent two species or two sexes? Or is it still a giant guess?
> >I'd appreciate whatever information anyone can provide.
> 
> This could be old news, but I remember reading an interesting piece
> comparing the skeletons of T. Rex with those of crocodiles. Apparently there
> is a very similar arrangement of arches under the first couple of tail
> vertibrae. Seemingly females have larger arches to give more room for egg
> laying. This would make the larger specemins the females.
> 
> I don't have the reference to hand, but I think it probably came from "The
> Complete T. Rex".
> 
> James
> 
> 
> 
        I don't know if this wil help or not, but here goes:
        A while back (2 years maybe) I was at a guest lecture by Pete
Larson on sexual dimorphism in T. rex.  One of his key points was the
existence (or lack of) an "arch" on one of the proximal, caudal vertebrae
(number 1 or 2, I don't remember exactly).  He said that, as in
alligators, this "arch" was the attachment point for a "penile retractor
muscle."  Obviously, the idea is that males have one, and females don't. 
I don't know of any other work on this, but I imagine it wouldn't be too
difficult to survey the few other T. rexes to see which had these bones
and which didn't.  Of course, if you have an incomplete skeleton, you may
have an "artificial" female.
        Does anyone know whether or not Larson or anyone else has 
actually confirmed this as a diagnostic character?  I don't remember how 
sure he was of this when he presented it.
        By the way, Sue didn't have one, and she's the robust morphotype.

-Mikiel