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Re: Sexual Dimorphism (was T.rex Q & A)


    I may be wrong with this, but I believe that I read somewhere that the
norm for sexual dimorphs throughout the animal kingdom is for the female to
be larger than the male.  There are major exceptions, the primates being one
of  them (Aren't they always?:-)).

    As to the evidence of sexual dimorphism in _T. rex_ :-
        "First, there is more room in the robust pelvis for the passage of
eggs (Carpenter 1990).  Second, the gracile morph has an extra bone (an
extra chevron) at the base of the tail (Larson 1994a,1995).  Male
crocodiles, distant relatives of _T. rex_, have a similar extra bone and it
is used to anchor the penis retractor muscles (Romer 1956, Larson 1993).  "
["The King's New Clothes: A Fresh Look at _Tyrannosaurus rex_" by Peter L.
Larson, published in the DinoFest International Symposium publication (1998)
pp. 65-71].  The accompanying figures (in the ref) show the attachment of
the chevron, and the size and shape of the chevron, and the second chevron
as well.  On the gracile morph, the chevrons are all of similar shape and
size (this is a very rough estimate, based on a small drawing).  On the
robust morph, the 'first' chevron is missing, and the second (?) chevron is
shorter than all the rest (about %50), and the end is 'squared' off - as
opposed to the pointed shape of all the other chevrons.

    As to sexual dimorphism in other dinosaurs, in the same volume ["Sexual
Dimorphism in Dinosaurs" by Ralph E. Chapman, David B. Weishampel, Gene
Hunt, and Diego Rasskin-Gutman, pp. 83-93], the authors find evidence for
sexual dimorphism (as noted by several other authors as well) in:
_Lambeosaurus_, _Plateosaurus_, _Apatosaurus_, _Diplodicus_, _Camarasaurus_,
_Coelophysis_, _Syntarsus_, _Tyrannosaurus rex_, _Parasaurolophus_,
_Stegoceras_, _Protoceratops_, _Pacyhrhinosaurus_, _Chasmosaurus_,
_Triceratops_, _Einiosaurus_, and _Achelousaurus_.
I believe that, in those species where the presumption of which sex is which
morph, the larger morph is generally thought to be the female.

    Allan Edels

-----Original Message-----
From: archosaur@usa.net <archosaur@usa.net>
To: dinosaur@usc.edu <dinosaur@usc.edu>
Date: Wednesday, December 09, 1998 11:00 PM
Subject: Sexual Dimorphism (was T.rex Q & A)

>Dwight writes:
>>    I shouldn't have used mammals as an example.  I thought about
>> that later. :-)  But, aren't male crocodiles larger than females?  I know
>there are birds in which the females are larger.   My point was that I have
>heard the African lion analogy used a few times to compare Tyrannosaurs &
>analogy seems wholly untenable.  African male lions outweigh the females by
>20-30%.  So, if Tyrannosaurus rex males were smaller (& I think the
>supports this) the "lion strategy"
>wouldn't seem to hold.  Or perhaps T.rex had no permanent social
>interaction with its own genus except at mating time???
>Sexual dimorphism in crocodiles can be just as extreme as in lions (maybe
>For instance in crocs like _C.porosus_ a male CAN clock in at 7 meters
while a
>female might not get much larger than 4 meters.
>Sexual dimorphism in other reptiles is equally diverse
>In turtles females are usually larger than males
>Same thing with snakes.
>Lizards usually have males being larger than females.
>So a larger female rex could happen (and it does seem to be the case)
>As for sexual dimorphism in other deinosaurs, it's probably equally as
>(in the species that have it)
>That actually brings up another question.
>What other deinos seem to show sexual dimorphism?
>Archosaur J
>Jurassosaurus's Reptipage: A page devoted to the study of the reptilia
>Get free e-mail and a permanent address at http://www.netaddress.com/?N=1