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Re: Bipedal lizards and pterosaurs




On Tue, 5 Dec 1995 DPterosaur@aol.com wrote:
> In lizards capable of bipedal progression two things stand out. The tail is
> longer, thinner, stiffer and has reduced hemals arches and transverse
> processes (see Snyder 1954, 1962).  In addition, a tiny little anterior
> process of the ilium is the source of a tendon that helps keep the body erect
> over the hind limbs.  In basal pterosaurs these features are taken to the
> extreme.  The tail is extremely long, thin and stiff and the hemal arches and
> transverse processes are vestiges.  In addition, the anterior process on the
> ilium is greatly lengthened, especially so in the larger forms.
        The use of the tail rudder- actually, more akin to a steering 
oar- can also explain the unusual modifications in the tail. And wouldn't 
the lack of a tail make full-time bipedalism difficult among anurognathans 
and pterodactyloids? 

> Sacral count is another indicator of bipedalism.  In bipedal lizards, the
> sacral vertebrae are more robust than those of their quad cousins.  In
> pterosaurs, the sacrals are not only more robust, more vertebrae have become
> sacralized, increasingly so in larger forms.
        Ankylosaurs get up to eight sacral vertebrae, titanosaurs also 
have extras in the sacrum. Bipedal theropods do get more vertebrae over 
time, but never as many as say, pterodactyloids or birds or armored
 dinos  (I have an eagle sacrum 
with some huge number of vertebrae, pterosaurs sometimes got up
to ?10?). So while development of the hips may be linked to bipedalism in 
some ways, it might not be what is going on in this case, or at least, it 
isn't the whole explanation.

> What happens in pterosaurs is this, the femur rotates at whatever angle to
> the pelvis (determined by the angle of the femoral head--which varies
> considerably), but the tibia, due to its ninety degree articulation with the
> femur, moves parasagittally beneath the body.  I have articulated skeletons
> of pterosaurs based on casts and original material on which this can be
> demonstrated. 
> 
> One more thing.  You do realize that when pterosaur wings are placed in the
> quadrupedal position the wings open ventrally.  If they are to open
> laterally, that is, to extend for flight, the pterosaur has to get off its
> manus to deploy them.
        Bipedal takeoff, I'll agree with (although animals as big as an eagle 
can get airborne simply by throwing themselves into the air). The rest is 
certainly stuff to think about.
        thanks, 
                 nick L.