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Re: Pubic Pendulum

To Michael Skrepnick:

> an attempt to lower the centre of gravity to 
> increase stability. 

<sigh> that's exactly how I should have said it. 

> I'm wondering however if the weight inherent in the
> distal shaft and boot of the pubis (little 
> musculature or soft anatomy present) alone
> would become much of a factor in stabilizing several
> tons of flesh overhead.

I think that ultimately hinges on the developement of
the air sacs in theropods. The shape of the body
itself would also determine how weight is shifted
around. To use the balancing bike analogy again, if
you layed flat across the seat with legs outstretched
behind, you would be more stabilized by the pendulum
than if you were sitting on the handle bars.
Obviously, theropods had fairly elongate bodies, so
they were probably pretty stable when compared with us
humans (who need a large cerebellum just to stay
upright) and the "pubic pendulum" could have increased
their stability.   

> but in terms of the more pronounced neural
> spines in the cervico-dorsal region, I wonder if 
> that is simply a way to help support the
> massive head in these dinosaurs. 

Oh yes, undoubtedly. Large neural spines could help
dinosaurs like Spinosaurus aegyptiacus lift their long
necks and heads including their very heavy meals (i've
heard the fish were HUGE....). Bison have a system
like this I think - elastic ligaments to help support
their big heads.   

> what other evolutionary advantage is to be gained in
> possessing a distally expanded boot, while many of 
> the more primitive theropod lineages ( comprising 
> both large and small animals ) seem to have gotten 
> along fine without it?

Basal tetanurans seem to have enlarged ischia to
support anything that the pubis didn't. Coelophysoids
were light and had thin ischia and pubes, so maybe
they could flex somewhat under the body's weight
(Dilophosaurus could be an important exception). If
i'm right about the opisthopubic condition evolving as
a compromise between a horizontal body and a shortened
tail, the orientation of the pubes might gives us
clues to posture. Other possibilities are that it
evolved to get the pubes out of the way for perching
(in the case of small dromaeosaurs) or shifted the
center of gravity rearward where the mobile tail would
make use of it during inter/intraspecific combat and
hunting. Troodontids throw a monkey wrench into this
idea, though. If it acted as a true pendulum, its
leverage would help the animal raise its upper body
from bending forward - in addition to the tail. Can
you think of any others?     

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