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Re: Pubic Pendulum



Hi Waylon,

In working on some recent restorations of Suchomimus and local fauna, the
fish ( including forms of coelacanths, gars and lungfish ) in many cases
indicate in and around the 3 to 6 foot range.
Nuchal ligaments seem to be a very probable system in holding heads high.
The case for opisthopubic condition is interesting as you say for
modifications to balance and also changes in locomotion.  For instance, how
does the re-orientation of the leg musculature origins and insertions for
the pife, adductors and so on effect leg excursion, power in protraction /
retraction of the hind limb in movement, etc...  While hanging onto prey is
the downward slicing and disemboweling stroke made more effective and
similarly could subsequent change in the animals posture have something to
say about behaviours ( going further out on a limb, me not the dinosaur ).
Could you infer that contraction some of the adducting musculature attaching
to  more backward oriented pubes / ischia are an adaptation to a more
vertically oriented body by velociraptorines descended from Utahraptor grade
predators, in order to maintain a higher visual vantage point utilized in
observing potential prey.  This in opposition to troodontids with a more
horizontal orientation that might better serve a predator hunting for
smaller animals within a more localized area, to be pinned to the ground
with its smaller raptorial claw and dispatched in a more conventionally
"birdlike" manner.

Mike S.


>
> 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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