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Re: Pubic Pendulum



It seems to me that in most theropods outside of the opisthopubic condition
that the following steps come into play as the animal moves into a resting
position.  First of all balance is maintained over the hips so there is no
appreciable shift in weight as the animal collapses it's limb elements in a
controlled descent while maintaining the upper torso in approximately a 45
degree angle.  The animal's weight is initially transmitted through the pads
of its feet, but as theropod trackways confirm, this is then distributed
further as the metatarsus contacts the ground.  Just before or after this
point ( depending upon the length of pubis in the theropod example and the
angle of attack ) the pubis is going to contact the ground, essentially
providing a "stop", which effectively prevents the upper leg musculature
from "overstretching" into an ungainly "squat" position.  It also would seem
that this is the most natural and efficient "ready" position from which an
ambush predator could most easily arise after resting and waiting for a prey
item to wander by.  At any rate, the weight is now distributed between 3
points of contact and also sometime during the above process, an area of the
proximal to midlength of the tail will also contact the ground and handle a
portion of the load through a dynamic stabilization provided by the caudo
femoralis and other caudal musculature.  With 4 separate contact areas
handling the dinosaur's weight, the actual amount of compression left over
to be sustained by the ischia must have been "comfortable".  If this was not
the case, I would expect to see museum collections over run with examples of
traumas and pathologies in large theropods and an evolutionary shift to
"something better".

Cheers,

Mike Skrepnick



: Pubic Pendulum


> Hooray! I got my Predatory Dinosaurs of the World book
> today.....beautiful illustrations, my hat goes off to
> you, HP G.S.Paul. One thing I would like to speculate
> on after reading page 118, is where it discusses the
> function of the distal pubic foot of many theropods.
> It says that the pubic foot could have been a "rocker"
> which theropods rested upon. In restorations of
> Allosaurus, Giganotosaurus, and Tyrannosaurus the
> angle of the pubic foot would seem too steep to
> support the animal. If you drew a line (which would
> indicate the surface of the ground contacting the
> pubes) from the tip of the foot to the proximal
> portion of the tail it would intersect the ischia. I
> suppose the distal expansion of the ischium in large
> carnosaurs would help support any weight not shifted
> to the pubic foot, but that of T. rex seems weak, and
> the estimated 7 tonnes of theropod mass might snap the
> ischium in half (which would undoubtedly make it
> painful for T. rex to relieve itself). Dr. Holtz would
> have to confirm this, because I haven't seen a rex
> skeleton in 14 years, but it seems like a safe
> assumption. But what if the pubic foot was being used
> as a stabilizing mechanism in these larger theropods?
> It might make them less top-heavy, and aid in faster
> locomotion. In essence, a "pubic pendulum". This
> effect would be even greater if the trunk was
> lightened from air sacs. In small bird-like theropods
> like Sinornithosaurus, the posteriorly-directed pubic
> boot would make up for the weight discrepancy created
> by a shortened tail. Anyway, just thought this would
> make for an interesting discussion....???
>
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