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Re: Pubic Pendulum



Waylon,

Sorry, I didn't address the second part of your commentary in my first
posting.  My focus was directed towards the act of attaining a resting
posture simply because as a paleo artist, I've pondered over the mechanics
involved and addressed this issue because you commented on the undue stress
that could have been placed on the ischia without other support.  I think
this might have been more problematic in smaller bipedal ornithischians.
I understand that what your referring to is in fact an attempt to lower the
centre of gravity to increase stability.  My understanding of theropod
trackways is that at a slower gait the tracks are offset from each other
forming a classic bipedal "zig-zag" pattern but that as the trackmaker moves
faster the separation between left and right footfalls draw in closer to the
midline as speed and momentum moves the animal forward. I think your correct
in assuming that the animal would be more vulnerable to an attack from the
side as it's progression forward is along a narrower path.  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.  As you have also
suggested smaller, feathered theropods with the assistance of aerodynamics
are going to be more nimble and able to dodge counterattacks from either
prey or pursuer.
Leg muscles must be able to retain elastic energy which is going to help the
animal manage it's weight as well as keep it balanced.  It would be counter
productive for a napping Tyrannosaurus to suddenly fall over on its side
because it couldn't retain tension in its leg muscles.
Leon Classens, a student from Denmark, has done a considerable amount of
work in the function of gastralia that seems to show an intimate
relationship to breathing.  Gastralia must also play a supportive role ( no
pun) in reinforcing the abdominal musculature supporting the viscera, etc...
The spinal column in most vertebrates is under compression whereas the ribs
and abdominal musculature work in unison under tension keeping the pelvic
and pectoral girdles from separating and therefore working to keep
compression on the spine like a bow and bowstring.( see guide to Vertebrate
Mechanics by Chris McGowan pg.120-124 for some really interesting anecdotes
on this ).  If this is the case then the forces of tension between the
girdles and gravity's pull on the guts wouldn't seem conducive to implying
the gastralia as a load-bearing structure from head through pectoral girdle.
Neural spines must help solidify the "dorsal arch" to handle the weight of
the body 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.  In tyrannosaurs the angle at which the
cervicals meet the dorsals is somewhat severe so that the weight of the head
is transmitted downward on a more vertically oriented column of cervicals,
perhaps resulting in less stress on cervical musculature trying to stabilize
the head over the body.  Giganotosaurus seems to have its head and neck
oriented in a more horizontal fashion, and even though the length of the
cervical series is less than the length of the skull, the sheer mass of such
a head ( regardless of air sinuses, etc...) might require additional muscle
mass ( and therefore attachment points) to functionally support those big
jaws.
Back for a moment to the pubic boot issue.  The question I have is that
other then functioning as a rest support system, 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?

Cheers,

Mike Skrepnick





>
> To Michael Skrepnick:
>
> I'm not debating whether theropods used their pubic
> boots to rest on. I'm saying that it could have a dual
> purpose and greatly increase stability when large
> carnivores attacked their prey. If T. rex had no boot,
> and was struck by a frightened hadrosaur or attacked
> by another of its own species, it would be easier for
> the beast to topple over from the force of impact.
> Have you ever seen that bicycle on TV which rides
> across a suspended wire with a pendulum beneath it?
> Well, i've heard that trackway evidence suggests many
> theropods walked in a bird-like one foot in front of
> the other style - analogous to the bike on the wire.
> Maybe it increased stride efficiency over great
> distances....which would be helpful if you were a T.
> rex following the migration of hadrosaurs. As for the
> small propubic species with somewhat short tails like
> the Oviraptorosauria, i'm not sure. They probably
> relied more on their primary/retrice feathers for
> stability. BTW, wouldn't the leg muscles need to be
> flexed when sleeping/resting to reduce stress on the
> pubic shaft? If they weren't, total body weight would
> be transferred to the pubis, possibly the ischium, and
> the tail. This gets me thinking about the role of
> gastralia in suspension of the thoracic portion of the
> body. Maybe the reason we see co-ossified gastral
> elements in Giganotosaurus is because the weight of
> the head, neck, shoulders, and rib cage were being
> placed partially on the gastralia and then to the
> pubis. This could also be the reason some of these big
> theropods have large neural spines....they act as a
> suspension bridge to help reduce the dorsoventral
> stresses on the centra/zygapophyses while resting. In
> fact, I think the "pectoral" or "cervicodorsal" neural
> spines of G. carolinii have been enlarged....possibly
> to make its upper body rigid? The thick neural spines
> of T. rex could probably do the same, and may be one
> of the reasons that species adopted a more compact
> frame. Your thoughts on this?
>
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