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Re: Theropod hips



Larry Febo wrote:

<The pterosaur body plan is "top heavy". When they landed on their feet
they had a strong tendancy to flop over onto their forepaws as well.
This is why they have no clavicles.>

  One could argue that the acute angle of the scapulocoracoid, fusion
of elements, oblique arrangement to the notarium or unfused pectoral
vertebrae and the size of the humeral flexors and extensors would be
able to act as a shock absorber to the flexed humerus as the animal
came to alight on its arms (assuming it did this -- I've looked at
pterosaur landings on outcrops and trees, but not on flat land, and the
bauplan differs strikingly from birds, so there is needed research in
this area) and legs, or just the arms (will not get into the legs,
yet).

<Such fragile bones would have broken if they were present.>

  It should be noted that some archosaurs and basal amniotes had pretty
massive clavicular systems (clavicle, interclavicle, cleithrum, etc.). 

<The forces of the landing were transmitted directly to the acetabulum,
and therefor the socket had to be deep.>

  This would be true only if the capitis femoris was aligned linearly
with the femoral shaft, which it isn't. In this case, the head in
inclined and the base of the capitis (trochanteric region) is massive,
and transmits forces. However, with the reorientation of the neck, the
forces are concentrated at the knee, not the hip joint, and for this
reason the cnemial process has increased in size in theropod dinosaurs,
for stabilizing the mm. iliotibialis.

<Muscles and ligaments attached from the tip of the pubis, and the
middle of the femur would tend to prevent the limb from moving outside
of the parasaggital plane, (thus preventing possible dislocation at the
knee or hip socket).>

  The more slender the shaft and offset the head, the more delicate the
mid-shaft, thus dinosaurs express wider femora than deep, to resist
lateral bending. More slender shafts in large forms develop accessory
intramuscular crests that probably acted like riflings on pillars, to
prevent torsion and lateral bending by concentrating force in
specifically strengthened areas, rather than in general or at the
sides.

<The long pubis of theropods puts the distal end of this bone in an
excellent position to perform this function. Plus by it`s connection in
the midline by the pubic symphysis, makes an excellant triangular brace
against the pelvic girdle.>

  There are probably two reasons for the pubic boot, as far as I can
see: forcing the pubic hepatic piston in breathing cycles, and bracing
the gastrocaudal muscles that facilitated this. The bigger the
theropod, the bigger the boot. Not so in sauropods, whose pubes being
wide and thickened at the lateral margins acheive this effect, or
ornithischians, which may operate more using the prepubic blade than
the posterior end (Carrier and Farmer, 2000). I see a certain opening
for research here :). The muscles for the control of the lateral splay
of the femur during the step cycle (compression of the leg as the
animal goes down on one or both would force the elements to move in the
easiest direction, which in these forms is outward) are the mm. pifi
and pife, as I described in my last posts: one (pifi) attaches the
femur to the pubes toward the distal end [above the boot in those which
have one] and the ischium towards the proximal end [at the obturator
process], the other (pife) attaches at the proximal pubes [near the
obturator foramen/notch] and the distal end of the ischium, if not near
the ischiadic boot. These keep the femur from pulling out too far
during the step cycle, and back in when the force of the falling body
is released as the other leg takes the compressive forces when the
stride shifts. Orientation of the pubes in some carnosaurs
(*Allosaurus*, carcharodontosaurines that have a pubes and ilium
preserved), tyrannosaurines (oh, take *Tyrannosaurus*), oviraptorids,
the "dinobirds," some prosauropods and most sauropods (especially
titanosauriforms, is mesopubic, and this essentially reduces the moment
arm of the pifi to the femur and thus increases the direct effect this
muscle has on the hip, but decreases the degree to which it can affect
it [it pulls harder, but has less leverage]. Opisthopubic theropods
have the anchors to both pifi and pife posterior to the femur, and this
has the added effect of reorienting the animal's stride and thoracic
posture, for it pulls the center of gravity further backward as the
femur shifts forward to compensate. The moment arms of both pifi and
pife increase to give added leverage, but reduced strength in the pife
is comparable to the mesopubic condition in the pifi.

  Someone tell me if I'm a little offbase here. And here I am leaking
my write-up, again ... I intended to use this in a larger perspective,
but, oh well, might as well use this list to refine some things.... I'm
still looking at muscles and force dynamics, and do not have the
fortune of having this drummed into my head in college, and there are
_no_ bookshops to acquire a college textbook, and Gray's will be of
fairly little help.... Darn. Anyway, Romer, Colbert, Osborn have all
written large pieces on the musculature of the hip in dinosaurs and
other reptiles, with specialty pieces in Borsuk-Bialynika for
titanosauriforms (*Opisthocoelicaudia,* to be exact, but the hip
anatomy does not vary by much, to my observation), Gatesy for theropods
and general avian transformations, and Maryanska for ankylosaurs. I'm
afraid that I must exhaust my meager savings on a few assorted pieces
before throwing a few quid out on the pile for the big musculature
studies that I do need. I don't have a student loan or an inheritance
to lean on :) Needless to say, I know what my thesis is going to be, if
I get to choose, and I get to disect and enjoy radiographies to do it,
too :)

  Anyway, there is apparently a different mechanism in pterosaurs: the
pubes do not contact each other, but the prepubic bones do in all forms
that have them, and probably essentially took over the function of the
pubes as part of the mobile breathing apparatus as in non-avian
reptiles. I don't want to say too much ... research research research.

  I hope I didn't bore any body.

------

  Oh, for those not on the Vert Paleo list, HP Doc Holtz' class is a
must attend if you even knew what goes on in that room.... ;) :P

=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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