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pterosaur and lizard walking



Jaime Headden wrote:
>>
Could you clarify for me? Much of my reasoning involves the very forward

center of gravity anatomy of the femur, which shows that unlike lizards
it
was not truly capable of a good deal of lateral direction; if it tried
to
walk while vertical, and its hip was vertical, it would fall. If the
animal walked bent-kneed to pull the CG closer to the ground and with
the
fulcrum of the feet better separated, this will cause its gait to become

very ungainly, and the feet to turn outwards, which will force the femur

to act in a lizardlike fashion and rotate outwards on the forward step;
such a step is at odds with ankle anatomy, and will cause a vertical
animal to swing side to side, which humans solve by having joints in
line
with the plane of travel, as do birds.

  I am sure that some pterosaur can solve the problem, but it seems that

essential anatomy is against this to some degree in rhamphorhynchoids
and
pterodactyloids that I have looked at, to any degree.
<<




I'm not sure what the precise question is, but it appears to revolve
around the pterosaur/lizard method of standing or running bipedally. If
so, here are some comments:

Jaime, go check out the bipedal lizard running mpgs I sent to the list
about two weeks ago. You'll notice that the whole body remains
relatively unmoving as the legs do their thing.

It's okay that the feet turn outward when quadrupedal. They move toward
the parasagittal plane while bipedal. The hips rotate axially to make
this possible. Rotodactylus tracks, which I ascribe to cosesaur- or
langobardisaur-like prolacertifoms are occassionally bipedal,
narrow-gauge and the toes point completely forward, despite their
lizard-like anatomy (laterally longer toes).  It surprised me to learn
that the most primitive pterosaur also had toes that were remarkably
longer laterally, despite haveing a shorter metatarsal IV than III. In
the next most primitive pterosaur, that situation appears to be
remedied.

The important joint in pterosaur terrestrial locomotion is the femoral
joint. Not much else had to move much. In basals forms it is not
well-developed, but by the Mid-Jurassic the ball joint with constricted
neck is well-formed. In my opinion, nearly all of the movement of the
hind limb occurred at the acetabulum, which when you add all the angles
of the pelvis and femoral head together, permit the femur (knee) to
travel in a parasagittal arc at varying distances from the sagittal
plane. You're correct in noting that the knee joint is hinged at an
angle to the parasagittal plane, as is the mesotarsal ankle, but that's
okay, because they don't move much. The major hinge lines of the toes
are also at an angle to the main axis of the foot, but in the above
scenario, the hinge lines become oriented at right angles to the
direction of travel, just where you want them to be.

I have a bipedal skeleton of Pteranodon that demonstrates this very
nicely. If you're ever in St. Louis swing on by.

Regarding the tilt of the spine: No pterosaur has a vertical spine, but
45 to 60 degrees up seems to be the range. Some bipedal lizards are
nearly horizontal surprisingly.

Regarding the splay of the femora: That varies widely in pterosaurs,
with some of the most primitive forms having the most parasaggital
femora. Later forms evolved a femur that could more easily remain
splayed in the plane of the wings during flight. The splay doesn't
matter though.

Even if the femora were lateral to the body, you could still achieve
narrow-gauge bipedal progression -- and you can show yourself how this
was done by doing this simple trick: Make two "guns" out of your hands
by extending the thumb and index finger only. Your thumbs are pterosaur
femora. Your index fingers are tibiae. Now touch the tips of your thumbs
together and rotate your new "pterosaur legs" as if walking them. Note
that if you keep the plane of the knee as high as your thumb tips (where
the axis of rotation passes through), then the legs are very widely
spread, as Wellnhofer has illustrated in his Encyclopedia. But if you
bring your index finger tips more or less together and continue "walking
your pterosaur legs" you will see how a narrow-gauge track can develop
rather easily by dropping the knees slightly relative to the acetabulum.
Continue doing the "pterosaur walk" but now swing your elbows out. That
should rotate your thumbs so they point back toward your chest,
approximating a dimorphodontid femur in which the femora are more
parasagittal. Keep the axis of rotation transverse. Note that you need
not move your index finger relative to your thumb in either case, so
knee bending is not required, except to cushion each step.

Hope this helps.

David Peters
St. Louis