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Re: patagia, etc. (fwd)



My thanks to Darren Naish for reposting Dr. Padian's fascinating reply on
pterosaur stance and locomotion, which I missed the first time around.  I
have a few questions, though, which Dr. Padian (or others) may wish to address:

I am a firm believer in functional analysis and the production of testable
hypotheses as opposed to random speculation about what extinct creatures -
particularly those with no living analogues - might have done (as those who
will recall our debate about ankylosaur "mimicry" - mentioned, BTW, in the
current issue of EARTH magazine, so I guess somebody reads this stuff - will
know).  However, the mere statement that the conservatism of pterosaur
hindlimb morphology proves that all pterosaurs were obligate bipeds (and I
know that is not exactly what Dr. Padian said) raises some caveats, at least
to me.

First of all, we do not know for certain that the pterosaur ancestors never
used their forelimbs in stance or locomotion.  Clearly they were bipeds, but
even a biped can use its forelimbs either as a prop for resting or as an
assist in climbing.  I am not aware, for instance, that the hoatzin, which
uses its forelimbs for quadrupedal climbing when a chick, has any functional
modifications to the pectoral girdle to allow for this (it may, of course -
my lack of awareness is just that, ignorance).  In fact, as the hoatzin is
the only living dinosaurian biped to do this, it might be an interesting
study for a functional morphologist.  So, testable hypothesis #1 would be,
limited use of the forelimbs in climbing need not require a functional shift
in the hind-limb structure, and the test (admittedly a not entirely
conclusive one) would be an analysis of hoatzins at various ages.

Second, if hypothesis #1 is correct, the question arises as to how much of a
shift would be required in the hind-limb structure to accompany a shift to a
more quadrupedal posture?  For instance, if pterosaurs ran bipedally but
rested on their forelimbs  when stopped, or used the forelimbs in climbing
to a greater degree, what sort of shifts would we expect to see - or, more
to the point, if the wings were just the right length to support the animal
when resting without a shift in the position of the spinal column relative
to the pelvic girdle, need there be any modification to the hindlimb
morphology at all?  So hypothesis #2 might be that pterosaurs may have been
able to adopt quadrupedal postures (at least when at rest) without such
modification.  One ought to be able to produce a functional model for this -
for example, what length would a pterosaur forelimb have to be to prop up
the animal in the position resulting in the least alteration of the hindlimb
posture? - and see if pterosaur wings are at all constructed to fit such a
model, consistent with their primary function as wings.  Probably a similar
model could be developed for a pterosaur climber.

It strikes me, further, that the problem of balance or top-heaviness is not
so much one of the size of the forelimbs but of the skull, which certainly
appears, in the later pterosaurs at least, to be very large for the body.
Admittedly the skull is hardly solid bone, but I still wonder if a bipedal
Pteranodon, for example, might not have had a tendency to pitch forward on
its nose, at least when at rest.  Once again, functional analysis could
answer this.  First, is the topheaviness an illusion; second, if it is real,
could postural alterations allow for this and are there any modifications in
pterosaurs supporting such postures (eg the ability to retract and flex the
neck to bring the center of gravity of the skull as nearly as possible over
the center of gravity of the postcranial skeleton when in a bipedal resting
posture).

This also makes me wonder about Pterosaurs like Pterodaustro and
Ctenochasma.  If these were indeed filter feeders (which seems highly
likely, I suppose) they must have experienced drag on the lower jaw whlie
drawing the skull through the water.  If they fed in flight, I would expect
structural modifications to prevent this drag from catapulting the animal
into the sea (for a living example see Dr. Richard Zusi's classic study of
the mechanics of the Black Skimmer, which also drags its lower mandible
through the water when in flight although it is not a filter feeder - the
reconstruction of Ahanguera in Wellnhofer's book shows it snapping up a fish
in a very skimmer-like manner).  If they fed while standing, the use of the
forelimbs as a brace to compensate for drag (as shown in the illustration in
Wellnhofer's book) seems reasonable and if that was not what happened I
would expect to see some  functional evidence of this.

Finally, though Dr. Padian is convinced of the bipedality of pterosaurs for
the reasons he gives, others are clearly not, and those of us not actually
working on these animals are left very confused (well, I am).  How do I
reconcile this statement of Dr. Padian's:

"Ornithodirans evolved from animals that were already small, bipedal, active
runners with long legs, as their synapomorphies show.  Through 150 million years
of evolution, pterosaurs NEVER altered the functional anatomy of their limbs
toward any other kind of locomotion, as far as preserved specimens show."

with this statement (and others like it, eg at p. 56) from Wellnhofer, p. 126:

"Three-dimensional reconstruction of the pelvis of Ahanguera revealed that
the hind legs could not be brought into a vertical position under the body,
but were splayed slightly to the side.  Thus bird-like, bipedal locomotion
on the ground was scarcely possible.  Orientation of the hip sockets
obliquely upwards and the slight bend of the articular head of the thigh
bone make quadrupedal locomotion on the ground more probable."

Is this in part an argument about which outgroup taxon (Scleromochlus v.
Lagosuchus, or whatever) represents the primitive condition of the pelvic
girdle and hindlimbs?

I trust Dr. Padian will excuse (and perhaps  even answer) these meanderings
from a might-have-been functional morphologist who is happy outside of
academe but occasionally misses days spent bent over a dissecting scope.
--
Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
International Wildlife Coalition              Fax/Modem: (905) 569-0116 (home)
Home: 1825 Shady Creek Court                  Messages: (416) 368-4661
Mississauga, Ontario, Canada L5L 3W2          Internet: ornstn@inforamp.net
Office: 130 Adelaide Street W., Suite 1940    
Toronto, Ontario Canada M5H 3P5