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patagia, etc. (fwd)



Following some recent discussion on pterosaur biomechanics, evolution etc., I
though some of you might be interested in seeing this message Dr. Kevin Padian
posted on the list some weeks ago, particularly as certain listers seem
determined to rouse Dr. Padian from lurker-hood. My apologies to those who
remember it from the first time round, and to Dr. Padian if he'd rather I didn't
re-cycle his material in this way. 

DARREN NAISH

Here it is:

Darren Naish is certainly entitled to his opinion, and the net should be a place
where free ideas can be exchanged.  It may be worth pointing out, however, that
dino-fandom, which is wonderful and healthy, can generate testable propositions
when it enters the realm of science.  Those who enjoy crossing the line from
science fiction to science may wish to read on.

Pterosaurs were a really interesting bunch of animals, but to understand
anything about them one has to study allometry, phylogeny, functional mechanics,
and aerodynamics, as well as general paleontology.  Pterosaurs were not related
to "ornithosuchids," which are close to phytosaurs and the other pseudosuchians
(sensu Gauthier 1986); rather, they are ornithodirans closely related to
dinosaurs, and Scleromochlus is their closest outgroup (Padian 1984). 
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.  To
claim that any set of derived pterosaurs did differently requires a complete
functional analysis, which entails not only first-hand analysis of the specimens
but also a detailed testable model of what they actually did -- which has never
been produced, to my knowledge.

Size increases in pterosaurs produced interesting effects.  Because mass
increases as a cubed function of any linear dimension, but wing area only in-
creases as a squared function, wing area must increase faster than mass in order
for wing loading to remain realistic; hence, shape changes with size.  This is
why the legs of large pterosaurs appear spindly; they are perfectly large when
compared to the torso (Padian, Carter, and Van Der Meulen, 1992). As pterosaurs
grew, they had two options to maintain bone strength. They could have retained
proportionally thick bone walls, but to increase the DIAMETER of the bone shaft,
while retaining very thin bone walls, the shape of these larger bones had a far
more advantageous second moment of inertia, which is the crucial factor in
resisting the stresses of flight (Van der Meulen, Padian, and Carter, 1992). 
Hence, the appearance of the hypertrophied fore-limbs of large pterosaurs is an
optical illusion:  they are all "mailing tubes and styrofoam," as Don Baird used
to say.

Where did the wing membranes attach?  Difficult question, in view of the fact
that the functional morphology of the hindlimb is so generally invariable among
pterosaurs.  So far, no studies of pterosaurs with wings apparently attaching
to hindlimbs have analyzed taphonomy.  Why for instance would the Vienna
Pterodactylus have membranes that attached to the BONES of the thigh?  David
Peters produced a quite reasonable alternative explanation of the classic Sordes
specimen in a recent reply in NATURE to Unwin and Bakhurina's paper.  If there
are other specimens supporting one claim or another, they have not yet been
described.  

Why did pterosaurs retain functional hands?  Well, let's put it another way. Why
didn't they evolve hooves, if they were quadrupedal?  :) All this is meant to
suggest that simple inspection of artists' reconstructions of pterosaurs is
hardly enough to decide questions of their paleoecology and functional
morphology.  However, a good painting does more to show us how realistic or
unrealistic an idea is (assuming the illustration is correct) than thousands of
words may.  So with the above caveats, I tend to agree with much of what Darren
Naish says.  The next step for Dino-Netters is to couch this stuff in testable
hypotheses and show us which research directions to take next! -- kp