Re: The Peters Strikes Back (pterosaurs)

["Jaime Headden" <ja_headden@qilong.8m.com>]

oops, I accidentally hit send prior to actually composing my message ... sorry.

Dave Peters (davidrpeters@earthlink.net) wrote:

<<You're assuming that pterosaur hind limbs are coupled. Hind limb diversity -- 
or _any_ kind of statistical analysis might not show
that. Only _evidence_ can show that.>>

Ron Orenstein (ornstn@rogers.com) replied:

<This is certainly true.  However, I think hind limb diversity is at least 
suggestive.  Has anyone done a statistical analysis of
the subject, comparing birds, pterosaurs and bats?  There is diversity in bat 
hind limbs too (think of the fish-catching talons of
the New World fishing bats, for example, or the limbs of mystacinids).  It just 
doesn't come within a mile of what we see in birds
(and if the hind limbs were not coupled, and pterosaurs were as aquatic as we 
think, I might have expected a few really long-legged
wading types to show up).>

  Yes. At SVPCA, 2002, Unwin presented data comparing bat, bird, and pterosaur 
limb proportions on ternary diagrams, sensu Gatesy
and Middleton for birds and non-avian theropods:

  "David Unwin provided a thorough analysis of pterosaur hindlimb morphometrics 
and locomotor modules. A la Gatesy and Middleton,
pterosaurs (from all groups) were plotted onto ternary diagrams and found to 
occupy a rather small morphospace which overlapped that
of both bats and birds (interestingly, the bird and bat clouds did not  
overlap). Bats occupy about 50% of the morphospace that
birds do, and pterosaurs occupied a small space than bats. Forms for which the 
wing membrane is known were scattered throughout the
cloud demonstrating that extensive patagia are not an unusual feature of one 
pterosaur subgroup." (From Darren Naish' report at
http://www.svpca.org/previousyears/2002/personalaccounts/dn2002-003.htm.)

  I would like to add something on this subject about possible means of 
recovering "coupled" limb mechanics. Though it is possible
for some animals to ander away from the trend of scaled hind and forelimbs 
relative to one another, and certainly this is true of
*Arthurdactylus* and *Nyctosaurus* with their ridiculously elongated forelimbs, 
comparative scaling does not have to occur because
of quadrupedal locomotion: inverse scaling can occur as one limb is favored 
over another, or they are more equal than not, as the
forelimb is grossly longer than the hind, or they are closer to one another 
than not; the basal condition favors relatively more
even limbs than in pterodactyloids, so that "rhamphorhynchoids" may alter the 
conclusions to favor a more "terrestrial" grade,
despite the absence of preserved terrestrial prints unequivocally 
rhamphorhynchoid (doesn't mean they didn't walk, though).
Extremely long hind limbs relative to forelimbs, as in some azhdarchoids, 
ctenochasmatoids, may favor an interpretation that they
were also quadrupedal "waders" or walkers, but this falls udner the same lines 
as "rhamphorhynchoids," such that one might think
having more closely equal limbs would mean "terrestrial;" however, given trunk 
lengths, one might draw different conclusions. While
its one thing to infer leg length has anything to do with terrestriality, I 
think this conclusion has a lot more to do with
physiology and myology than it does to do with just relative limb length or 
lengths of limb components. It is most likely that all
pterosaurs could assume a fully quadrupedal standing posture on the substrate, 
ground or otherwise. The limbs could certainly move
while on the substrate, so they could "walk;" but for some reason, it seems odd 
to some that walking was not habitual ... even bats
will walk if they must, however puny their hindlegs, and no pterosaur seems to 
be as badly designed for walking as the loon is,
quadrupedal or bipedal.

  CoG studies favor a quadrupedal stance is more stable and should be more 
favorable for the given pterosaurs, including
*Nyctosaurus,* and though any pterosaur can assume a stable CoG while standing 
bipedally, no study has yet to explain a stable CoG
that would prevent toppling while walking bipedally in pterodactyloids, where 
"rhamphorhynchoids" have a stabilising long tail.

  Cheers,

  Jaime A. Headden

  Little steps are often the hardest to take. We are too used to making leaps 
in the face of adversity, that a simple skip is so
hard to do.  We should all learn to walk soft, walk small, see the world around 
us rather than zoom by it.

  "Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)