Re: The Peters Strikes Back (pterosaurs)

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

Chris Bennett (cbennett@bridgeport.edu) wrote:

<Most humans seem to be able to stand AND walk bipedally without toppling even 
though they lack a stabilizing tail [even I can do it
a lot of the time!].  All it requires is that one keep the center of gravity 
over the polygon enclosing the hind feet or temporarily
put it in front of that polygon so as to move the body forward.  Ignoring the 
question of whether any pterosaurs actually were
habitually bipedal, I would ask whether there is any reason to think that some 
pterosaurs could not have been bipedal. The point I
made about Nyctosaurus in my 1997 terrestrial locomotion paper (JVP 17:104-113) 
was that the forelimbs were so long that attempting
to put Nyctosaurus in a quadrupedal posture made the trunk and hindlimb almost 
as erect as they would have to be in bipedal
locomotion AND also put the center of gravity really close to the polygon 
enclosing the hind feet.  If the hindlimb and trunk are
that erect and the hindlimb is bearing almost all of the weight, then why not 
walk bipedally?>

  My argument is that, unless the pterosaur is vertical or nearly so, then the 
stable CoG (either static or dynamic) when standing
could not be maintained unless it perhaps had to take really long strides ... 
the head must point forward, and provides a
de-balancing effect aht doesn't seem possible to compensate with the 
wing-finger is the rest of the arm is folded forward of the
hips. Even in *Nyctosaurus,* if it stands or tries to walk, would this not 
still provide a better stability if it used its arms to
walk with, especially since the CoG is still forward of the footprint? That 
when the animal is in motion, it will noticeable shift
the CoG even further in front of the feet, even if at some point the CoG comes 
above the feet, and would more or less stay in front
of the feet.

<Do the numbers?  You don't need to do any numbers, just use your common 
sense!.  Extend the hindlimb and balance the center of
gravity over the polygon enclosing the hind feet.  What is so hard about that?>

  It would have to do this every time it took a step, squat-walking perhaps. It 
is possible that the effect pterosaur leg was the
tibia and pes, rather than inclusive of the femur, as in birds. This would 
place the CoG during bipedal stance much closer if not
between the feet durning the walking phase in some of the pterosaurs like 
ctenochasmatoids or azhdarchoids, having relatively long
legs.

  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)