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Re: Thou Shalt Not Climb!

Tim Williams (twilliams_alpha@hotmail.com) wrote:

<In order to get from the propubic to the opisthopubic condition, the pubic 
shaft had to swing through the "no-man's-land" of mesopuby.  Many bird-like 
maniraptorans and some basal birds have a mesopubic pelvis, such as 
_Archaeopteryx_, unenlagiines (including _Rahonavis_), scansoriopterygids, and
jeholornithids.  A mesopubic pelvis would seem to be the worst possible
configuration for a climber, given that the pubic shaft is directed vertically
(or subvertically).>

  But the mesopubic condition does confer some precipatory advantages that
propuby would no for avian-like dinosaurs: It gives the animal more gut space,
and it brings the pubo-ischio-femoral musuclar complex more medial and
posterior to the path of femoral rotation than in the propubic condition. It
requires the femur to be held more horizontally in stasis than the propubic
condition, and enforces a stonger crouch to maintain balance relative to the
center of gravity. These features would seem anticipatory to the modern avian
condition. As for climbing, the pubis appears to maintain the same relative
distance given a horizontal sacrum and the knee when the femur is positioned
exactly vertical, as when the pubis is procumbently oriented and even in those
animals when the pubis is opisthopubic. The difference in how the body is held
is that the chest can more closely contact the substrate without the pubis
projecting between the knees in the last case, but the mesopubic condition
would seem to be easier to crouch and bring the chest to the substrate than the
propubic condition. Brooding may have even assisted this process.


Jaime A. Headden

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

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