Re: More hand rotation

[GSP1954@aol.com]

Some mammals like us can rotate our hands via radius and ulna rotation a full 
180 degrees, which is why we have such well developed adaptations for this 
action. For prosauropods to have been able to held their hands with palms 
directed medially when bipedal versus the palm directed postero-medially seen 
in trackways would have required only 70 or so degrees or rotation. Perhaps 
less if a little humerus rotation was involved. Because the shoulder socket 
faced strongly posteriorly and lacked a major lateral component it was not 
possible to use the crocodilian system and use strong humerus rotation and 
outward bowing of the elbow to achieve the backwards facing palm recorded in 
prosauropod and other dinosaur trackways, so their lower arm bones must have 
been crossed when walking on all fours, albeit to a modest degree. These 
factors indicate that lower arm rotation was possible in those dinosaurs in 
which the radius and ulna did not interlock. This limited rotation was 
probably achieved by sliding the distal end of the radius relative to that of 
the ulna. It is difficult to tell what was going on because we have no living 
archosaurs with backwardly directed shoulder joints and that walk both 
bipedally and quadrupedally. Curse that K/T extinction. In any case the 
rotation may have amounted to only a few degrees in strongly quadrupedal 
examples. The only dinosaurs definitely known to lack any lower arm rotation 
are hadrosaurs since their radius and ulna tightly interlock. 

GP