More hand rotation

[GSP1954@aol.com]

Concerning Nicks oberservations on lower arm rotation in dinos and mammals. 
He note that in mammals in which lower arm rotation is not possible that the 
radius and ulna are locked together. It has been little appreciated that the 
same is true of hadrosaurs. At the distal end there is a peg-in-notch 
articulation between the two bones that, in association with a tight proximal 
fit of the radius into a sulcus of the ulna, renders the radius and ulna 
immobile relative to one another. This is a running adaptation that shows 
that hadrosaur arms were used primarily for locomotion rather than 
manipulation of objects that their short, pad encased fingers were ill suited 
for. It also indicates that hadrosaurs remained quadrupedal even at high 
speeds. 

In any case, if dinosaur lower arms were inherently unable to rotate, then 
why did hadrosaurs go to the trouble of locking the radius-ulna? Converesly, 
if dinosaur lower arms were inherently unable to rotate, then why weren't 
they all locked up? That most dinosaurs kept the radius-ulna loose suggests 
the retention of some degree of rotation, albeit less than primates and so 
forth. 

Prosauropod lower arms, for instance, articulate with the palm facing 
medially. But trackways show the palm facing strongly backwards, with only a 
modest medial orientation. This probably reflects rotation of the lower arm 
when using them to walk compared to when they are inert. Rather like how 
people often walk with palms inwards but direct the hand forward when on all 
fours. 

G Paul