Re: More hand rotation

["Matthew Bonnan" <mbonnan@hotmail.com>]

Hi Greg, all:

Greg said:

"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."

Well, the radius and ulna of elephants are typically "loose" in the sense 
that they are not fused or locked together, and yet the manus of elephants 
is permanently pronated.  In some older individuals they may become fused, 
but typically both bones are "free" of each other.  The proximal end of 
their radius is triangular in shape and fits snugly into a triangular radial 
fossa on the proximal end of the ulna on the anterior and lateral side.  
This precludes any rotational movement of the radius. Pronation is achieved 
by the shaft of the radius crossing the ulna about 1/3rd to 1/2 way down the 
forearm.  The shaft of the radius is practically sigmoidal and this allows 
pronation of the manus.  In fact, elephants do not have the pronator or 
supinator muscles typical of many mammals.

In alligators, the radius and ulna are not what you would consider "locked" 
either, and yet the radius does not actively rotate about the ulna -- in 
fact, mammals are really the only terrestrial verts I can think of that 
really develop pronation and supination to any great degree using radius 
rotation about the ulna.  And even here, it's really only well developed in 
a few groups.  As an interesting sidenote, the fused radioulna of frogs can 
rotate to pronate their manus!  Amazingly, the distal end of their humerus 
has an almost spherical articular surface that allows the fused bones to 
rotate -- cool!

I agree with Greg that the strong locking of the radius and ulna in 
hadrosaurs is for stability of the forearm during quadrupedal locomotion, 
perhaps at higher speeds, because you wouldn't want your arm to twist on you 
when trotting or whatever -- it might cause serious damage! =)

Greg continues:

"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."

Perhaps, but then the proximal end of the radius is triangular or 
wedge-shaped in prosauropods.  Even though the radius and ulna do not lock 
together in the sense they may in hadrosaurs, there is still a 
part-counterpart relationship between the two bones and if you try to rotate 
the radius about the ulna you'd either have to break the bones or severely 
dislocate them.  If you can imagine holding your hands so that they face 
inwards toward each like you're praying, but now imagine that you cannot 
rotate them so that the palms face down -- you're stuck with your palms 
facing in towards each other.  How would you pronate your hands?  By 
pointing your elbows outwards -- try it yourself dinolisters to get an idea 
of what I'm getting at.  Maybe when prosauropods dropped to all fours their 
elbows were somewhat flexed and pointing outwards.  Because the hands would 
angle in, this would still work with the known trackways.

In animals like ourselves where pronation and supination are well-developed, 
a rounded radial head and an almost spherical radial condyle (capitulum) on 
the humerus are required for these movements.  Granted, Greg is not claiming 
anything like this for dinosaurs, but we just don't see anything close to 
this condition in the large herbivores.  It is on this basis that I am 
doubtful that the radius was capable of much rotational movement about the 
ulna in the large herbivorous dinosaurs -- prosauropods, sauropods, 
thyreophorans, ceratopsians, etc.  In fact, this makes the functional 
morphology of dinosaur forelimb so interesting to me -- what on earth were 
these guys doing to get the pronation we see in the trackways, and how 
interesting that it would be different from what mammals typically do.

Interesting topic, Greg.

Matt


Matthew F. Bonnan, Ph.D.
Department of Biological Sciences
Western Illinois University
Macomb, IL 61455
(309) 298-2155
mbonnan@hotmail.com
MF-Bonnan@wiu.edu
http://www.wiu.edu/users/mfb100/

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