ALIMENTARY MY DEAR HOATZIN

["Matthew Troutman" <m_troutman@hotmail.com>]

<<Okay, forget the hindlimbs, let's examine the ulna, radius, and
wrist of theropod dinosaurs.  It is not what we would see in a climbing 
mammal.  Why not?  The ulna and radius, while crossed to varying degrees, 
are not very mobile (in fact, in most cases almost complete immobile!), and 
therefore the hand and forearm cannot be manipulated well to accomodate 
efficient  grasping and climbing.  Further, the wrist bones in the most 
bird-like  dinosaurs (maniraptorans) allow motion essentially in one plane 
-- that's the whole deal with the semi-lunate carpal maniraptoran wrist 
being similar to  birds.>>

Hoatzins, perhaps the only avians that we can get a guess towards the 
climbing forelimb motions of maniraptoran (or maniraptoriform) theropods 
from, need not be flexible in their wrists or ulna and radius when in their 
juvenile climbing/swimming stage.

Anyway, although most arboreal mammals have very different wrists from 
theropods, they do allow the motions of the maniraptoran wrist, including a 
"flick" towards the end of the climbing stroke in sloths functionally 
identical to that of the maniraptoran wrist motion.

<<In huge contrast to this, primates, squirrels, and other tree-climbing 
mammals have large degrees of supination and pronation in their arms -- the 
radius ACTIVELY crosses the ulna allowing the hand to swivel back  and forth 
freely, and furthermore the carpals in the wrist allow several degrees of 
freedom at that joint.>>

We're talking apples and oranges here.  What you are describing is a system 
of climbing similiar to that of a monkey climbing up branches when the 
adaptations of the maniraptoran forelimb indicate something different, 
perhaps tree creeping or trunk hugging.  The colugo (_Cynocephalus_) has a 
forelimb very similiar to that of a maniraptoran: long, narrow digits; 
small, almost reduced wrist; elongate radius and ulna; elongate humerus with 
a triangular deltopectoral crest.

Some are working on the issue of tree-climbing in maniraptoran theropods, 
and are coming to surprising results in that theropods have many climbing 
features that in almost any other vertebrate they would unequivocably be 
considered tree-climbers such as their proportionally elongate forelimbs, 
recurved carpal claws, and the shoulder apparatus.

Matt Troutman
m_troutman@hotmail.com

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