Re: Gliders to Fliers? (Was Re: Ruben Strikes Back)

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

Dinogeorge said:
> Nobody says bipedality is an adaptation >for< arboreality. Rather, it is a
> natural outcome for an animal >previously adapted< to climbing, that has
> essentially lost the locomotor aspects of its forelimbs and has taken the
> easier route to a terrestrial lifestyle as a bipedal animal instead of
> re-evolving a quadrupedal stance. This almost certainly happened with 
> humans
> (bipedality came before massive cranial enlargement and may even have
> permitted it) as well as with theropod dinosaurs.

This is where I would say we have to be careful.  As I mentioned, the 
cylindrical femoral head and hinge-ankle of theropods would not lead me to 
say "almost certainly" theropods are descendants of arboreal animals.  What 
of bipedal ornithischians?  All of these forms are bipedal because of 
arboreality as well?

What are the functional/structural features of the typical theropod skeleton 
that would suggest arboreal ancestry/habit?

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.

If your supination (palm side up) and pronation (palm side down) motions are 
restricted by BOTH your forearm AND wrist, this seems to me to be a poor 
adaptation for arboreality, nor does it suggest an arboeal ancestor if 
maniraptorans are terrestrial versions of once-arboreal dinos (or birds).  
The only thing maniraptorans have going for them is the laterally facing 
glenoid (shoulder socket) which allows the humerus to move out and in like a 
bird wing.  Most other theropods have ventral (down) and posterior (back) 
directed glenoids, which would restrict total arm motion even further.

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.

How does a stiff-legged, stiff-armed, restricted-handed animal evolve from a 
tree-dweller in which one finds very different anatomical characters?  
Again, this has nothing to do with systematics or cladograms of any kind.  I 
can just tell you from articulating the forearms of sauropods and theropods, 
you ain't getting the motions that would appear to be necessary for a 
climbing animal.  While some theropods could have pronated and supinated 
their hands, the range and degree of this would have been far below that 
seen in climbing mammals, and was, in fact, affected by somewhat different 
articulations.

Again, this is not to outright strike George's bipedality idea as invalid, 
but I'm genuinely interested in why the anatomical features found in both 
dinos and birds are not the ones found in climbing, arboreal mammals or even 
reptiles.

My several dollars and two cents,

Matt Bonnan

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