CHAMAELEON ARMS

[darren.naish@port.ac.uk]

On the subject of chamaeleon limb posture and its relationship to 
what dinosaurs did, Ralph Miller III wrote...

> Right.  Old World tree chameleons do set down their feet directly 
> under their bodies when they walk along narrow branches-- eschewing 
> the typical reptilian sprawl -- but they do not walk with straight 
> legs.

If you are saying this makes them different from 'erect stanced' 
dinosaurs, think again because no one is saying that dinosaur legs 
are necessarily held out straight beneath the body. An erect stance 
does not preclude legs/arms bowing outward and in fact numerous 
erect-stanced tetrapods - birds including ratites and anseriforms 
are great examples - actually have legs that bow outwards when their 
skeletons are viewed cranially or caudally. The important thing is 
that the body is perpetually raised off the ground during all modes 
of terrestrial locomotion. Thus reptiles like crocs and varanids, 
which can assume erect-stanced postures, are not fully 
erect-stanced.. any distinction has become blurry since we have 
learnt more of biomechanics and tetrapod gait however (e.g. sprawling 
limbs in MTBs). Pillar-like erect legs that really don't bow outwards 
are seen, not surprisingly, in big graviportal tetrapods like 
sauropods and elephants.

What is important and superficially dinosaur-like about chamaeleon 
forelimbs is that they have a slab-sided thorax where the scapula is 
stuck flat against it and the glenoid fossa is not directed 
laterally, but more ventrocaudally (actually, it is kind of 
lateroventrocadually in the chamaeleons I've seen..). It's well known 
that some quadrupedal dinosaurs have elbows that articulate in a 
flexed position, and not in a straight vertical line perpendicular to 
the spine. Chamaeleons approach this complex better than other 
lizards.

What chamaeleons do not have, and what ceratopians apparently do, is 
a lateral lip to the glenoid fossa which prevent lateral torsion of 
the humerus. If this is correct, it shows that ceratopian forelimb 
motion is not totally like that of chamaeleons. I only know the 
basics of ceratopian anatomy so I'll leave it at that.

"Look at me, I'm a kitty kat: meoww-moww!!"

DARREN NAISH
darren.naish@port.ac.uk