Re: semilunate carpal

["Tim Williams" <twilliams_alpha@hotmail.com>]

ptnorton@msn.com wrote:

> But holding onto struggling prey requires a powerful grip, which the
> sideways folding wrist significantly limits.

No, the grip was powerful enough.  It just wasn't dexterous.  Think of the 
two forelimbs of a maniraptoran as the twin arms of a pair of tongs.

And, to repeat Dan Varner's point regarding _Velociraptor_ vs 
_Protoceratops_ - it obviously worked.  The semilunate carpal was put to 
lively (or deadly) use by these predators.


> That noncoplanar arrangement
> of joints and muscles simply doesn't make sense as an innovation related to 
> grasping or holding anything.

The constraints on rotation (pronation/supination) provided by the 
semilunate carpal ensured a *firmer* grip.  Don't confuse ourselves with a 
_Velociraptor_.  We need an opposable thumb in order to grasp - but how 
useful is the thumb when gripping a large object with *both* hands.  In 
_Velociraptor_, the two hands were "bracing" the prey while the jaws and 
teeth ripped into the prey, and/or the raptorial pedal claw disembowelled 
it.

For this purpose, three coplanar digits exerting a strong force were more 
useful than two coplanar digits and an opposing "thumb" which would only get 
in the way.

To quote from the YPM site on the Chinese Feathered Dinosaurs:

"All maniraptors, flighted or not, possess a unique wrist bone, the 
semilunate carpal, that moves with the hand in a broad, flat, 190 degree 
arc.  Powered by heavy chest muscles, the bones of the arm link together 
with the wrist so as to force the grasping hands to spread out toward the 
prey during the forestroke/downstroke and fold in on the prey during the 
backstroke/upstroke."


> > the jaws and feet (especially in dromies) did most of the work.<
>
> They had to, since the hands were becoming much less useful in that regard.

Zounds!!  No!  The hands were becoming less useful as prehensile devices.  
But as grappling devices they excelled, since their purpose was to hold the 
prey in place while it was subdued by other means.  And in many 
maniraptorans, the "other means" was very formidable.  The "Fighting 
Dinosaurs" (thanks Dan) shows this clearly.

http://www.amnh.org/exhibitions/fightingdinos/gallery/fd.jpg


> The hypertrophy of the arms and hands could also be explained as selection
> for increased lever arm length and turning moments.

You mean it's originally a flight-related character exapted for predation in 
a "make-the-best-of-a-bad-thing" fashion?  Like George's BCF, I think this 
is putting the cart before the horse.  The flight stroke developed from the 
predatory stroke, not the other way round.


Tim



---------------------------------------------------------------------

Timothy J. Williams

USDA-ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163



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