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Re: semilunate carpal




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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