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Re: semilunate carpal
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
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
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.
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.
Timothy J. Williams
Iowa State University
Ames IA 50014
Phone: 515 294 9233
Fax: 515 294 3163
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