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Re: evolution of theropod wrist motion
Mike Keesey wrote:
If BCF folks are incorrect, then why would _Deinonychus_ even have >limited
To clarify: the semilunate carpal actually increased the mobility of the
wrist in the mediolateral plane. In dromaeosaurids and other swivel-wristed
theropods, the wrist could execute an arc of up to 190 degrees. Pretty
impressive, I think.
However, flexion in the perpendicular (dorsoventral) plane was certainly
limited. The deep trochlear groove precluded anything we might call
supination or pronation. This may actually have been a boon when both hands
were holding struggling prey, by reducing involuntary movement at the wrist
joint: lack of mobility was actually an advantage in this situation.
Both models require that limited wrist mobility be somehow advantageous to
a terrestrial animal -- it's not something that BCF alone must explain.
How about this. The hands of maniraptorans became dedicated to clasping
prey. The forelimbs became accustomed to being used in tandem (as in the
flight stroke of birds). In maniraptorans, both hands were used in
opposition to grasp ("clamp") the prey. Think of dromaeosaurid arms as the
twin arms of a tongs. In paravians (at least primitively) the sickle-claw
became a major instrument for dispatching prey - ably assisted, of course,
by the jaws and the clawed fingers (death by blood loss and/or suffocative
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