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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 wrist mobility?

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 bite, etc).



Tim

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