Re: evolution of theropod wrist motion

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

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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