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Re: semilunate carpal



On Tue, Aug 07, 2001 at 03:43:11PM -0500, Tim Williams scripsit:
> The semilunate carpal is an element which constrained the movement of the 
> hand (manus) to a mediolateral arc (the "swivel wrist").  There are two 
> major exaptive hypotheses for the presence of the semilunate carpal in 
> terrestrial predatory theropods and flighted birds.
> 
> (1) Predatory stroke: The "swivel wrist" allowed the hand to spread out 
> toward the prey as the arms moved forward and downward, and (if necessary) 
> then fold in on the prey - anticipating the downstroke and upstroke, 
> respectively, of flighted birds.  According to this hypothesis, the function 
> of the semilunate carpal in the flight stroke of birds is the derived 
> condition.  The long arms and hands of these predatory theropods was also 
> oriented toward the same function - seizing prey.

Some maniraptorans are obviously evolved to attack prey as large or
larger than themselves; in these cases, seizing isn't going to be a good
description of the interaction with the prey animal.

The other thing is that the greatest relative strength would have had to
evolve in small forms, since those are the ones that became volant, and
minimal levels of useful for flight arm strength would already have to
be enough to move *them*; locomotor strength levels, but in a limb that
couldn't be used for walking.  Hard to explain if it's an adaptation
primarily to grab much smaller mammals.

Is there a known biomechanical reason why the use of the 'predatory
stroke' might not have been to align the maniraptoran, relative to the
prey, rather than to move a small prey animal toward the jaws?  That's
reasonably consistent with tree climbing, and a steady increase in
stength to the point where the arms were capable of becoming locomotor
organs.

-- 
                           graydon@dsl.ca
               To maintain the end is to uphold the means.