Re: semilunate carpal

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

Dinogeorge wrote:

> There is simply no reasonable explanation for why the
> forelimb of a cursorial, non-flying predator that used its forelimbs > just 
> to grasp and manipulate its prey would evolve into a form >less< > useful 
> for this purpose. Imagine how useless your hands would be if they could not 
> rotate on their wrists and if the bones of the palms were pretty much 
> frozen into one position without an opposable thumb! > And if the fingers 
> had become too long and too stiff to wrap around an object to hold it.

I disagree.

I'm going out on a limb here (so to speak), but it all may depend on what 
you mean by "grasp and manipulate".  As primates, we tend to regard a 
prehensile manus with an opposable thumb as a sort of crowning glory of 
evolution.  It works for us _Homo sapiens_ really well, because our 
lifestyle depends on a high degree of manual dexterity.  I couldn't write 
this message without it, or peel a banana, or work the remote control of my 
TV (which forms a much larger part of my own ecology than it should!)  So 
when a lineage actually reduces the mobility and prehensile ability of the 
manus, it's regarded as a retrograde step.  However, in the context of a 
lineage of consummate predators, it might actually be quite advantageous.

If tetanurines were primitively "grapple-and-slash" ambush predators, there 
was not much need for enhanced manipulative abilities of the manus.  The 
hands (both at the same time) were used simply to sieze the prey and hold on 
to it while the jaws and/or feet dealt the killer blow.  As such, a 
relatively inflexible manus capable of limited movement (as a result of a 
less mobile carpus, with the semilunate carpal capping metacarpals I and II) 
could assist the predator in resisting the struggling prey.  The elongate 
manus and forelimbs were evolved for much the same purpose in 
maniraptoriforms - grabbing and siezing prey.  I would aver that most of the 
"manipulation" of the carcass (i.e. getting the chunks of flesh into its 
gullet) was done by the jaws - perhaps anticipating the way birds feed 
today.  I'm willing to consider that the arms were folded against the chest 
during feeding; the jaws and feet (especially in dromies) did most of the 
work.

In other words, in maniraptoriforms, the hands and arms became instruments 
dedicated to prey capture, not feeding.

Also, somebody mentioned that the relatively immobile wrist was integral to 
the "predatory stroke" - the rapid forward-and-downward movement of the 
forelimbs which was exapted to the flight stroke in birds.  When the ambush 
predator sprang forward, ready to sieze the hapless prey, the forelimbs 
swung forward and the hands swung inward as part of the same lithe motion.  
I think the listmember was asking if this was also connected at all with the 
semilunate carpal?



Tim




---------------------------------------------------------------------

Timothy J. Williams

USDA-ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163



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