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Re: Theropod limbs - how mobile?




David Marjanovic wrote:
that immobility is a great thing for such a predator, as this
makes it much more difficult for prey to escape, relatively much easier --
no muscular effort is required -- for the predator to hold it.
 
*** Not to belabor the issue, Ostrom '69 sums it up as follows:
"The manus of Deinonychus was a highly perfected and powerful grasping structure quite unlike that of any other adequately known theropod.  The most important features confirming this conclusion are the long and stout first and second digits with their large trenchant and strongly recurved claws, the slender abductable third digit with its unique restrictive joints, the very large flexor tubercles on all unguals, the highly perfected carpus that provided extensive and precise abduction and supination of the hand, the unusual length of of all forelimb components and great size of the deltopectoral crest and internal tuberosity."
 
***He also makes mention of the strong collateral ligament pits that suggest strength between phalanges and therefore digits as a whole.  As a related aside, I was at the library today looking at a book on mammalian predators which included a photograph of a leopard in a tree with one forelimb outstretched and hanging from the tip of a single claw was an entire carcass of a baboon it had captured earlier on.  Transpose that sort of digit strength onto a Deinonychus.
 
 
Can somebody please tell me why they think this is so? But first, try these simple experiments. You'll need a pair of hands and an angry cat. Holding your fingers rigid, and your hands facing inward, try to grip the struggling cat.  After applying ointment and plasters, try again, this time with your full range of mobility. Which works better?
 
*** Now, lets modify your first experiment. Lets implant nasty recurved claws with large flexor tubercles (indicating attachment to strong digit flexor muscles in my forearms) into the ends of my fingers. Let's lock together my first two metacarpals, give me enlarged deltopectoral crests which will be attached to significantly strong pectorals and other adducting chest / shoulder musculature.  For good measure throw in a sharp recurved oversized second pedal ungual and a set of small but sharp serious teeth, a respectable appetite. . . Suddenly your struggling cat has been reduced to "puppy chow" in less time than it took you to read the words "puppy chow".
 
 
Secondly,  partially bend your right index finger. Now press it against your  left hand. Move your left hand about a bit. You'll notice that the flexibility of your digit improves both contact and safety. If you try it again, this time with your finger straightened and bent back till it is immobile, not only is it less use, it is also in serious danger of getting broken!
 
*** From Ostrom's description and what I understand about Alan Gishlick's paper ( I don't have a copy but have read references to it), although the digits are frozen or restricted in respect to some interphalangeal articulations, their is still  flexion between metacarpals and digits and digits to unguals.  This then, does not parallel your example of imposed digit hyper extension above.  Even if their was complete immobility of the hand beyond the wrist  (consider Captain Hook for a moment with a snap on, triple bladed, gardening implement instead of his standard steely single claw) and a penchant for performing bust improvement exercises. When Freddie Kruger or Deinonychus are looking for a "hug", it's time to leave the party!
 
Cheers,
 
Mike Skrepnick