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Re: Campbell's even crazier than a MANIAC? (archeopteryx climbing)



Scott Hartmann wrote:

<Well wait here, "tomial" is the cutting edge itself (e.g. the cutting edge of 
raptorial bird beaks) and itself depends on the morphology of the keratinous 
sheath.  If you were to strip the ramphotheca from a falcon skull I doubt very 
much that it would tear through meat with the same prowess as the living animal 
does (with ramphotheca intact).  The #2 pedal claw morphology in Deinonychus 
and Velociraptor certainly constrict ventrally, leaving an inverted teardrop 
shape that is very derived compared to the primitive condition (and indeed the 
condition of the other pedal claws) which are sub-triangular in crossection 
with the widest portion at the base.  Also, in the Thermopolis Specimen and in 
Chinese dromaeosaurs that preseve impressions of the keratinous sheath or the 
second there is more keratin on the ventral surface, so unless the sheath 
cross-sectional morphology differed radically from the bony core it the already 
narrowing claw would have to
 pinch out to an edge.>

  We're dealing with some interesting assumptions here:

  1. Neither felid nor falconid/acciptrid unguals have ventral tomia, but are 
more rounded in section. The "cutting edge" must neccessarily derive from the 
keratinous sheath overlying the bony ungual. This argues that the main 
inferrence for a cutting edge on the claw MUST come from the keratin sheath.

  2. Felids and falconids use their claws to grip, primarily, rather than cut. 
The bulk of the claw is used to pierce and hold prey, and cannot be used to 
slice (given the lack of a cutting ventral edge). This is not the case with the 
tip, which, while also bearing a rounded ventral edge, was sharper relative to 
the claw width due to its aspect. The amount of force generated by the claw has 
a greater effect when the body of the ungual has reduced relative to the speed 
at which it's being used, which provides the cutting power. Claw tips in cats 
also have a tendency to break off repeatedly.

  3. Regardless of the ventral constriction of dromaeosaurid unguals, the 
unguals lack a sharpened edge. The conclusion that the keratin sheath should 
follow and develop on the shape of the underlying ungual is likely, but to 
exagerrate the shape is based on a previous assumption of function. No animal 
uses the bulk of its claw to cut with, as noted above, and to affirm the 
presence of such a cutting edge further basally than the tip is more fiction 
than fact. It's not that I disagree, but the evidence doesn't back up that 
conclusion.

  4. Virtually all clawed animals have a keratinous sheath that extended 
further ventrally than dorsally to the ungual core itself. This is simply 
logical given the resistance exerted on the ungual is certainly almost always 
ventral, and not dorsal, as well as apically. It would be as simple as to argue 
that dogs, ostriches and elephants also possess keratinous "nails" extending 
ventral to the ungual, yet these animals do not use them as cutting/slicing 
devices.

  Cheers,

Jaime A. Headden
http://bitestuff.blogspot.com/

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)