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Re: Quo vadis, T. rex?



On Thu, 1 Feb 1996 Dinogeorge@aol.com wrote:

> In a message dated 96-02-01 20:37:01 EST, pharrinj@PLU.edu (Nicholas J.
> Pharris) writes:
> 
> >I didn't mean to suggest that they did.  I just don't believe it takes 
> >that much to lose a finger.
> 
> I know--but think about how long those vestigial metacarpals and metatarsals
> just hang around. It seems as if the first, or outermost, digits are the
> easiest to lose, but once you're down around three, losing them becomes very
> difficult. It surely has something to do with the way the homeobox genes (or
> whatever complex it is that governs limb development) work.
> 

Actually, digits one and five are theoretically the easiest to lose.  
Embyological studies have shown that the first digit to form is number 
four, which arises from a linear segmentation of the ulna, followed by 
three, two, and one, in that order.  Digit five is formed by a separate 
branching sequence off number four.  See Stephen Jay Gould's essay 
"Eight Little Piggies," in the book of the same name, for a more complete 
explanation and illustrations.

While this pattern does not necessarily dictate which digits will be lost 
(advanced theropods appear to have completely gotten rid of manual digit 
four, the first to form and presumably hardest to lose), it does explain 
the primitive amniote phalangial count of 2,3,4,5,4:  The earlier the 
digit forms, the more phalanges it accumulates--unless something 
prevents this, as in mammals.  I just thought it was interesting.


Nick Pharris
Pacific Lutheran University
Tacoma, WA 98447
(206)535-8204
PharriNJ@PLU.edu

"If you can't convince them, confuse them." -- Harry S. Truman