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Re: Quo vadis, T. rex?
On Fri, 2 Feb 1996, Nicholas J. Pharris wrote:
> 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.
Now, I believe that in crocs, only digits 1-3 are clawed, and I
thought that this is also the case in duckbills, Othnielia, ceratopians, and
theropods... Is this true of all dinosaurs, or at least, in theropods? If it
is, it might help explain why digits IV and V are reduced in theropods, they
wouldn't be of too much help in grasping without the claws.