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Digital loss ... or ... The Monster with ridiculous little fingers

Do tyrannosaurids' fossils along the time show clearly the gradual reduction of their arms?
Why would the cause of good-armed theropod have their arms reduced to ridiculous protuberances? What's the evolutionary explanation?
On Fri, 8 Jun 2001 13:03:37  
 Dinogeorge wrote:
>In a message dated 6/8/01 11:48:07 AM EST, twilliams_alpha@hotmail.com writes:
><< Tyrannosaurs had ceased to use their
> hands for grappling or manipulating prey.  Therefore it doesn't matter how
> many fingers a tyrannosaur had.  Heck, a few more random mutations and
> _Tyrannosaurus_ may have had no fingers at all - just a pair of fleshy
> protuberances attached to each humerus. >>
>I don't think digital loss is >anything like< this trivial an occurrence.
>Digital development is not governed by one or two genes that can be turned on
>and off or deleted or suppressed by evolutionary whim; it is governed by
>entire systems of genes. It took some 50 million years of evolution for
>horses to lose their digits, for example, and whole groups of dinosaurs
>shared almost exactly the same fore and hindlimb digit configurations for
>scores of millions of years. Humans today still have the same digital counts
>as amphibians from the Carboniferous Period. Digital counts and phalangeal
>counts persist mightily in the face of immense selection pressure and in
>spite of sometimes drastic limb modifications. It takes an awful lot of
>evolutionary work to make even a single phalanx go away in a lineage.

That's another good point (wow, I'm agreeing with George a lot today...).  Digit development is governed by a very large number of genes, and an equally large number of proteins coded by these genes, plus cartilage and osteocytes also figure into the mess. 

Once again, if I may bring up Larsson's talk, in which he discussed the formation and reduction of digits.  Digit formation and reduction is odd.  Not only can digits be lost, but also positions in the manus/pes can be lost.  For example, digits can shift positions and forge a new identity through ontogeny.  Much of this can be traced to that wonderfully-named Sonic the Hedge Hog Protein (SHH), which makes tissue able to form digits.  Based on what Larsson said (if I recall correctly), about a day after SHH is released to the manus/pes region in an embryo, digits begin to form. 

Digits also have no identity before cartilage is formed, which means that a whole number of genes can shift positions.  In other words, digit formation is different than digit identity.  So, in order to lose a digit, the animal must lose two processes: digit identity and formation.  Therefore, the mutation in the genome of a dinosaur (or any other animal that loses digits) must basically remove two complex processes, processes which are governed by a plethora of genes. 

To me, it is strange that random mutations could simply just remove these genes so easily, and so continually throughout the history of life.  Therefore, there must have been a distinct advantage to those organisms that lost digits. Once again, these digits do not re-evolve.  Why?  Probably because it would be very hard to re-evolve all of the complex processes required to form a digit.  The odds are a million-to-one.  However, the odds are also against the trivial deletion of all of these digit-forming genes.  There must have been a reason why theropods lost their digits, and this process must have taken millions of years, and required many mutations, not a few trivial ones (as George puts it). Is it because a wing was forming?  This is where I don't necessarily agree with George so readily.  I think the wing is a possible answer, but my gut tells me that it may have had to do with something else.  What, I don't know.

Anyhow, the Sox and Cubs are coming on, so I'm logging off until tomorrow.


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