Digits, Digits, Digits!!

["Steve Brusatte" <dinoland@lycos.com>]

Hey,
I would have chimed in on a few of the individual posts over the past two days, 
but I've been watching and playing too much baseball to have time...

I enjoyed philidor's post on extra digits in cats and such.  This reminds me 
of, dare I say it, a baseball analogy (am I turning into Gould or what?).  
There is a relief pitcher on the Florida Marlina named Antonio Alfonseca.  
Other than being an excellent closer, he also has six fingers!  If anything, 
his six finger resembles a pinkie (digit V) most, although I cannot be quite 
sure.  

Many listmembers have commented that it would be very difficult to provide 
enough mutations to create a totally new functional digit after that digit has 
been lost (this was a response to me asking: why didn't tyrannosaurs and other 
theropods re-evolve digits?).  Obviously, the mutations were there to create 
this totally new finger for Alfonseca, as they were to create the sixth finger 
of philidor and Jaime's cats.  Of course, in Alfonseca, it is likely that this 
finger may be a copy of the pinkie finger, which means that a mutation provided 
for the copying of the pinkie genes.

It may be close to impossible to produce an entirely new digit after all five 
digits have been lost, but I wouldn't be surprised if it were quite easy to 
duplicate any digit if that digit is still present.  In many cases, six digits, 
even if two are basically identical, may be an advantage.  However, we see none 
of this in theropods.  This, once again, makes me believe that something was 
holding the re-evolution of digits back.  What?  Again, I don't know.  

For some reason theropods, none of them-throughout their lineage-could use this 
extra digit, a digit which at some point had to have evolved.  If these extra 
digits evolved in cats, giraffes, and relief pitchers, then why would a 
theropod be an exception?  Even if a digit is complex, as Dan said, it is quite 
easy to copy an existing set of genes that codes for a different digit (a la 
Alfonseca).  But, we don't see any digits re-evolve in theropods.

Also, as I said before, it isn't quite as easy to lose an entire digit.  It may 
be somewhat easy (speaking in an evolutionary tone) to lose a gene that codes 
for a protein, such as SHH, that creates digits, but it would be very difficult 
to lose the SHH gene, the cartilage gene, the osteocyte gene, the muscle genes, 
the nerve genes, etc., etc.  Digit formation is something that is very 
polygenetic, as several genes code for it.  It may be somewhat "easy" to 
duplicate these genes, as with Alfonseca, but it is probably difficult to lose 
all of them (not impossible, as is evident, but difficult).

However, we do see a recurring theme in digit loss.  We don't see a recurring 
theme in digit re-evolution.  Why?  To me, it seems as if there was some sort 
of pressure, a pressure that led to the loss of digits and then prevented their 
re-evolution.  Just don't ask me what.

Steve

P.S.: Jaime, hasn't there been a few recent studies which have shown that the 
appendix may play a minor role in digestion, such as housing a population of 
symbiotic bacteria.


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Steve Brusatte-DINO LAND PALEONTOLOGY
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