More on Digit Loss in Theropods

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

Hey,
Awhile back (oh, probably June) we had a nice discussion onlist about the loss 
of digits in theropods, and whether or not there was an evolutionary "pressure" 
keeping these digits from re-evolving.  I remember supporting the "side" (with 
George Olshevsky!) that pointed to some pressure keeping these digits from 
re-evolving because, after all, there is a trend throughout theropod evolution 
for digits to disappear, but there is no trend that sees the re-evolution of 
these digits.  It was a really good debate, and I may have some new information 
to rev it up again.

A few weeks back I picked up a nice book (The Shapes of Time, by Ken McNamara) 
that discusses heterochrony and the evolution of certain body shapes.  There 
was a short passage discussing the evolution and loss of digits. McNamara and 
his colleagues Emily Gale and Peter Alberch tried to correlate the relationship 
between digit number and body size in certain mammals (especially dogs), 
finding that in larger dog breeds more digits were often present.  

This is great, but it is hard to correlate these data with dinosaurs.  However, 
McNamara also discussed research on early embryonic amphibians.  They found 
that when a certain chemical was added (I don't believe the name of the 
chemical was given, which is a bummer), a limb developed in the adult amphibian 
that was not only much smaller, but also contained less digits. 

Interestingly, though, McNamara mentions that some animals-among them cats 
(which I believe one listmember did use in the argument)-often develop extra 
toes.  While he believes that the development of extra digits in dogs may be 
related to large body size, McNamara hasn't yet found any such correlation in 
cats, meaning that perhaps the development of digits in felines may be entirely 
dependent of any major ontogenic factor.  

We also see this in humans.  I recall mentioning Florida Marlins pitcher 
Antonio Alfonseca, who possesses six digits on both hands.  I used this 
example, as did a fellow listmember (philidor??) with cats to show that, at 
least in some animals, developing extra digits may not be that difficult.  
However, in theropods there is no trend to re-evolve digits.  I believe George 
Olshevsky said that this was due to the evolution of the wing.  I can't say 
that I support that viewpoint 100 percent, but I still must say that something 
was likely holding the re-evolution of digits back in theropods.  

After all, at least in many modern animals, it isn't too hard to develop extra 
digits.  Once there, if these extra digits hold any advantage they may be 
selected for.  Perhaps in theropods there was no advantage for extra digits and 
they were simply neutral.  However, in dinosaurs that relied on hunting this is 
difficult to fathom, although not impossible.  

Steve

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Steve Brusatte-DINO LAND PALEONTOLOGY
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