Re: Tyrannosaurid arms (was RE: Forelimbs of "Terror Birds")

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

On Fri, 14 Dec 2001 22:16:43  
 David Marjanovic wrote:
>> [...] begs the
>> question: why did tyrannosaurids have only two fingers?
>
>2 are apparently enough for the function suggested in Mesozoic Vertebrate
>Life (holding prey). A thin 3rd one doesn't add much force and brings in
>useless mobility, I'd say. Well, who knows.

Who knows is right.  I would say, based on many previous on-list discussions 
regarding digits in theropods, that something was holding back the re-evolution 
of digits in the theropod clade as a whole.  We've discussed this many times, 
so there is no need to revisit that (check the archives).  

I like Carpenter and Smith's hypothesis.  I like it a lot.  The calculations 
really seem to show that a prey-holding function was a strong possibility, 
which also may be a possible reason that only two digits remained in 
Tyrannosaurus (along with George's BCF hypothesis, which I am also intrigued 
by).  

A third digit could, in some cases, offer a competative advantage, but probably 
not in Tyrannosaurus.  Two digits most likely sufficed, and evolving a third 
would have been a valuable waste of precious tissue and energy.  Of course, 
this is only true if Carpenter and Smith's hypothesis, which is well supported, 
is indeed correct.

Steve

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