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Re: Digit Loss



In a message dated 6/8/01 12:19:35 PM EST, twilliams_alpha@hotmail.com writes:

<< I disagree.  In most lineages (including the ones you mention) the hands 
 were actually *used* for something.  Not always the same thing, of course, 
 but *something*.  Not in tyrannosaurids though.>>

How do you know what the forelimbs of tyrannosaurids were used, or whether 
they were useless? It has been shown that they were pretty well muscled and 
didn't just dangle there. I strongly question this assumption.
 
<< Basically, what we are arguing about is whether tyrannosaurids lost their 
 third finger before or after the forelimb was reduced (vestigialized, 
 perhaps).  I say it happened after, and that the loss of the third digit was 
 not prompted by any selection at all.  The loss of a finger made no 
 difference at all to tyrannosaurids - as long as his teeth and toes didn't 
 disappear, a _Tyrannosaurus_ could still bring down a hadrosaur irrespective 
 of the number of digits on its puny forelimbs.>>

Again, you are arguing about the use of tyrannosaurid forelimbs when we 
really don't know anything much about tyrannosaurid behavior, etc. The hands 
may look useless to you, but they probably weren't useless to the 
tyrannosaurids. Also, this "gee, it just got lost" explanation doesn't 
explain why the whole forelimb wasn't lost, or why one or both of the other 
digits weren't lost. After all, tyrannosaurids were around for some 30 
million years--plenty of time for more digits to go--but retained the same 
digital count and manual anatomy as far as we know throughout this period. 
And why digit III, the next in the series to go, starting outermost first? 
Just a coincidence, or was it connected with the loss of digits IV and V in 
the creation of a wing?
 
<< You claim that the loss of the tyrannosaurid's third finger was somehow 
 connected to the aerial performance of an (as-yet-unknown) dinobird from 
 which tyrannosaurs are supposedly descended.  I would ask, how does a 
 three-fingered basal tyrannosauroid like _Eotyrannus_ fit into this 
 scenario? >>

It's in a clade of three-fingered tyrannosaurians that is the sister group to 
the two-fingered ones. (And, do we >know< it had three fingers? I read the 
paper, and all we have is the proximal part of metacarpal III, so we don't 
yet know whether there actually was a digit on it. Maybe Eotyrannus was 
two-fingered also.) The describers keep Eotyrannus outside Tyrannosauroidea 
(whatever that is these days) as a sister group, so to this extent our 
phylogenetic hypotheses agree.