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Re: Ah ha! That's where therizinosaurs came from



Mickey Mortimer <mickey_mortimer111@msn.com> wrote:

> I don't know what the function of a carpometacarpus is, but I doubt it's 
> correlated with a "lack of use".  In addition to the above taxa and the 
> previously noted alvarezsauroids,
> Therizinosaurus, Oviraptor and Mapusaurus also show fusion among flightless 
> taxa.  Therizinosaurus, Oviraptor and Balaur have large well developed hands, 
> and while those
> of Heyuannia and Avimimus (Tsuihiji et al., 2009) are more robust and slender 
> respectively, neither shows obvious reduction in utility.  The fusion in 
> Mapusaurus shows that
> unless you subscribe to BCF, the feature can arise in nonvolant theropods at 
> least sometimes.  That said, the other taxa may be neoflightless, especially 
> Balaur if Cau is
> right about it being an avialan.


Ah no - I don't subscribe to BCF.   ;-)


By "lack of use" I meant the manus' lack of use in terms of its
traditional function in grasping - and it's this that could
occasionally lead to carpometacarpal fusion.  As Csiki et al. say of
_Balaur_: "The hand is fused, atrophied, and poorly suited for
grasping."  Add to that, the forelimbs of _Balaur_ are reduced in
length - although as you say, the manus is still fairly large.


Given that many maniraptorans are unlikely to have used their
forelimbs for prey capture, carpometacarpal fusion is not all that
surprising to me.  We don't need to invoke secondary flightlessness to
account for carpometacarpal fusion among non-avialan maniraptorans.


_Mapusaurus_ has second and third metacarpals fused; don't know why,
but it might have something to do with forelimb reduction and/or
limited use in prey capture.  The related _Giganotosaurus_ has a
pectoral girdle proportionally even smaller than that of
_Tyrannosaurus_.


Alvarezsaurids are a special case, because carpometacarpal fusion was
probably tied up somehow with the specialized digging function of the
forelimbs, including the hypertrophied ungual.  So I was careful not
to associate carpometacarpal fusion in alvarezsaurids with a "lack of
use".  However, I believe it likely that forelimb truncation preceded
the advent of a specialized, functionally monodactyl manus in this
group.






Cheers

Tim