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



  My back-of-hand hypothesis, which has yet to be assessed, is that it is to 
relieve muscles and ligaments from strain while supporting additional elements 
of the manus (namely, elongated feathers, digits, forces acting against the end 
of the limb not typically during rest, etc.) This has nothing to do with 
expansion of the semi-lunate carpal block, which can occur without fusion 
(e.g., *Acrocanthosaurus atokensis*). Arthrodesis is a pathology of the limbs 
in which fusion at the various joints in the wrist occur due to strain and (I'm 
stretching here) an immune response to "heal" the damaged area. Anticipatory 
fusion can be exapted and thus evolutionarily useful when such fusion is 
between two elements where the lack of strain can enable another preferred 
behavior. Animals preferring limb motions in one range where the fusion does 
not inhibit can be selected over the other, as this motion can be useful. For 
large manus-based graspers, such fusion and increased joint size at the wrist, 
and the development of the distal humerus and proximal forearm indicate 
enlarged and maybe even slightly ossified arm ligaments, more strongly 
anchoring the muscles and preventing avulsion. I suspect this is the initial 
cause of the carpal fusion. Note that only in *Avialae* does it seem to expand 
to the third metacarpal, while in other theropods with carpo-metacarpal fusion, 
it's between the distal carpals 1 and 2, and between MCI + II, or between all 
four elements at once (I am presuming homology of the block derives from those 
specific distal carpals).

  In mammals, horses, colugos, bats and various fossorial insectivores exhibit 
natural, non pathologic carpal fusion, and in non horses this is usually 
focused on the scaphoid and lunate, which are proximal carpals. Inhibition of 
intercarpal movement that is produced by this fusion appears to stabilize 
movement of the distal ulna and radius, rather than enable it in more flexible 
carpal arrangements. This would seem to indicate a greater capability of 
withstanding torsional forces, and forcing the limb to move in _just_ such a 
way.

  Again, this is a preliminary hypothesis.

Cheers,

  Jaime A. Headden
  The Bite Stuff (site v2)
  http://qilong.wordpress.com/

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


"Ever since man first left his cave and met a stranger with a
different language and a new way of looking at things, the human race
has had a dream: to kill him, so we don't have to learn his language or
his new way of looking at things." --- Zapp Brannigan (Beast With a Billion 
Backs)


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> Date: Sat, 13 Aug 2011 23:29:44 -0700
> From: mickey_mortimer111@msn.com
> To: dinosaur@usc.edu
> Subject: RE: Ah ha! That's where therizinosaurs came from
>
>
> Tim Williams wrote-
>
> > Somebody (in this case me) will point to the fused carpometacarpus in
> > the oviraptorosaurs _Avimimus_ and _Heyuannia_, and in the
> > deinonychosaur _Balaur_. In these cases, fusion of the
> > carpometacarpus might very well be associated with lack of use.
>
> 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.
>
> Mickey Mortimer
>