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Re: dino-lice



2011/4/19 Tim Williams <tijawi@gmail.com>:
> Dann Pigdon <dannj@alphalink.com.au> wrote:
>
> I have no doubt that alvarezsaurids used their truncated, monodactyl
> forelimbs for *something* - and as with the tyrannosaurids, it was
> probably a trophic function.  But unlike David M., I associate
> forelimb truncation in alvarezsaurids with a lack of day-to-day usage,
> rather than being an optimization for digging.

I was thinking that digit reduction may support viewing the extremity
as helping nest (or bark) destruction. For vermilinguans have one
digit with a claw more powerful than the rest, and it seems the same
is the case for pangolins
(http://upload.wikimedia.org/wikipedia/commons/e/ed/Chinese_pangolin_skeleton.jpg).
Perhaps digital reduction is good for nest destruction, because of
concentrating all the force in just one claw. It seems reasonable that
in the generally mesaxonic mammals, this digit is III, while in the
generally entaxonic dinosaurs, this digit is I. Forelimb shortening
may be adaptive to reduce shearing forces generated by the apparently
high forces generated. Even mammals, where the forelimb is also used
in locomotion as David indicated, can reduce the lenght of their
forelimbs by shortening all the limbs. Tamanduas, pangolins,
armadillos, and even moles and some mustelids, requiring powerful
extensor exertion, seem to have relatively short limbs. However, the
specialized giant anteater has relatively long limbs, which may be
related to its relatively greater size and probably the need for more
energetically saving locomotion by increasing the stride, to patrol a
larger home range (may this be the reason by which ungulates, bears,
cats, hyenids and canids seem to generally have proportionally longer
limbs than mustelids, viverrids, and rodents? - also why dinosaurs,
relatively long-limbed, are larger than most other amniotes?). If we
talk about breaking bark, we do not need to hypothesize unique
reliance on social insects, for arthropods in general on rotting trees
may be made available.

> I also have a
> suspicion that the compact arm-folding mechanism (semilunate carpal,
> etc) of maniraptorans was evolved for the same reason - it represented
> an alternative to forelimb truncation, rather than being associated
> with a specialized function per se.  I know many will disagree with me
> on this...

May be, it may give a similar result, namely, reduction in the
prominence of an appendix from the body. This looks to me like
possibly good for a more aerodynamic profile in running animals.
However, some of the relatively faster dinosaurs, ornithomimids, have
neither particularly reduced nor able to fold forelimbs (I do not know
much about the shape of the glenoid articulation, but it may be
observed if they may have aligned the forelimbs with the body). If
folding has to do with possibility of fastly changing aerodynamic
aspects, then it would not mean the same as reduced forelimbs, or if
the reduction has to do with nutrient economy or weight reduction.