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RE: Alvarezsaurid arms was Re: JFC-Bloodiest Battle ??
David Marjanovic wrote:
> Instead, I propose -- and I don't think this is an original idea of mine
> even just when applied to alvarezsaurs; it has been suggested for
> tyrannosaurids in any case -- to go back to elementary-school physics. To
> the ratio of force to lever length, that is. [snip] Now, quadrupedal
> animals, let alone burrowing or climbing ones, cannot afford to shorten the
> forelimbs indefinitely. This is why all modern myrmecophages retain fairly
> long forelimbs.
No way. The giant pangolin (_Manis gigantea_) is mostly bipedal, and it
certainly doesn't have reduced forelimbs. Pangolins in general often move
around bipedally when on the ground. Their forelimbs are specialized for
digging (which is why bipedality if heavily favored when walking), and show no
signs of being shortened. So the assertion that bipedality would promote the
digging forelimbs of ant-eating mammals to become shorter is not correct.
Besides, I've never heard it said of *any* myrmecophagous mammal (echidnas,
anteaters, armadillos, pangolins, aardvark, sloth bear) that large forelimbs
confers a *disadvantage* to tearing open insect nests. Myrmecophages have long
forelimbs not because they were foisted upon them by quadrupedality, but
because they allow greater reach when ripping open nests.
> For the same reasons, modern myrmecophages are never cursorial, while the
> alvarezsaurids were: being cursorial requires cursorial limb proportions,
> and a quadruped cannot have cursorial and fossorial limb proportions at the
> same time, while a biped can.
Myrmecophagous quadrupeds have forelimbs highly specialized for digging
(including breaking open insect nests), which prevents quadrupedal
cursoriality. (There's also an issue that a diet consisting solely or
predominantly of ants and termites is woefully poor in terms of nutritional
value, which forces myrmecophages to have a mostly 'slow motion' lifestyle....
but I won't dwell on that.)
> But the alvarezsaurids were bipedal and thus free to shorten
> the arms further, till the biceps inserted right next to the wrist and the
> serratus or whatever arm retractor inserted right next to the elbow. The
> shortness of the forelimbs is not a bug, it's a feature.
Alvarezsaurs were cursorial bipeds because their ancestors were cursorial
bipeds, and there was no reason for them to abandon this form of locomotion.
It's as simple as that. There's no way that their absurdly stunted forelimbs
represent some pinnacle in fossorial/myrmecophagous evolution. Something else
is going on with alvarezsaurs.
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