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Alvarezsaurid arms was Re: JFC-Bloodiest Battle ??
Nevertheless, the shortness of the alvarezsaur forelimbs is especially
weird. Even if the forelimbs were used for digging, why were they so
(You almost certainly know this, but I'd just like to clarify that "digging"
is meant here in the biomechanical sense, not necessarily in the sense of
digging holes into the ground, an activity that the alvarezsaurid body shape
rules out at extremely high levels of confidence.)
It's possible/lilely that alvarezsaurids evolved from theropods in which
the forelimbs were already reduced (as in compsognathids - or maybe
compsognathids themselves were the ancestors of alvarezsaurids). Perhaps
alvarezsaurids inherited forelimbs that were on the way to being
vestigial? Then, for one reason or another (maybe to do with the shift to
a myrmecophagous diet), the forelimbs acquired a new function in digging.
But by this point the alvarezsaurs were stuck with their puny forelimbs,
and had to make the best of them. And, for some reason, the forelimbs
could not be re-enlarged - although fortunately a strong upper arm
musculature was retained.
This does not make sense.
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. For maximal speed of movement,
it is desirable that all muscles insert as far proximally as possible; the
tradeoff is reduced force. For maximal force, it is desirable that all
muscles insert as far distally as possible. That's clearly where the
selection pressure for a myrmecophage goes. Unfortunately, there's a
constraint in development: the upper arm, for example, cannot become as
broad as the forearm is long -- the insertion points of muscles cannot
migrate distally indefinitely. The solution is to let the distal end migrate
toward the muscle insertion instead -- to shorten the arm. 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. 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.
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.
Carnotaurines have tiny forelimbs that were probably useless, but the
scapula and coracoid are still very large and (presumably) well-muscled.
So alvarezsaurid ancestors may similarly have drastically shortened the
forelimb (associated with loss of function), but left the arm musculature
mostly untouched (?to promote ribcage and pectoral girdle mobility, as
suggested for _Carnotaurus_).
Flightless birds often have large shoulder girdles. This holds even for
moas, which completely lacked forelimbs including the shoulder joint.
Regarding the meaning of "ecological niche", I was taught to never say an
organism occupies a niche, but instead to say that it _forms_ it. This is
IMHO exaggerated, but it overshoots in the right direction.