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Re: SVP 2004

T. Michael Keesey wrote:

(Note that the palms even face inward a bit, as in theropods.)

Of course, the myrmecophagids have forelimbs that are about as long as
their hindlimbs, while the mononykines have very long hindlimbs and
very short forelimbs, so that's a big difference. But if you look at
the forelimbs alone, the similarity in structure is quite striking.

Here's another explanation for the bizarre forelimbs of alvarezsaurids: In the ancestors of alvarezsaurids, the forelimbs lost their grasping function, e.g., because the alvarezsaurid ancestors did not need their forelimbs for catching prey (and the forelimbs were accordingly reduced), and/or alvarezsaurids evolved from ancestors that had wings (and the forelimbs were reduced in secondarily flightless forms). Thus, the alvarezsaurids inherited absurdly short forelimbs in which the hands were not much good for anything. The highly derived forelimbs of alvarezsaurids might therefore represent an attempt to "revive" some form of grasping function from what was left of the forelimb. One claw per manus was enlarged, and the paired claws acted together to grasp objects - sort of like tongs or pincers. The musculature of the upper arm was expanded, as were the olecranon process and the sternal keel for attachment - increasing the adductive strength of the truncated forelimb.

As David notes, there are elements of similarity between alvarezsaurid and tyrannosaurid forelimbs, in that both groups have forelimbs that are at once extremely small and extremely strong. For tyrannosaurids, one explanation was that the forelimbs were used to hook into large prey held by the head. Plainly, this doesn't work for the alvarezsaurids.

Hopefully one day an alvarezsaurid specimen will be found with preserved stomach contents, telling us what they did eat: ants or termites (and the forelimbs were used to rip open nests), grubs (forelimbs used to strip bark), eggs (forelimbs used to carry and break open large eggs - and the long legs were used to run away from angry mothers), maybe carrion (forelimbs used to tear open carcasses)?

It's also possible that the forelimbs were used for different functions in different alvarezsaurid taxa. After all, there is an impressive range of size variation in the group, from little _Parvicursor_ (maybe under 1m in body length) to _Rapator_ (possibly over 6m).