Dan Chure wrote:
I remain agnostic on the myrmecophagus habits of Mononykus. The
forelimbs are very short and I can't see how that would work
functionally with the long neck.
The alvarezsaur bauplan is certainly unlike any modern myrmecophagous mammal. However,
between them I think Senter (2005) and Longrich & Currie (2008) present a strong
case for considering alvarezsaur forelimbs as being specialized for breaking into
insect nests. The combination of a long neck and short forelimbs might make sense if
(as Longrich & Currie propose) alvarezsaurs fed on wood-nesting termites. So as
the claws tore into rotting trees, the alvarezsaurid jaws would snatch up the termites
scurrying up the surface of the trunk. A long and flexible neck would help in this
case.
Nevertheless, the shortness of the alvarezsaur forelimbs is especially weird. Even if the forelimbs were used for digging, why were they so short? 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. Carnotaurines have tiny forelimbs that were probably useless, but the scapula and coracoid are still very large and (presumably) well-muscled. So alvarezsaurid ancestors ma
y
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_). This gave the alvarezsaurid forelimb 'wiggle room' to take on a novel function, although in a less-than-ideal way given how short the forelimbs had become.
Cheers
Tim
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