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Re: Alvarezsaur spurs (was Re: dino-lice)

Jason Brougham <jaseb@amnh.org> wrote:

> When I looked at dorsal views of Shuvuuia (MGI 100/1001) next to the
> Myrmecobius one on Digimorph I felt, subjectively, that there was an
> uncanny convergence of the superficial skull shape.
> http://www.digimorph.org/specimens/Myrmecobius_fasciatus/
> They seem to have converged on a wine bottle - shaped skull with a
> slender, tubular, snout.

I think there's a lot to recommend for your numbat (_Myrmecobius_)
comparison for alvarezsaurs.  At least insofar as the skull is
concerned.  Although it is a myrmecophage (termites and ants make up
nearly 100% of the diet), the skull and forelimbs are relatively
unspecialized for this behavior.  The numbat has been described as an
"amateur anteater", which I think is a little harsh, but probably
justified.  As for the long tongues of numbats - the tongue can be
extended ~ 100mm.  Not bad for an animal with a skull length
(condylobasal) of ~ 56mm.

One of the best summaries of myrmecophagy in mammals is in Patterson's
1975 paper on fossil aardvarks.  According to him (and I'm pretty much
paraphrasing here) there are two routes to full myrmecophagous
specializations: (1) from a generalized insectivorous-carnivorous
ancestor; (2) from an insectivorous-omnivorous ancestor with fossorial
adaptations (the most common route).  In either case, an increasing
dependence on termites and/or ants for food would have provided the
basis for specialization.  The essential requirements to initiate this
shift are the ability to extend the tongue, and an ability to scratch
or scrape the substratum (earth or wood).

The numbat is an example of (1), given the lack of specialization in
the forelimbs, and its phylogenetic affiliation with dasyuromorph

If alvarezsaurs were myrmecophagous, I'm not sure which category they
would fall into.  The morphology of _Haplocheirus_ suggests (1); but
if the 'fossorial' (digging) adaptations in the forelimb came before
the derived craniodental changes, then possibly (2).  I doubt this
happened, so I favor (1).  My hypothesis (which requires testing
against as-yet-undescribed intermediate forms) is that alvarezsaur
forelimbs were first reduced (as in compsognathids) along with a shift
to a generalized insectivorous-carnivorous diet, and later adopted the
highly derived digging morphology seen in Late Cretaceous