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Alvarezsaurs



A couple days ago, Tom Holtz referred to a "'mononykine'
alvarezsaurid."  I'm pretty suspicious that Mononykus probably isn't
an alvarezsaurid at all, having pored over Bonaparte's description of
_Alvarezsaurus_ (thanks again, Tracy!).

The foot of this animal is highly distinctive.  TIn basic form, it
resembles that of an ornithomimid, but there are several indications
that the second pedal digit in _A. calvoi_ evolved from a raptorial
digit:

It is apparently divergent, pointing away from the other two toes at
an acute angle.

Phalanx II-2 is slightly shorter than phalanx II-1, as in
dromaeosaurs.

Metatarsal II is considerably shorter than mtt IV (although digit II
has been lengthened so that the tips of the unguals of digits II and
IV are about even).

And metatarsal IV curves strongly away from mtt III, as in
dromaeosaurs, to distribute the animal's weight between digits III and
IV.

Some of these features are also found in troodonts, but metatarsal III
is only slightly pinched, the glenoid socket faces backward and down,
and the pubic peduncle extends ventrally farther than the ischiadic
peduncle, all indicating that _Alvarezsaurus_ is closer to
dromaeosaurs than to troodonts.
 I would put it in a clade Dromaeosauria along with _Itemirus_ and the
Dromaeosauridae.

_Alvarezsaurus_'s foot, then, is apparently a dromaeosaur foot
jury-rigged to work like an ornithomimid foot!  If the pedes of
_Mononykus_ and _Patagonykus_ lack this distinctive morphology, then I
doubt very much that the latter two genera belong in the
Alvarezsauridae.

Nick Pharris
Olympia, WA