Alvarezsaur Polarity and Features

["Jaime A. Headden" <qilongia@yahoo.com>]

David Marjanovic (david.marjanovic@gmx.at) wrote:

<I had to "infer" the primitive condition for Alvarezsauridae because many 
characters are only
known for *Alvarezsaurus* _or_ *Patagonykus* _or_ Mononykinae.>

  Mickey and I differ on the interpretation of the tarsus of *Patagonykus*; 
this is an essential
dichotomy that comes from the morphology of the cited elements. Nothing will 
resolve this except
examining the specimen itself. Until then, all we have to go on is 
*Alvarezsaurus*, which
certainly lacks a derived proximally pinched third metatarsal, to the degree 
seen even in
oviraptorids or dromaeosaurids. It indicates that this is essentially a 
reversal from the
primitive state, no matter where alvarezsaurs go in the Coelurosauria mix. In 
this manner, derived
alvarezsaurs (Mononykidae) bear the apparent outgroup condition of the 
proximally pinched and/or
reduced third metatarsal, which does not appear to be present proximally in the 
tarsus of
*Mononykus* and is certainly not present in *Parvicursor*. There are two 
choices for the evolution
of the tarsus that a matrix like PAUP would not resolve, and this is 
essentially an evolutionary
complex that only the human brain can come up with:

  1) the tarsus of *Alvarezsaurus* is basal to _all_ alvarezsaurs, thus the 
condition in
mononykids are derived and convergent to the condition in other arctomet 
theropods; the arctomet
is essentially only a complex of advanced theropod cursorial adaptations, 
rather than indicating
any real phylogenetic signal.

  2) the tarsus of *Alvarezsaurus* is unique to Maniraptoriformes, indicating 
that the tarsus of
the Mononykidae is the plesiomorphic condition; the arctomet pes is 
plesiomorphic to the group,
and appears to indicate a phylogentic signal as to the origins of the group 
being nested within
true arctomet theropods like Troodontidae, Ornihomimosauria (ignoring that fact 
that a basal
ornithomime has a more generalized arctomet than even ornithomimids do), or 
Tyrannosauridae.

  Either condition is plausible, one is more parsimonious than the other based 
on reduction of
transformations (reversals or acquisitions).

  Finally, in regards to the arms used for myrmecophagy, there is little else 
that these arms
could do but dig (see Nick Longrich's abstract in the 2000 JVP abstract volume, 
20(suppl. to 3)).
It is unlikely that such powerful limbs were adapted for grasping based on the 
form of the manus
having a distinct form related to fossorial earth-moving. No other hypothesis 
satisfies the
function of the arms, despite the leggedness of the animal. It simply appears 
to be an ecology
(cursorial myrmecophage) not encountered before: we have arboreal, small and 
large subcursorial,
and larger mediportal myrmecophages. As for the carriage of the arm, please 
look at the arm
skeleton of the golden mole, such as *Amblysoma*, which holds the arm neutrally 
having the manus
facing each other volarly. The humerus is capable of adducting, and personal 
observation indicates
that there was adductorability in the humerus and pectoral girdle of 
*Mononykus* as in
*Carnotaurus*. These features indicate the manus may have volarly faced 
ventrally (palm down)
relative to the earth, but without manipulating the arm itself (something I 
plan to do, if no one
gets to it first) to determine actual degrees of movement.

  Food for thought,



=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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