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Re: FYI -- Microraptor gui, Burnham and Alexander, PNAS...



> Importantly, however, those posts explicitly don't argue for the
> 180°-sprawled pose that Burnham et al. wanted to squeeze *M.* into. I was
> there at their talk at the SVP meeting of 2008, and they had a
> reconstruction where the femoral head pointed not into the acetabulum but
> ventrally (vertically downwards), and the trochanteric crest touched the
> ilium, leaving _no_ space for the very muscles that would have pulled the
> legs up into this pose! That was cringeworthy.

I do not know about the figures in the meeting, but, suppossing they
opened the legs so that the distal end of the femur directed
laterally, you have still a large area of origin of the derivative of
the iliofemoral muscle (mostly M. iliotrochantericus caudalis) which
would not be covered by the trochanteric crest, in the preacetabular
process. However, this muscle would rotate the femur more than
abducting it.

Regarding articulation, it is true that the femur would be mostly
disarticulated. Cracraft (1971) however indicated that in retraction,
if the facies articularis antitrochanterica of the femur
disarticulates from the antitrochanter, it can be abducted a maximum
of 35° in pigeons. However, he considers this movement to be uncommon
in nature. I do not believe such a degree of disarticulation to take
place in Microraptor, because it would be dangerous being there a
force of air resistance which would tend to dislodge the hip joint
even further (in Cracraft's manipulations, there was still articulated
the head with the part of the acetabular surface which is not the
antitrochanter).

Ref.
Cracraft, J., 1971. The functional morphology of the hind limb of the
domestic pigeon. Bull. Amer. Mus. Nat. Hist. 144: 171–268.