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Re: Ornithischian opisthopuby (was RE: Giant birds)

Matt Bonnan posts:
"John Hutchinson and Steve Gatesy presented some interesting info on just
this sort of muscular/pelves stuff at SVP.  The iliofemoralis (IF) muscle in
'gators was shown to help swing the femur out during locomotion, and this
outward swing was resisted and protracted by three puboischiatic adductor
muscles.  In birds, some of these muscles now act to aid the animals in
stance, particularly on one leg."

Thanks for the kind words, Matt.  I wish I could explain this more with
pictures, etc. right now, but I'd prefer to wait until the paper on this is

One correction on our SVP abstract is that the posterior trochanter in
maniraptorans and other theropods is actually probably not related to the
insertion of M. pubo-ischio-femoralis externus 1+2 (PIFE1+2); it's another
story entirely.  The lateral surface of the greater trochanter is the
PIFE1+2 insertion in both extant clades of archosaurs.  Tetanurine
theropods rotated the proximal femur medially, drawing the greater
trochanter (and PIFE1+2 insertion) laterally and hence increasing the
lateral rotation moment arm of the PIFE1+2.

Plesiomorphically the PIFE1+2 protracts, adducts, and laterally rotates the
femur while the foot is off the ground (swing phase).  The avian homolog
only laterally rotates the femur; protraction was lost with pubic
retroversion, and adduction eventually diminished.  Lateral rotation of the
femur draws the lower leg medially (i.e., adducts everything distal to the
strongly flexed knee; try it at home).  This adduction brings the foot back
to the midline of the body before the foot contacts the ground.  I would
not be surprised if ornithischians evolved along a similar path.

Exactly why ornithischians and maniraptorans retroverted the pubes, I don't
know.  We focus on one probable functional consequence of pubic
retroversion, parts of which others (e.g., Romer) have commented on as well.

John R. Hutchinson
Department of Integrative Biology               Phone:  (510) 643-2109
3060 Valley Life Sciences Bldg.                 Fax:    (510) 642-1822
University of California
Berkeley, CA 94720-3140