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Re: Heel to Toe. Thanks!
Gregory S. Paul's book, _Predatory Dinosaurs of the World_, goes into the
anatomical and locomotory differences between non-avian theropod dinosaurs
and extant birds, one principal difference being that birds generally hold
their femurs more horizontal and nearly immobile when walking, moving much
more at the knee joint than at the hips. Non-avian theropod dinosaurs, by
contrast, don't hold their femurs as horizontally, and swing their legs
freely from the hips as well as the knees.
This is due in part because of the caudofemoralis muscles which the
non-avian theropod (bipedal) dinosaurs use (and which birds lack) and
because non-avian theropods have a substantial tail which more closely
balances the body which lies forward of the hip joint (acetabulum).
Because a bird's center of gravity is so much further forward, it had to
move its pivot point forward to keep from falling on its face all the time,
so it pivots principally at the knee. On the other hand, cursorial birds
(ostriches, for example) do swing the femur a fair amount during a run.
And, as Paul points out, the reflexive toe clenching action so familiar to
us in bird locomotion (whereby the bird automatically flexes the toes when
the foot is picked up off the ground) is restricted to perching species,
and would not be reflected in their Mesozoic non-avian non-perching kin.
Ratites do not exhibit toe flexing during locomotion; their toes just droop
Emus and other ratites have been the subject of research by Jim Farlow in
an effort to better interpret the identity (darn near impossible to assign
beyond the "theropod" or "duckbill" or "ankylosaur" level) and activity
implied by fossilized theropod dinosaur tracks, because ratites appear to
be the best analogs extant. But, as others have pointed out, such
analogies are of little use when comparing birds to dissimilar groups of
dinosaurs (birds versus quadrupedal dinosaurs in particular).
-- Ralph W. Miller III firstname.lastname@example.org