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Re: Ornithischian opisthopuby (was RE: Giant birds)
Martin Barnett asks:
Are there any advantages to the Saurischian hip design? In the early
of hip development are there any fundamental morphologic reasons why an
ornithischian hip could not be built straight off?
While Tom Holtz has already addressed some of these points, let me throw my
hat in here for what it's worth. Dinosaurs represent a group of archosaurs
that become bipedal (with some forms becoming secondarily quadrupedal) and
get away from a sprawling stance by assuming a para-sagittal and erect
Anyone who's dissected 'gators will tell you that they have some nice big
femoral adductor muscles. These muscles act to keep the limb from splaying
out too much during the course of locomotion. The pelves of many archosaurs
and lizards and such are therefore relatively wide to provide decent
In dinosaurs, the femur is essentially locked into a parasagittal plane with
little chance of spalying outward. Therefore, adductor musculature for the
femur is less needed, and the pubis and ischium become laterally compressed
and thinner structures since the adductor muscles are smaller.
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.
Hutchinson and Gatesy found that the big iliofemoralis muscle probably acted
in stance rather than swing in dinosauriformes -- i.e., it now acts to
prevent the femur from being pulled in too much by the adductors rather than
swinging the femur out in an arc like in gators. Since the pubis is the
propubic condition, the adductors on the pubis help to draw the femur
forward (protract it) during locomotion.
Where is this all going? Still with me? In maniraptoran dinosaurs, the
pubes become retroverted, meaning that some of these adductors of the femur
shifted posterior to the shaft of the femur. Hutchinson and Gatesy found
that the insertion of the puboischiadic muscles (the group called PIFE1+2:
puboischiofemoralis externus 1 and 2) shifted laterally, marked by a
posterior trochanter on the femur. This means that these adductors now
caused the femur to rotate out (laterally) and in (adduction) while walking.
Simply put, the retroversion of the pubis in maniraptorans moved the
adductors of the femur in such a way as to cause the thigh to turn out while
being drawn inward during locomotion.
Being less familiar with ornithischian pubes, perhaps a shift from the
propubic condition caused a similar sort of muscular shift which perhaps
aided the animals in certain aspects of their locomotion while also (as
Holtz has already noted) providing more gut space.
In sum, the "advantage" of saurischian pelves is that they narrow the hips
and the pubis is positioned (pubis pointing forward) such that adductors on
it help to protract the femur (pull it forward) during locomotion. This
condition is modified in maniraptorans, birds, and ornithischian dinosaurs,
and may have aided in numerous things, one of which is modifying the gait of
Since the saurischian hip appears to be the primitive condition, the
ornithischian condition most likely arose from it. Could it built straight
off? Not sure what you mean by that question, but I hope this helps out in
some strange way.
Sauropod podiatrist and so much less.
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