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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 stages
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 stance.

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 adductor anchorage.

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 these animals.

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.

Matt Bonnan
Sauropod podiatrist and so much less.

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