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Re: Proportion of torso devoted to pelvis query

On torso-pelvis ratio, Gregory Paul has some ideas in "Dinosaurs of
the Air" (2002), mainly related to the size of the thigh musculature,
and probable correlation of this with activity level and high
metabolism. Regarding sacral number, always though the more sacrals
there are, the stronger the attachment because of an increase in
contact between pelvis and vertebral column, and that this would be
more useful to active creatures standing above the limbs a great share
of their days (and not with the belly on the floor). So the
craniocaudal expansion of the ilium may correlate with both larger
muscles and greater sacral attachment, which can be both in turn
related to greater activity.

However, yesterday I was seeing in a veterinary book that the sacrum
of many mammals, including bipedal humans and such large mammals as
cows and horses, attaches to the pelvis apparently only by the ribs of
the first or first two sacrals, all the other sacrals being caudal to
the pelvic contact.

It seems the more extensive attachment of dinosaurs, chatterjeeid
poposaurs and birds relative to mammals would have to do with the
bipedalism of the former and the general quadrupedalism of the latter,
although there are many contrary facts: humans do not differ from the
condition in other mammals, and as far as I know, quadrupedal
dinosaurs such as ankylosaurs and ceratopsians have also increased
sacral counts; also, basal sauropodomorphs ("prosauropods"), which
present more bipedal features than sauropods, generally have less
sacrals than sauropods.

I suppose the fusion between vertebrae not contacting the ilium in
mammals decreases the movement of these vertebrae with respect to the
pelvis in mammals in relation to the pyriform muscle, which is
sometimes considered a reduced homolog, or at least analog, of the
caudifemoralis. Or may provide some wider, motionless origin for some
tail muscle. Would not say any of these are the reason for the fusion,