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Re: caudofemoralis and flight



><<>Pubic retroversion, however, may simply be related to giving the
>>body a more aerodynamic shape, as it occurs in Theropoda only in the most
>>avian-like forms.
>
>I don't know how that would work, or if its prevalence in maniraptoriformes
>is causally linked to flight at all. The fact that similar changes occur in
>therizinosauroid theropods and basal ornithischians seems to hint at a
>similar pattern related to locomotion, but it's tough to say without
>rigorous biomechanical testing of such hypotheses.>>
>
>"Therizinosauroid theropods"?? What are those?
>
>Since neither segnosaurs nor ornithischians are particularly closely related
>to theropods, there is no reason to believe that pubic reversion in those
>groups had a similar origin to the pubic reversion of theropods. In pre-avian
>theropods, pubic reversion surely helped give the rear and underside of the
>body an aerodynamic shape, as it does in virtually all living birds; in
>segnosaurs and ornithischians, pubic reversion probably helped to accommodate
>the more intricate digestive system that herbivores seem to require. This is
>simply an example of convergence of form.

I'm quite tired of the "segnosaur wars," so I won't comment on that.  It's
self-evident that this is a case of convergence, at least with
ornithischians. But convergent patterns aren't necessarily dismissable with
simple explanations, like "herbivory requirements," and in any case I don't
see convergence as necessarily "simple." There's no reason to believe that
pubic retroversion _didn't_ have a similar, but not identical, origin in
those groups. There's probably a far more complex nexus of mechanisms
underlying pubic retroversion -- shifting the pubis posteriorly would
probably change the center of gravity of the body, move the muscles and
other internal organs around, and probably more. It doesn't take much
thought to figure that much out. I don't think we're at the point where we
can separate out which evolutionary mechanisms shaped the retroversion of
dinosaur pubes and which did not, or which was even a primary influence.
It's hard to do that with living animals; I don't know how we can claim
omniscience about the long-extinct ones.

I'm not so confident in our knowledge of the evolution of dinosaurs to say
that herbivory was the fundamental source of selection pressure that drove
the retroversion of the ornithischian pubes. As in many cases in
evolutionary biology, it's far safer to be honest about what we know and
what hypotheses are falsifiable based on that data. In this case, we can't
say much for now, and I don't see any way around that based on current
data... at least any scientific way. We can speculate till the cows come
home, but speculations are just ideas and don't justify making bold
declarative statements about our actual knowledge of the past. So let's be
honest and scientific about those dinos, please.

                       --John R. Hutchinson