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JD Harris writes: <Thus, just why the development of a synsacrum [in large
pterosaurs]?  Almost certainly for rigidity and muscle attachment -- possibly
something to do with flight
(fine control of the tail and feet), and possibly some unguessed-at function
of the feet.>

Thank you for your reply, but your answer leaves some of the same questions
behind.  i.e. : why rigidity, except for bipedal excursions? Flying dragons
have no more than two sacrals and they are never bipedal. And what muscle
attachments would require a synsacrum? Longitudinally the epaxials are
located in that area. It seems reasonable that additional sacralized
vertebrae would tend to stiffen this area, structurally relieving stresses
otherwise born by the epaxial muscles (longissimus dorsi, etc.).  Those
stresses may have their source in bipedalism.