Hmmmmm thats what I was afraid of. Youre right, of course, in saying that bearded dragons do not have a diaphragm, but thats because no lizard does. The most 'evolved' in that regard have a proto diaphragm, but thats about it. Lizards breathe by contracting muscles to expand the ribcage, which thus sucks air into the lungs. Its the same thing as we do with our diaphragm, just a different starting point. Its also why many lizards cant run for more than a short burst of time. The same muscles they use for locomotion are the ones that are used for breathing, so its usually a trade off to a certain extent. (some families of lizard employ what is called gular pumping to compensate for that shortcoming) There is nothing about being on their backs that would negate this breathing apparatus either way though. With the examples you have given the more interesting question is why the lizards were not able to right themselves on their own. Unlike a turtle, a lizard with long appendages and tail should have no problem in righting itself. I would guess with the person whose dragon died that other issues were to blame. As for your Jace, it sounds like you fell prey to the dreaded thanatosis!