I am actually questioning a bit of it.
Firstly the evolutionary development of the skeleton (what did it arise from for example) would have to strongly be considered..
Also the ribs, I assume that for whatever reason the Plents use a diaphragm for breathing? Where does it attach?
I'm honestly curious as to why they have ribs at all, this is only my suspicion but aren't ribs derived from spinal growths for swimming?
as far as i remember the first plents evolved their breathing organs similar to how insects do, they just processed air from their butt nostril, eventually certain hazards put them to evolve a breathing system more akin to that of vertebrates, i expect species like buttblasters to have very powerfull structures in order to well, you know...
as for the skeleton, again the first plents were like a moving lamp i mean, they had to move on those tiny structures, inmobile plents got it right since they only had to extend their tendrils on the ground. movile plents had to develop a support structure, as they didnt have any bone structure they must had to evolve it so my hypothesis is that they created hardened tissues inside the body and around the organs. eventually that hardened flesh became the wooden "bones" we know today.