One of the topics that interests me is about how tetrapods could evolve extra limbs. I prefer to use this because the biological explanations can be more interesting than, say, "is an alternate world in which the common ancestor of all fishes' were hexapod".
But as it is almost impossible to develop new limbs for vertebrates it is necesary to go with the pseudo-limbs, completly new mechanisms evolved from other body structures to supplement the function of a limb, like elephant trunks or prehensile tails.
So in the case of dragons I searched for similar things and I found there are two similar questions but I think they do not consider other important aspects What would the changes necessary for powered flight using rib-derived "wings" look like? and this Kuehneosauridae and powered flight?
Different from those two questions, I decided to use as base the Coelosauravus, the reason being that this species, is that instead of using the ribs to glide, evolved dermal ossfied rods (or osteoderm-like things) to hold the skin membrane which I think is an extra so don't have problems with removing the ribs.
But I realized that to get dragons bigger than 20 centimeters requires even more strong articulations, and muscles attachments are not enough to fold the wings behind the body because the size would increase and would become a hindrance.
Wings would need to be folded like bird wings or maybe like pterosaur wings, the problems is that wings evolved from rigid bone rods don't have articulations to permit to be folded and the required muscles either.
How could dragons evolved in this way solve these problems? or alternatively, how could evolve the required new articulations and muscles?
Things to consider:
- Apparently it is relatively easy to evolve new boney structures and skin extensions.
- Apparently evolving new tendons and muscles is not so common and even a strange event.
- The wings need to be conected to the principal skeleton.
- I remember reading that fishes don't have sinovial articulations (with lubricant fluid and variety of shapes) but this evolved in tetrapods.
And here is an image which I think can examplify this kind of evolution but obviously doesn't solve the mentioned problems. https://www.deviantart.com/speculative-zoology/art/Draconian-Evolution-57276938