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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.

I have thought that muscle wings can be made from current reptile muscles

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

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    $\begingroup$ "Apparently is relatively easy to evolve new boney structures" You're referencing something I said are you? :) I wouldn't exactly characterise it quite like that but it has happened many many many times (starting with the first limbless chordate with nothing but a spinal column & on ever since through every other addition to our skeletal structure), if it hadn't none of us would have the skeletons we do would we // & what has happened b4 can happen again, especially when you already have all the genetic coding for bone etc on hand. $\endgroup$
    – Pelinore
    Aug 20, 2021 at 7:23
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    $\begingroup$ A good relatively recent evolutionary example of that ^ might be the moles 'extra thumb' // it can actually wiggle so seemingly is at least part way to having a new joint. $\endgroup$
    – Pelinore
    Aug 20, 2021 at 9:14
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    $\begingroup$ That's Coelurosauravus = grandfather of hollow-tailed lizards. (Macaronic Greco-Latin.) **Coelosauruvus is meaningless; the first part may mean hollow lizard, but the **uvus part has no meaning. $\endgroup$
    – AlexP
    Aug 20, 2021 at 12:15
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    $\begingroup$ @Willk not really, that one is asking about the correct or best mechanical placement / morphology, this one wants to know how you get there through evolution, $\endgroup$
    – Pelinore
    Aug 20, 2021 at 13:34
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    $\begingroup$ On evolving new articulations: in humans who break a rib it can be hard to let it heal, especially when combined with lung diseases that cause a lot of coughing. If this happens continuously the body will actually evolve a full articulation there. If the bony protrusions are based on the ribs then an initially rudimentary musculo-tendon system could develop with it, if necessary as part of another mutation like humans with a functional polydactyly (nature.com/articles/s41467-019-10306-w) $\endgroup$
    – Demigan
    Oct 2, 2021 at 17:36

4 Answers 4

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Perhaps a single set of limbs would split into two?

You know the radius and the ulna? That's some prime real-estate for more limbs, even if they'd still only connect to the main skeletal frame with a single socket. Observe, yes I'm using a human arm, but it could be applied to most if not all vertebrates(snakes don't apply, they'd be able to get rib wings or something I'm sure). The tendency for reptiles to have some sort of webbing will lend itself greatly for this, as marked in red.enter image description here

You also get to have some manipulation with the remaining relatively unaltered fingers/digits, which is nice.

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  • $\begingroup$ Is an interesting idea but how could happen it? literally dive the half of the arm $\endgroup$
    – Drakio-X
    Oct 3, 2021 at 4:41
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    $\begingroup$ @Drakio-X At first you have to split up the arm for my example to happen. Sexual selection is an option, with the partner needing to stimulate many places at once but they've been driven further and further apart over the generations, with the partner's evolution tending towards arms that can reach many places at once. Split established, you can have the reptile follow a similar evolutionary bath of the bat. Pressures could be flying prey and lofty nests but it can also be sexual selection, the wing part keeping a partner warm while they can use the arm part to protect it and its partner. $\endgroup$
    – Hearsay
    Oct 3, 2021 at 6:44
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My theory is that the wing structure would evolve from shoulder blades extending and segmenting, while the skin maintained a membrane between the shoulder blades and the hips. While there is no biological pressure to start this transition, perhaps there is a societal pressure?

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  • $\begingroup$ With social pressure that might means sexual selection? $\endgroup$
    – Drakio-X
    Sep 26, 2021 at 20:16
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Look at Drepanosaurus

Drepanosaurus was an extinct reptile with a unique forelimb anatomy, which could be adapted into a hexapodal-seeming structure

Specifically, if the humerus was shortened and/or fused into the torso, perhaps as an adaptation for swimming, then the zeugopod (lower foreleg) would appear to come directly out of the body. An aquatic history would also be a good step towards wings, as both fins and wings would require a more even digit structure to work well

The next adaptation would be for the arm (or really the wrist, with the Drepanosaurus) to split into two. This could start as some adaptation for gripping, becoming a natural form of Krukenberg hands. Once you have the split limbs, it would be rather trivial for these split limbs to become independent and for the radial wrist bone to extend into a long form. Once you have that, all you'll need are a few more radial digits and you'll have a dragon

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    $\begingroup$ This would make more sense with illustrations and links. $\endgroup$
    – JDługosz
    Nov 3, 2021 at 22:22
  • $\begingroup$ It's a good idea, I just think that would be better with some images and sources. $\endgroup$
    – Drakio-X
    Nov 8, 2021 at 2:23
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Your original idea is fine. Just add rib alignment, thoracic girdle, muscles and joints to taste.

Your choice of starting point is superb - I didn't even realize there was a tetrapod species that had evolved new bones for wings (more illustrations). This is by all means the beginning of a bold new experiment, which apparently may have been pursued further - that site I linked leads me to Mecistotrachelos apeoros, in which the dermal ossicles seem to have made a one-to-one alignment with the ribs. To my surprise, this species was Triassic, having survived across the P-T extinction event from the Coelurosauravus.

I don't actually know why it didn't "take off" in the longer term, but were I to propose further honing of the evolution, well, presumably we need a thicker, stronger girdle for large flight muscles, anchored by some extra dorsal bone(s). If the ribs are too weak an attachment and we need direct articulation with the pelvic girdle, we can introduce other joints if we desire relatively easily - remember that joint development is essentially a matter of adding an interzone to an existing bone, though naturally there are details that take a geologically long time to get right. The outer bony segments might well end up becoming much more specialized, with significant differences in size and shape, but without a supercomputer (and people to program it) I quail to suggest precisely how that might go.

But you basically have the beginning of a good story, and the majority of the details of the evolution from that point depend on the exact appearance and function you would like for your dragon.

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  • $\begingroup$ Definitly this can help to polish some other questionings about my creature design, but I think this is a little bit out of the focus of the question. I completly know that new muscles and articulations are necesary to improve flight, the problem is that I don't how plaussible is this, that's why I need to know if its possible to develop completly new articulations and muscles from "nothing" (after all evolution just transforms structures to other function not create new completly ones). I need antecedents that this has already happened that some animal develops a new joint and musculature $\endgroup$
    – Drakio-X
    Nov 8, 2021 at 6:46

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