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I'm new to asking questions so please forgive me if i'm strange.

I have a species of plant-based dragons. Small, covered in feathers and intelligent. I just want one thing backed up for me because I haven't been able to find anything to back me up:

Can a wing and an arm merge together?

Dragons are naturally six-limbed, but the species I'm talking about has the skin and muscles of the arms merged into the wing. They walk on their knuckles, leaving the wings folded up. Is this feature able to be sensibly evolved?

Edit:

I realize I could've done better with explaining so I pulled out this old image.

enter image description here

Originally two separate limbs, bonded together by skin and muscle, but not bone.

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    $\begingroup$ If you subscribe to the definition that Dragons have 4 legs and 2 wings, then I belive Wyvern is the specie you're looking for. Wyverns typically have 2 legs and 2 wings, and would walk on the ground as described. $\endgroup$ – Spoki0 - Reinstate Monica Dec 18 '19 at 13:35
  • $\begingroup$ I know what a wyvern is-- And these are based on them! But since they are related to other creatures with six limbs I just want to see how well their recent evolution would be reasonable. $\endgroup$ – RadioGoblin Dec 18 '19 at 13:38
  • $\begingroup$ In your drawing, that fore-arm looks pretty meaty to be the leading edge of a wing. The limb in a wing tends to be very thin so as to minimize weight. Plus, the muscles in the fore-arm would not be used all that much, the main effort coming from the chest and shoulders, and to a lesser extent the upper arm. $\endgroup$ – puppetsock Dec 18 '19 at 14:13
  • $\begingroup$ I suppose it doesn't matter so much as just curiosity, but when you say plant-based dragon, do you just mean its diet? $\endgroup$ – BKlassen Dec 18 '19 at 16:27
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    $\begingroup$ Eventually, yes. Whales and dolphins did it $\endgroup$ – nzaman Dec 18 '19 at 17:24
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It's a bit of a "just so story" but here goes. The big deal is, suppose the dragons liked to nest in cliff faces. That would mean it needed to be able to do rather serious maneuvers. The colloquial term is "catching an egg on a plate." It would fly directly at the cliff, a little low, then at the last moment swoop upwards and put on the brakes, perching on the edge of its doorway. Some birds can do this, such as the barn swallow. It nests in places like the edge of beams.

enter image description here

This is a flying squirrel. He does not really fly but mostly glides, with some significant ability to "duck and turn." He's doing this to go from tree to tree without having to touch the ground, thus decreasing his exposure to predators. By the way, notice how thin and "birdlike" his limbs are.

enter image description here

Imagine your dragon started with four legs and wings. Then it added some skin folds, or possibly secondary feathers on the fore-limbs, that it could extend for control on landing. That gives you a pattern with both wings and skin folds. Birds have tails that they use very effectively for control. Maybe the dragon's wings were good for ordinary flight but not quite the best for detailed control, and sudden changes in direction, that landing on a cliff face might require.

Nesting in a cliff face has fairly obvious advantages. It's hard for most potential enemies to even know there is a dragon nest there. Getting to it would require either similar ability to fly, or outrageous ability to climb. Maybe the dragon does some remodeling of the cliff face specifically to make that more difficult. Possibly making it over-hung and very smooth. Then the dragon's eggs are in a fairly safe spot while the dragon is out hunting its preferred food.

So the dragon might start out with folds of skin, or even secondary feathers on the fore-limbs. The current dragon might well show some "vestigial" remnants of this evolutionary path, maybe with some skin folds that it "fluffs out" when it's doing particularly difficult flying moves. Or perhaps they have been re-purposed for signalling such as in the mating season.

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    $\begingroup$ this is actually really cool! thanks for the help bro $\endgroup$ – RadioGoblin Dec 18 '19 at 16:00
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Gobies fused their hind fins.

https://en.wikipedia.org/wiki/Fish_fin

The paired pelvic or ventral fins are typically located ventrally below and behind the pectoral fins... They are homologous to the hindlimbs of tetrapods. The pelvic fin assists the fish in going up or down through the water, turning sharply, and stopping quickly. In gobies, the pelvic fins are often fused into a single sucker disk. This can be used to attach to objects...

goby with fused pelvic fins

https://en.wikipedia.org/wiki/Pelvic_fin

Fish have more limbs (fins) to work with and have had more evolutionary time to work so in fish you can see more of the phenotypic evolutionary variation possible for chordates. If fish can fuse functional fins into organs with a completely different purpose, that should be possible for your hexapod dragons.

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  • $\begingroup$ I'd have thought rays would be better example, as rather than left and right pelvic fins merging, similar to manatees and mermaids, in rays pelvic and pectoral fins on the same side merged, closer to the dragon in question. $\endgroup$ – Pete Kirkham Dec 19 '19 at 14:28
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    $\begingroup$ @PeteKirkham - you are right that is a better example. I was not sure, though, that those long finned fish had undergone a fin merger - I thought it might be enlargement of one fin and regression of the other. A bilateral structure becoming a central structure is definitely a merger. $\endgroup$ – Willk Dec 19 '19 at 20:59
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Dragons are not a real thing. So there is no general rule that they have to have 4 legs + 2 wings.

Regarding the evolutionary aspect of the question: Birds have done pretty much that. They "gave up" 2 legs in order to evolve wings. As evolution works extremely slow, there probably was a time when the predecessors of birds limbs that were kind of like legs and wings at the same time.

Or you can look at bats. They have claws on their wings and they walk pretty much the way you described it.

Edit: Regarding the question if 2 limbs can merge.

If it is possible, then I guess it is extremely rare. For a mutation to spread it must provide an advantage. If you start with 4 legs + 2 wings then having 2 legs + 2 wings is no advantage at all. It means flying just as before but walking much slower.

Also, even if there was some advantage that I just can't think of, then it would not be a merging of wings and legs. If the front legs were not needed anymore and therefore only dead weight, then they would gradually become smaller until they pretty much disappeared. Then for whatever change in the environment, having some frontlegs would have to be important again, so those individuals who can somehow manage to use their wings for that would have the advantage and therefore propagate more.

These wings would then -over many generations and only if the ability to walk like this stays important for survival- reshape into something that makes it easier to walk with. Being able to fly would also have to be important all that time, as otherwise the wings left after some generations might not enable flying anymore.

Anyways, that would not be a merging, but a evolution to lose the frontlegs first, followed by another one to reshape the wings.

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    $\begingroup$ Weight is generally an issue with flying, and losing 2 limbs, with the mass that follows, could easily provide a big advantage there. $\endgroup$ – Spoki0 - Reinstate Monica Dec 18 '19 at 13:39
  • $\begingroup$ @Spoki0-ReinstateMonica you are right. weight is a big issue. walking on 2 legs instead of 4 is a big issue too though. even standing on 2 legs is. It also very much depends on how big the legs are etc. but you are right on the matter that less weight is an advantage for flight. $\endgroup$ – elPolloLoco Dec 18 '19 at 13:47
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    $\begingroup$ "Or you can look at bats" or manatees. Somebody correct me if I'm wrong, but didn't they use to have two legs that were then combined in one tail? Or did they just lose the legs instead? $\endgroup$ – VLAZ Dec 18 '19 at 15:12
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    $\begingroup$ "there probably was a time when the predecessors of birds limbs that were kind of like legs and wings at the same time" indeedy. Have a look at Microraptor, which had feathered hindlimbs. $\endgroup$ – Starfish Prime Dec 18 '19 at 15:20
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    $\begingroup$ @VLAZ Cetaceans (whales, etc.) and manatees evolved their tails from a regular tail, with a continuous spine to the tail. They lost their legs instead, with vestigial leg bones left behind. We do have an example of what you describe though - seals! They have two completely conjoined legs forming the lower part of their body, with two feet (flippers) at the end. Sealions have followed a similar trajectory, but still have a little more independence of movement so are only partly conjoined. $\endgroup$ – Graham Dec 19 '19 at 10:08
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You should have a look at how the wings of butterflies and moths work :)

From Wikipedia:

Some four-winged insect orders, such as the Lepidoptera, have developed a wide variety of morphological wing coupling mechanisms in the imago which render these taxa as "functionally dipterous" (effectively two-winged) for efficient insect flight. All, but the most basal forms, exhibit this wing coupling.

The more primitive groups of moth have an enlarged lobe-like area near the basal posterior margin, i.e. at the base of the forewing, called jugum, that folds under the hindwing in flight.

Other groups of moth have a frenulum on the hindwing that hooks under a retinaculum on the forewing. The retinaculum is a hook or tuft on the underside of the forewing of some moths. Along with the frenulum, a spine at the base of the forward or costal edge of the hindwing, it forms a coupling mechanism for the front and rear wings of the moth.

The frenulum mechanism mentioned above looks like velcro when you look at it. There are drawings in the wiki.

In the butterflies and in the Bombycoidea there is no arrangement of frenulum and retinaculum to couple the wings. Instead, an enlarged humeral area of the hindwing is broadly overlapped by the forewing. Despite the absence of a specific mechanical connection, the wings overlap and operate in phase. The power stroke of the forewing pushes down the hindwing in unison. This type of coupling is a variation of frenate type but where the frenulum and retinaculum are completely lost.

Replace the fore wing with an arm or a paw and you get what you wanted, specially if you go for a frenulum. Velcro is strong enough to stick an adult man to a ceiling by the soles of the feet if you use the best quality fabrics, and it has the advantage that if your creature ever gets the limbs disconnected, they may reconnect again. The merge might be permanent (requiring medical help to reconnect right) or temporary (so the creature may remerge at will).

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    $\begingroup$ The thing which I think could make a difference here is that the two pairs of wings serve the same function, while wings and arms serve different functions. Merging the wings together improved flight characteristics, while merging a wing and an arm likely reduces the utility of both. This example does show that it can happen though, there just needs to be a good evolutionary reason for it. $\endgroup$ – Turksarama Dec 18 '19 at 22:35
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Have you looked at bats before? Their wings have actual wrists and thumbs. If you can justify the merger to make them walk on two wings and 2 legs by making them functioning claws allowing the creature to walk on 4 legs while only having 2.

Like this poor little creature :

https://www.youtube.com/watch?v=OZC-qYDNVvc

With this "advantage" front legs become obsolete and removing them would be evolutionary beneficial due to the reduction of weight.

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In the Disney Series "Gargoyles" the character of Lexington had a structure where his six limbs (two wings, two arms, two legs) had a membrane that connected his arms, wings, and legs together and was similar to a bat or flying squirrel when compared to his "human with wings" counterpart. This layout was distinct, but the larger gargoyle species had plenty of examples if you look at background characters.

In Avatar (the blue people, not the airbender) all vertibrate life on Pandora have Six Limbs, though the Navi'i's are vestigial. There is a briefly seen "monkey" that has it's fore limbs split at the elbow rather than the shoulder, which is supposed to be an evolutionary forebear of the Navi'i and show the gradual loss of the third pair of limbs. In teal life, several species such as snakes and whales retain bone structure for vestigial limbs they no longer use and never develop (all four of the snake's limbs and the whale's two hind limbs) so it could be your "four Limb" dragon does have a bone structure that shows that they have not fully shed the now unused third pair.

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Yes they can, and it's very likely that it would happen.

As I've noted in questions about angelics, the muscles of the arms and wings are likely going to be shared. Assuming the European dragon the torso isn't elongated enough to have seperate pectorals for the arms and the wings. So to fly the muscles of the wings would pull on the arms and the pectoral muscles would pull on the other end, causing the end result to be that both arms and wings move downwards with the full strength of the Pectorals which is needed to fly.

Since this motion is simultaneous and there is a lot of evolutionary pressure to simplify this by reducing the amount of muscles and mobility of the arms in favor of a more stable and stronger pectoral muscle movement it is likely the arms will eventually merge with the wings.

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First of all, I totally love your drawing.

It's actually completely feasible to have a species that has wings and arms merged. There are definitely a few things to consider, though. When you say that their wings and arms are merged, I'm assuming you're implying that the arms are functional and can be used like normal limbs.

Pretty much any animal with wings does technically have a limb with the wing part attached to it, but the arm isn't very functional. This is because it needs to stay thin, aerodynamic, lightweight, and isn't used as an arm. Having a functional arm would make flight extremely difficult.

So, consider these details about the species:

  • The arm is a fair bit weaker as a sacrifice for weight

  • The body is smaller and/or the wing and arm are much longer and bigger so as to account for the size of the body

  • Flight cannot be sustained for large amounts of time

Now, onto another detail you asked about. Having the species walk around on their knuckles is also feasible, although, and this is just my opinion, this would be a bit more awkward and different than what's normal. Their bodies would be closer to that of an animal in terms of limb function, and thus may even have paws, and thus, it would completely negate the fact that they have wings.

So, instead, consider this:

The species walks on two legs, but because they live in difficult terrain with jagged and jarred, varying heights, and as a result, their fists are born calloused and able to withstand having all of the species' body weight on it for extended periods of time in order to navigate the terrain.

From an evolution standpoint, having the environment of their old home as a height-differing terrain, a tall forest would make sense, can explain the need for wings. Say their food grows super high on trees, so their wings evolved in order to reach the food and to hunt more effectively like birds do.

Awesome idea!

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Maybe not quite what you were looking for, but there are a number of animals whose limbs have over time shrunk to the point of uselessness and become merged into their bodies. The ancestors of whales and dolphins had hind legs that disappeared over time. There are still remnants of them that can be seen in the skeletons of modern animals.

Likewise, snakes lost all of their limbs over time, but these too can be seen in their skeletons as vestigial bone structures. And there are even a few cases of snakes found with actual visible (though non-functional) legs.

Ostriches and emus have visible arms/wings, though they have basically no muscles in them and cannot use them for anything. Give it another few eons or so, and their descendants may have no visible external arm structures at all, similar to the legs of whales or snakes. Or they might go the other way and evolve some musculature if it proves advantageous for some purpose.

Even humans have vestigial tails, which in most of us have no visible external structure, but occasionally manifests on some people due to mutations.

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