In my story there are scary monstrous sea serpent like creatures of all shapes and sizes. What the people don’t know, is (reverse whale evolution theory) they came to shore and turned into something like a dragon. My model idea was a flying fish, but I don’t know that it scientifically would have enough muscle in its near transparent wings if its wingspan is 60ft. I could hollow out the bones, but should her body look less narrow and snake like? Maybe take away the transparency idea from flying fish wings?[
Adding a little magic can often help situations where science fails.
As this is low fantasy, you may want to make this a thing of wonderment, an impossibility in the midst of more grounded reality. And, the people living in your world will simply be experiencing it, so they, and your readers don't need to be privy to the exact track of their evolution. Nevertheless, as a world-builder, you are looking to build that out and have an understanding of it as an underpinning.
Your order of evolution is:
water animal, then land dragon.
This is less far-fetched than you believe. Because all evolution according to current modeling is that literally EVERYTHING started in the water. All it really means is that there are simply extra steps in-between water animal to the dragon.
But since you already have serpent water monsters, the steps in-between are actually far fewer and less of a stretch than say, between a fishy creature and a human.
Your creature can look any way you like, and as for flight requirements, my opinion is that you should look less to science and how it might get to this point, and more to magic, plus inspiration from legendary water dragons.
In the Japanese tradition, what they call dragons are what the western world would call sea monsters or sea serpents. They do seem to get air and it looks as though they fly but the majority are wingless and associated with bodies of water.
I am not suggesting that your dragon look anything like this, I just thought it was an interesting find.
This is more about what you want than what is. If you would like a through-line connecting them back to their fishy origins, keep the wing transparency. I say this for a couple of reasons:
- This makes your dragons less typical than the standard dragon.
- It keeps a connection that you want, even if it is never explained outright.
- Like, how cool would it look?
The only dragon I can think of with a transparent wing are the small pseudo dragons, fairy dragons, which have wings based on butterflies. Your version is much more metal!
You tagged your question low-fantasy, good!
I absolutely, positively love this idea!
The answer to your question: I believe using a flying fish as your model with some good rationalization of how the evolved dragon would need to change to accommodate things like greater bulk and eating (e.g.) deer rather than bugs is a great idea. The fact that what a flying fish is right now isn't what a dragon would need to be is irrelevant. Stick with that low-fantasy tag, start with the flying fish, and simply change as little as you might need to arrive at the dragon with your (hopefully) predetermined attributes and capabilities.
Please do NOT get too wound up with the idea of scientifically justifiable flying dragons. Unless you count things like pterodactyls, there have never been dragons upon the Earth, which means we don't have anything to draw from to rationalize your creation. It's all guess work.
Please do NOT get too wound up with the differences between flying fish and dragons. Evolution theorizes that all life on Earth started with a primordial soup. That soup didn't have the attributes necessary to justify, for example, fingers and toes. You can't erase a connection between two points along an evolutionary path simply because the attributes between them are, apparently, too different. From a simplistic but practical perspective, the only thing required to evolve anything is time.
It's not only tempting to believe everything about your fictional world must have a scientific basis, we're kinda trained to believe that nowadays. Remember that there's a HUGE difference between suspension-of-disbelief and hard science. I believe a dragon modeled after a flying fish is one of the most creative ideas I've seen in years. Whether or not the dragon you design meets the rigid and often boring requirements of science is irrelevant beyond suspension-of-disbelief, is it not?
short answer: Your dragon is too big (and heavy) for flying on an earth-like planet
When considering what hypothetical species could do, it is always good to look at reality to see what is plausible and what is not. You already mentioned whale evolution theory. It is not even necessary to go that far, since life was coming from the ocean in the first place, so a sea animal going to the land (or into the air) is definitely plausible since it has happened before.
As it was pointed out before, flying fish do not really fly, they glide. However, flying was developed independently by multiple species (birds, bats, insects) and flying fish are already halfway there, so this is plausible as well.
If we are looking at big current flyers, we can see that all of them are quite small compared to big land or sea animals. The albatross is the biggest flying animal, has a wingspan of 3,5 m (~11,5 feet) and weighs around 12 kg. Even the albatross has trouble starting from the ground and needs a runway (https://www.youtube.com/watch?v=gKTfcs6LL6A). I guess your dragon could not be much larger than that, since the force necessary to lift off would be just too high (assuming earth-like gravity and pressure). Hollow bones will not help significantly. All animals mostly consist of water and water is heavy.
So if you are open to reduce gravity and increase pressure significantly, I think this can work (https://what-if.xkcd.com/30/ the last part). In this case, flying would be the favored mode of transportation for most animals.