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The fossil record indicates they evolved far back in their ancestry from something not dissimilar in appearance to a lobe finned fish, but with an extra pair of bilaterally symmetrical fins, though they now look surprisingly similar to the appearance of the more serpentine mythological dragons of earth.

They have electricity generating organs similar to an electric eels that no doubt originally developed under similar evolutionary pressures for similar reasons but now serve a different purpose, they use it to extract hydrogen from water by electrolysis which is stored in evolved swim-bladders that have now become a series of valve linked chambers storing hydrogen under varying pressure.

Much like some fish they retain both lungs & gills.

The hydrogen can be released through the valves from the high pressure bladders into 'normal' swim-bladders for more buoyancy in both air or water & (if there's time) it can also be pumped back through valves into higher pressure champers through muscle contractions to reduce buoyancy.

If they need to lose buoyancy quickly the swim bladders can be vented through the gills, nostrils & mouth though if in flight the mouth is closed & the nostrils pinched shut much like a camels to avoid slowing their passage with the rapid expulsion of the gas.

The Question : If the animal has a body mass similar to a typical race horse, so about 1,000 pounds, how big do the fully inflated unpressurised swim-bladders need to be to allow the animal to fly by flapping rather than gliding in a similar gravity & air pressure to earth's?

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    $\begingroup$ A cubic meter of hydrogen on Earth is capable of lifting ~1.2 kilograms. So for a 1000 lb animal to reach neutral buoyancy requires roughly 377 cubic meters of hydrogen. Of course, this isn't considering how much the wings would contribute but you haven't given us any way to estimate that. Either way, it's entirely impractical for an animal that large to float on Earth. $\endgroup$ – Mike Nichols Sep 13 at 20:02
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    $\begingroup$ Eating food, then burning calories to generate electricity, then using the electricity to electrolyse water seems a bit of a roundabout way of doing things. There are various biological pathways that involve liberating hydrogen that might be more reasonably used instead... $\endgroup$ – Starfish Prime Sep 13 at 21:13
  • $\begingroup$ They jump out of the water out onto low planet orbit. Making them look like mini solar flares. $\endgroup$ – Dehbop Sep 14 at 6:05
  • $\begingroup$ @MikeNichols : So a cubic balloon a smidgen less than 7.3 meters a side? doesn't sound too bad :) $\endgroup$ – Pelinore Sep 14 at 8:04
  • $\begingroup$ @StarfishPrime : I'm definitely open to alternatives :) can you name them so I can Google? $\endgroup$ – Pelinore Sep 14 at 8:12
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Assuming I am understanding the question correctly, the evolved creature is going to be flying by using a hydrogen balloon. Based on the comments above, suggesting the creature needs @ 377 m^3 of lift gas, the overall size would be a cylinder about 3.5 m in diameter and 10 m long. For a more "dragon" like appearance, you could go for a narrower diameter but much longer cylindrical form, for example a creature 2 m in diameter would be 30 m long.

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Nice model or photoshop of a traditional Chinese Dragon

Energetically, it will likely use some sort of chemical path to releasing hydrogen, perhaps using bacteria in the microbiome in the creature's gut, and sending it to a series of bladders evenly distributed along the length of the creature. Physically, it may resemble a Zeppelin internally, with a long rib cage to protect the bladders, and te ability to move like a snake in water and air for propulsion (it would need a dorsal and ventral fin along the length, and the ability to massively extend the fins in the atmosphere). Alternatively, it could mimic the flying fish, jumping into the air and extending wing like fins along the side instead. The amount of energy needed to compress hydrogen is likely too large to make sense, the creature is most likely going to vent small amounts for buoyancy control instead.

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Snake skeleton

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Flying fish

What isn't really clear from the description is why a creature would evolve this way? It is very large, but fragile and unwieldy. Based on the description it does not seem to be a predator (or is preying on even weaker and slower moving creatures), but it does not make much sense as a herbivore either.

While a balloon like creature may make some sort of sense, you would have to define the ecosystem much more clearly to understand why such a creature would exist and how it interacts with the rest of the environment. Based on the sort of information given (i.e. none), I might conclude the best form for a balloon based creature isn't a "dragon" at all, but something like a jellyfish which floats over an area and stings its prey and consumes it by sending digestive chemicals down the tentacles and absorbing nutrients back up

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A 4.5 m radius sphere as the top portion supplies the necessary lift

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