How do marine dragons evolve flight without any feathers and no hollow
Symbiotic Rocket Power.
Symbiosis is defined as:
any type of a close and long-term biological interaction between two
different biological organisms, be it mutualistic, commensalistic, or
Your dragon has many stomachs and pseudo stomachs.
- One such supports and nourishes a colony of the bacterium Nitrosomonas Eutropha which can filter the ammonium ions from the dragon's blood (as the liver does in us) and:
oxidize ammonia in the absence of dissolved oxygen, replacing
molecular oxygen by nitrogen dioxide or nitrogen tetroxide [Dinitrogen tetroxide]
Liquid at room temperature, the Dinitrogen Tetroxide is drained away and stored in a muscular pouch adjacent to the dragon's anus.
It is unstable and is easily hydrolyzed to the toxic compound
Similarly liquid at room temperature, the Monomethylhydrazine is drained away and stored.
The Monomethylhydrazine and Dinitrogen Tetroxide are held in muscular pouches beneath the tail of the dragon, when the pouches squeeze, the sphincters release, the jets of these compounds mix in a fiery and fierce hypergolic stream of high pressure flame providing directable forward thrust and enabling immediate flight.
This is the same fuel mixture that the Apollo moon landings used for landing and takeoff to orbit and the LEM didn't even have wings.
This is a reaction which can occur in a vacuum and with the appropriate adaptations at the exit port, there's every reason to suppose it could be turned to usefull effect under water. As to the evolution part, just as a squid's defence mechanism is to squirt ink to cloud the water, a natural extention of this adaptation - providing a quick escape from predators would be a burst of rocket speed.
The jumbo jet wingspan of your dragons would support enormous weight. In the case of the jet, 320 short tons. 151.2 tons of Jet-A fuel would take the aircraft 10,800 km at speeds of 907 km per hour.
If the dragon were to fly at 100 km per hour, a five hour flight would take it 500 km, and use much less than the 7.5 tons of fuel that simple arythmetic would indicate because drag would be so much less at lower speeds - lower drag than a jumbo would mean that the lift to drag ratio could be improved over the aeroplane's to help carry those heavy bones.
The difference in the specific impulse of the aircraft's engines and the dragon's fuel is so small as to be negligible.
The dragon's use of thermals to gain height for extra glide length would extend the range without extra fuel cost and make up for the extra fuel cost at takeoff.