The show suggests the idea that dragons could have overcome their weight problem by using hydrogen filled bladders for lift. This would not work at all. In the show, they estimate the capacity of the juvenile dragon's bladders as 30 cubic feet. 30 cubic ft of hydrogen will only reduce the dragon's weight about 2 pounds! Even if an adult had a capacity of 200 cubic ft, it would only reduce its weight by about 15 pounds. In an animal that weighs at least 1000 pounds, this is negligible. In fact, the animal would have to be so much larger to accommodate the bladders that it would actually be far heavier with them than without them.
There is a problem, the blimps weigh far more than the dragons, and they can fly.--Hulk10 (talk) 17:47, February 23, 2016 (UTC)

Then there's the fire-breathing thing. First, a dragon would never evolve fire-breathing because it wouldn't really need it. After all, it has jaws, talons and can fly, so it could easily catch prey and escape predators anyway. Second, if it did breath fire, it wouldn't have wanted to use hydrogen or methane (as in the movie) because they both burn too quickly. The gases would just flare right in front of the animals face (actually, a liquid fuel like alcohol would be better). Platinum would lower the ignition temperature somewhat, but not down to body temp. And of course platinum is extremely rare and never found in high concentrations in nature.

Then there's the idea that the dragons survived the K/T extinction by living in the water. They would probably have lost the fire-breathing capability? It certainly wouldn't work on fish! Besides, hydrogen bladders would make them bob like a cork. I suppose they could have re-evolved it later, but then, with no T-rex around, they need it even less than before.

How about having to keep their eggs in an oven? No other animal has to keep their eggs that hot, so why would the dragons? It's a senseless waste of valuable energy.