evolutionarily impossible
The flick starts very well, it seems to have a lot of thought put on it. It depicts dragons as being related to crocodiles rather than to dinosaurs, if I understood it well, it tries to explain scientifically every single thing. It fails for a moment when stating that the dragons survived the meteor impact by means of the aquatic species (what for? aquatic species at that time also became extinct, due to massive pollution of the waters), which suggests that the "current" dragons are just the aquatic dragons re-evolving their flying abilities, which is entirely implausible but passable.
But then it fails altogether with no hope of redeem when it depicts a four-legged dragon. The first dragons had their limbs with membranes to form wings - ok, very realistic. But how come the later ones gained four legs and two wings? There's no credible evolutionary path for that, as any good student of evolution theory can tell you.
They should have consulted better informed biologists to formulate the movie. It has shown such a promise!