- If the Forest and the Mountain Dragons are the descendants of the Sea Dragon, unable to breath fire, how did they evolve exactly the same fire-breathing ability as the Prehistoric Dragon? In evolution, if something that complex disappears, it is almost impossible to reappear exactly the same way (for example, that's why whales breathe air: their fish ancestors lost their gills when they came to land, and gills couldn't re-evolve when the whales returned to water).
- The program never specifically stated that the marine dragon lost its fire breathing ability, but did explicitly inform that they retained the flight bladder (which is what allows them to use breath fire) that they inherited from the last common ancestor they shared with the prehistoric dragon. Also the fish to whales example isn't completely apples with apples given that some fish ancestors evolved to amphibians, some amphibians evolutionary descendants include land based reptiles, some reptiles evolved to synapsids (AKA mammal-like reptiles), Synapsids evolutionary descendants include mammals, of which a line would eventually lead to Ambulocetus which in turn is the ancestors of whales. In contrast, the evolutionary line of Dragons remained with the same Taxonomic class of Reptilia. Thus the forest, desert, and mountain dragons retaining the ability to wield fire is less of a stretch than it seems at first.
- I feel I should point out that despite all appearances, the Marine Dragon wasn't actually proven to exist in-universe. Even in the context of the show, the Marine Dragon was completely speculative, its existence based entirely on the presence of sea serpents in mythology, the palatal valve of the Mountain and Forest Dragons (which, unlike the whole process of fire breathing, IS a simple enough structure to have evolved more than once). That's not much to go on. So perhaps the reality of things was a little different? Perhaps dragons survived the K-Pg extinction by some other means (maybe by being small like mammals and birds) and the Marine Dragon (if it did exist) was more of a distantly related offshoot than a direct ancestor of the more recent species? It seems possible that, even after he's been proven to be right all along, Dr. Tanner's still making mistakes.
- The way he connects the mountain dragon to a marine dragon is by a previous find of his with the same palatal valve. He misidentified it as an extinct crocodile ancestor.
- Convergent evolution, simple as. Note that unlike the examples offered the dragons still retain the organs associated with the process; a more accurate comparison would probably be the limbs of a whale, which went from the fins of a fish ancestor to the legs of a mammal to back into propulsion organs once they returned to the water.
Headscratchers / Dragons: A Fantasy Made Real