User blog comment:BannedLagiacrus/Monster Appreciation Week: Molten Tigrex/@comment-4702773-20160124053148/@comment-17735225-20160124155812

Ah, I see your point. That coupled with low gravity theory works wonder on explain MH's ecology.

But that type of lung would still have limited oxygen intake that is suit to its normal habitat though, meaning it shouldn't benefit much more from increased oxygen. Lungs evolving to take advantage of more oxygen isn't common either.

I'm speaking from Tigrex/Brute >>> Molten theory though. If Molten comes directly from Primal Tigrex or Akantor line then your explaination fits perfectly.