User blog comment:IamJakuhoRaikoben/Chapter 412: Dance Unto Ishgar, Review./@comment-1359322-20141208144403/@comment-9595671-20141208160821

There's that, but there's also one other thing that makes that up for debate. Acnologia's killed Dragons too, how do we know if Dragon Slayer magic doesn't leave bones behind?

In fact how do we even know if those were dragons who were killed but their bodies stayed intact? That means when they died their bodies were still there to rot after death, Atlas Flame was confirmed dead since he's the eternal flame and I don't recall seeing his bones in the area, that seems to hint his body was fully destroyed.

Basically we don't really have a full idea just yet.