User blog comment:RennaGraceCarter/Chapter 327 My Part Too - Review/@comment-203.82.92.26-20130414012153/@comment-5951335-20130414042057

Hmm... I agree with the Dragon slayers coming from 400 years ago part. Layla Heartfilia may have used her magic to open the Eclipse gate to bring them through, which resulted in her death. And I think the reason that the dragons seemingly disappeared is because after the dragon war (which resulted in the bones under the arena) the few survivors escaped to the present time, making it seem like they went extinct.

Another theory I saw somewhere is that Wendy, Natsu, and Gajeel are actually the dragons Grandine, Igneel, and Metalicana, as a result of Dragon slayer magic having a sort of side effect that turns its users into dragons. When they thought their dragons disappeared, it was actually them going through the gate. The images of their dragons shown in the manga and anime were simply their dragon souls locked within them. This one has some holes in it, but it could be possible...