I think the Devon is a time period, which is about 359 - 416 million years ago ( and Silurian 416-443), which means that the Devonian Dragon and the Silurian Dragon are very old dragons. This could be a reason, why they can´t be the result of mutation.
whaaaat!?!?!?
you're implying that they can't be result of a mutation because they're too old?
maybe you mean that evolutioning speaking, a mutation is a new patern on an individual that isn't really intended for adapting, but may be heritable.
but this is more like a sci-fi kind of mutation, a insta-mutation, as we see in game like a skill