This thread is about the Ancient Files. The author says this:
"The wyvern is actually probably the adloscent form of the Western Dragon, though Western Dragons categorally deny this".
this supposition comes, IMO, from the appearance of baby dragons, looking like wyverns.
However, as the majority of 4th era people thinks of dragon as dangerous mind-less murders, I came to think that this belief originates from adolescent dragons (which, hence, can't be wyverns).
I think that wyvern are a form of degenerate cousins of dragons (like monkeys and humans-no I don't imply here that monkeys are degenerated humans...).
To highlighten my opinion:
Dragons spends 100 years in baby form. If wyvern were adolscent dragons, and if dragons awoke after 2000, no wyvern would appear before 2100. Thus adolescent dragons (roaming around and dangerous) are not going to appear before some time.