In the Dragonology Field Guide to Dragons by Ernest Drake, it states clearly that a Wyvern is a type of dragon, not a whole different species altogether.
"The biggest of all dragons, with larger scales and fewer spines than other species; up to 50 feet long; 18 to 20 feet high, their central toe alone can be the size of a small child.
Not to mention, it's in the 'Classification Of Dragon Species' section of the book, and the specimen name,
Draco africanus, refers to Draco obviously as dragon, and africanus where it lives, in Africa and Arabia, not to mention the Sahara and Kalahari Deserts.

Oops. I've said too much, haven't I? Sorry, I am quite a dragon freak. I don't mean to offend you. Also, I would like to know where you got your information, just curious.
~JX