It's one of those silly "in play" things. There actually used to be a rule that you could choose to mulligan if a fossil was the only thing in your hand that would count as a basic. With the newer rules, if you have a fossil, you have to play it as your basic. That is the only time that card is looked at as a basic while not sitting on the bench or playing field. Roseanne's can't even pick it up in the deck, and Dialga G's deafen prevents it from being played.