User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-4212769-20120302173919/@comment-4212769-20120524234931

Now I know where I read about the Tigrex being the ancestor of all Wyverns. It is in the Quest details of the Absolute Power quest of the Elder Quests. I want to quote it:

"The Tigrex, said to be an ancestor of today's wyverns, has finally made an appearance..."

So either Capcom made a mistake about the biology in MH or another theory can be:

The Tigrex hasn't changed much in appearance the last thousands or tenthousands of years (like sharks and crocodiles in real world). So it was referred to be the same like when he was found and still could be called "Tigrex". So it can be considered as ancestor of the wyverns.