Sky Beasts

Describe the fossils of an ancient species, do they still exist:
There is an off branch of whales that are rumored to exist. These great beasts however were never found at the bottoms of the seas, they have only ever been found along coast lines and mountain tops. There bones are mostly hollow, but it is unknown if their bones were actually bone. They had large bat-like wings that spread for acres with (relative to them) thin bony protrusions that would be used to support their skin on the wings. Their main body did resemble a humpback whale, however they have not been found to have flippers, or even any kind of landing gear at all. This has lead to the argument that they never land, or legs have just never been found.
Natural historians have been having a rough time justifying how this beast actually flew, or if it did. Some believe that the beasts were filled with a buoyant gas that kept them a float, and their wings were more used to turn direction. Some believed that this beast stayed aloft by a use of magic, this is somewhat credible as the bones have higher than usual properties in gravity magic. This line of thought is highly debated however, as they cannot know whether the fossilization process did this, or if it was innate property of the beast itself.
There are a few specimens of this species that are known to exist. Colossal leviathans of an unbelievable scale. There a few complete fossils that have been found along the coast. The ones found on mountains usually have eroded pieces. They are hunted by the Druidic Circles of all walks to add to their natural histories. It is believed that a few possible off shoots have evolved into various different species over the eons, but the sight of this creature has never been recorded.  This hasn't dissuaded the Circles though, as the Lion turtles were thought to be extinct not so long ago as well. Until that village found it's new home on its back. Perhaps they are incredibly long lived, or live very high in the atmosphere where no one can see them. Their hollow bones, and massive wings had to be used for something.