Winged Beasts, a squandered resource?

[Gordis]

Originally Posted by Boromir88

I'm pretty sure there is some mention of them having feathers, but there's not much else.

It had no feathers:

And behold! it was a winged creature: if bird, then greater than all other birds, and it was naked, and neither quill nor feather did it bear, and its vast pinions were as webs of hide between horned fingers; and it stank.-LOTR

Now wings as "webs of hide between horned FINGERS" is a feature of mammals, for instance bats.
See this comparative anatomy illustration comparing wing structure in pterodactyls (upper picture) in bats (the picture in the middle) and birds (bottom picture)
Wings

Edit: Here a better drawing: Wing Bones
with explanations here
The question is whether Tolkien himself was aware of these facts. Probably he wasn't.

The absence of feathers speaks against the FB being a bird. All the rest, (except the wing structure) indicates a reptile.