The "fossil" reminds me of something I have seen many times at the new-to-me lake I have been fishing. I see this floating "stuff" big as a softball, even a soccer ball, that looks like it's made of rock but it floats and the waves take it across the surface. It looks a bit similar to what you have. The only explanation I have come up with after inspection is that it might have something to do with nutria or beaver lodge material. There are a lot of nutria on this lake and probably some beavers because there are several what appear to be beaver lodges. I am not aware of nutria building lodges, I thought they just dug burrows. Anyway, this "stuff" looks like it could come from the mud and stick concoction that beavers manufacture when building a lodge. It doesn't seem possible for it to float but it sure does. I was thinking that the plant material and also maybe air bubbles within give it the buoyancy. Then again, I do not know what this stuff is. I have never seen it anywhere else.