Weren’t they already frequenting the island?

As someone else said, no cloning from a fossil. To expand on that a bit, a fossil is when all the organic material, all the actual parts of whatever is getting fossilized, is sloooooowly washed away and replaced with stone. For macroscopic things like bones, that means we get to keep the shape and maybe texture, but for microscopic things like DNA, it’s just gone. Waaaaay too fine to get locked in the fossil most likely, and even if it was there, how do you even read it? It’s rock sitting in a blob of other rock, except there’s some structure there.

Since they’re fossilized, there’s no longer any biological material; cloning wouldn’t be possible.