The criteria used must be 'philosophical', and cannot be based on a
survey or public opinion. What makes art 'art' is that it is:

1) Representational (more or less)

2) Not causally linked to anything else for its subject matter

By criterion 1, a fossil could be art because it is representational
(that is, it is a 'likeness' of something, such as a sea-shell.

By criterion 2, a fossil cannot be 'art' because it is causally linked
to the existence of something else.

So, fossils are not 'art'.

A man-made object torned out on a lathe or whittled that looked exactly
like a fossil would be art. One could imagine someone who is gifted
enough to be able to turn out very good pieces that look exactly like
fossils. He sculpts raw materials by hand. Is that art?

Yes. Why? Because there is no causal connection between the existence
of the fossil and the artwork.

No, let us consider someone who makes copies of fossils (through
molding or a similar process) and mass-produces them.

Is that art?

No. Why? Because there is a causal connection between the existence of
the fossil and the reproduction.