I hope this is not going into too much detail.
Can we take two fossils of different species and say that one is the ancestor of another ?
Well for a start we can't say that the individual animals were ancestor and descendant. The best we can manage is that the species are related as ancestor and descendant.
Often we don't have the fine-grained record that would allow us to say even that the species of the later fossil is the descendant of the older fossil. Sometimes we do, but usually we don't.
So usually we are restricted to saying that the older fossil is either the ancestral species or closely related to the ancestral species of the more recent fossil.