Fossils are formed over millions of years. How do scientists quantify the extent to which a particular fossil has changed in shape/relative tissue density/etc. over geological timescales (if at all)? I ask this (as a non-biologist) because we are often presented with fossils as being exact copies of the original species, as it was when it was alive. I wondered if this is indeed the case and imagine that the answer will vary for fossils of soft versus harder tissue composition.
If organisms do indeed retain their shape almost entirely, what experiments are carried out to mimic geologic timescales and processes to demonstrate this?