Impression and pseudomorphism in fossils?

Question

In a book on invertebrate paleontology I have (Treatise on Invertebrate Palaeontology, A) there is a diagram showing possible fates of a shell after the death of the organism. One of the possible fates is "true fossilization". Another is "impression and pseudomorphism". What's the difference between these two?

Answer 1

Impression would be an impression of the original organism.

Pseudomorphism would be a duplication of the dimensions of the original organism except the organism would have been replaced by something else such as rock.