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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?

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Impression are just that, an impression, an imprint of the fossil but the fossil that created it is gone (or just separate) such as this impression of a ammonite.

Pseudomorphism is just a new term for a natural casting it can be a cast of the fossil or a cast of the fossils impression, again the shape is preserved the the original fossil is seperate or has dissolved and been replaced, for instance the natural casting of the braincase of hominids can be used to reconstruct missing parts of the skull. This snail shell is also a pseudomorph, the entire thing has been replaced with agate.

In true fossilization some component of the original material is still present this can preserve internal structure. With bone for instance a ture fossil of a bone often has some of the original calcium/phosphate with pore spaces around it filled in with other minerals. Of course like anything in nature pseudomorph and true fossil are not completely discrete but ends on a spectrum, partial replacement is the norm.

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I disagree with John that a "true fossil" still has a component of the original material. A trilobite shell that has had its chitin replaced with silica is a fossil even though all the original chitin is gone.

I am not sure what your textbook's author is saying about a fossil that is the result of a shell's diagenesis (change in form as it fossilizes) could create a pseudomorph or impression.

Usually a pseudomorph is a term for a false fossil, something that looks like it was alive but was produced only from inorganic chemical reactions - like rust staining of sandstone producing a shell-like pattern.

An impression is usually the term for something like fossil tracks or burrows that show evidence for life in the geologic past, but are not themselves the preservation of a life form.

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    $\begingroup$ trilobite shells are not only made of chitin (chitin by itself is soft and flexible) there's quite a large mineral component as well, mostly calcite and calcium phosphates. $\endgroup$
    – John
    Jul 12, 2017 at 5:26
  • $\begingroup$ I didn't say that trilobites were only chitin, I said that if all the chitin (that is, all the organic matter) were replaced with silica the result would still be a true fossil. Sorry if that wasn't clear. $\endgroup$
    – verisimilidude
    Jul 12, 2017 at 13:22
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    $\begingroup$ Provided some of the original minerals were present, otherwise it is a casting. $\endgroup$
    – John
    Jul 12, 2017 at 14:06
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    $\begingroup$ Burrows and tracks are generally considered trace fossils, not impressions. trace fossils also include coprolites and feeding marks. $\endgroup$
    – John
    Jul 12, 2017 at 14:35

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