<p>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.</p>

<p>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.</p>

<p>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.</p>

<p>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.</p>

CC BY-SA 3.0
2017-07-12T05:26:16.150Z
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.
John
CC BY-SA 3.0
2017-07-12T13:22:18.450Z
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.
verisimilidude
CC BY-SA 3.0
2017-07-12T14:06:11.423Z
Provided some of the original minerals were present, otherwise it is a casting.
John
CC BY-SA 3.0
2017-07-12T14:35:01.617Z
Burrows and tracks are generally considered trace fossils, not impressions. trace fossils also include coprolites and feeding marks.
John
CC BY-SA 3.0
2017-07-11T04:19:04.933Z
2017-07-11T04:19:04.933Z
verisimilidude
<p><strong>Impression</strong> is just that, an impression, an imprint of the fossil, but the fossil that created it is gone (or just separate) such as <a href="https://www.bas.ac.uk/wp-content/uploads/2015/04/by-43-1_cm.jpg" rel="nofollow noreferrer">this</a> impression of a ammonite.</p>
<p><strong>Pseudomorphism</strong> is just a new term for a natural casting. It can be a cast of the fossil, or a cast of the fossil's impression; again, the shape is preserved, and the original fossil is separate 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 <a href="https://s-media-cache-ak0.pinimg.com/originals/ff/db/a8/ffdba83c45d383067abd78aae078cd67.jpg" rel="nofollow noreferrer">snail</a> shell is also a pseudomorph, the entire thing has been replaced with agate.</p>
<p>In <strong>true fossilization</strong> some component of the original material is still present. This can preserve internal structure. With bone, for instance, a true fossil often has some of the original calcium/phosphate with pore spaces around it filled in with other minerals.  Of course, like anything in nature, pseudomorphs and true fossils are not completely discrete, but ends on a spectrum; partial replacement is the norm.</p>

CC BY-SA 4.0
2017-07-10T22:49:06.360Z
2024-05-27T03:53:18.087Z
2024-05-27T03:53:18.087Z
John
<p>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. </p>

<p>One of the possible fates is "true fossilization". Another is "impression and pseudomorphism". What's the difference between these two?</p>

<geology><terminology><fossils>
"Impression and pseudomorphism" in fossils
fdgsdg
# "Impression and pseudomorphism" in fossils

(Asked by: username_0 on 2017-07-10)

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?

**Tags:** geology, terminology, fossils


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## 2 answers

### Answer by username_2 on 2017-07-10. Score: 3 | Accepted answer

**Impression** is just that, an impression, an imprint of the fossil, but the fossil that created it is gone (or just separate) such as [this](https://www.bas.ac.uk/wp-content/uploads/2015/04/by-43-1_cm.jpg) 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 fossil's impression; again, the shape is preserved, and the original fossil is separate 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](https://s-media-cache-ak0.pinimg.com/originals/ff/db/a8/ffdba83c45d383067abd78aae078cd67.jpg) 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 true fossil often has some of the original calcium/phosphate with pore spaces around it filled in with other minerals. Of course, like anything in nature, pseudomorphs and true fossils are not completely discrete, but ends on a spectrum; partial replacement is the norm.


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### Answer by username_1 on 2017-07-11. Score: 1

I disagree with username_2 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.

> **Comments:**
> **username_2**: 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.  
> **username_1**: 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.  
> **username_2**: Provided some of the original minerals were present, otherwise it is a casting.  
> **username_2**: Burrows and tracks are generally considered trace fossils, not impressions. trace fossils also include coprolites and feeding marks.  

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