Reply To: Mold, Cast and Steinkern

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Jack Kallmeyer

@dave-carlson Regarding your cephalopod question.  We have that same kind  of preservation here in our nautiloid fossils.  We may be getting to splitting hairs here and I don’t know if it’s worth making the distinction of calling them steinkerns or not.  I would guess that these are some kind of combination steinkern and body fossil.  I am guessing that the siphuncles may have been calcitic in life while the rest of the shell was aragonitic.  That would explain why the difference in preservation of the different parts although there is a lot of chemistry that affects the animal after death that determines what the eventual fossil preservation becomes..

In our area we have one particular Member in the Kope Formation where all mollusc shells are preserved rather than being steinkerns.  Even in other areas where the external shells are gone, sometimes even parts of the septa are preserved.  Other times, it may just be the siphuncle.