Hello Jack (@jkallmeyer),
Out here we get cephalopod fossils in the Ordovician strata that have no shell material but retain the internal structure of the siphuncle. The entire mass of the fossil is solid (seems like a steinkern) but the fill matrix can be separated from the siphuncle, even though they look essentially the same. This suggests a process more complicated than with a bivalve or brach. Is there a better term than simply “steinkern” for this?