Mold, Cast and Steinkern

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    John Christian

    I consider steinkerns formed from sediment that entered a hollow natural object (such as a bivalve) to always be internal molds. Am I correct?

    If a hollow natural object (bivalve) is not filled by sediment but is later filled by a precipitated mineral such as calcite  then the calcite should be considered an internal cast when the original shell material has eroded away. Am I correct?

    Here’s a picture of the bivalve Protarca tramitensis from the Cretaceous Woodbine sandstones near Dallas Texas. They were deposited into a sandstone but were not filled with sand and were left hollow for awhile. Later they were filled with crystalline calcite. The fossil on the right has no shell left. What should the calcite nodules be called when the shell erodes away: an internal cast or an internal mold?

    What is the best scientific source for the definitions of “mold”, “cast” and “steinkern”? It seems that the dictionary definitions of a steinkern call it both a cast and a mold. Also, mold and cast are often used interchangeable in scientific literature.

    Any thoughts? @rleder, @bdattilo, @bmacfadden, @crobins, @rebecca-freeman, @jkallmeyer

    Thanks for the help.



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    Benjamin Dattilo

    I am not authoritative, but I am picky. With steinkerns there is confusion because internal molds are tricky. A mold is a negative impression of the original formed by contact with the original.  A cast is a positive impression of the original formed by contact with the mold. Negative = cast, positive = mold.  Material or mode of filling does not change this. That is level 1 basics, it can get complex…but, maybe later…


    To understand the confusion with steinkerns, consider (for e.g, a statue that an artist makes in clay, molds in plaster, and casts in bronze) that typically a mold is a hollow space and a cast is a solid object. In the case of internal molds, the original is a hollow space and the mold is a solid object. No wonder people get mixed up: a steinkern is a solid object. Negative = mold, positive = cast.

    John Christian

    Thanks Benjamin,

    So to simplify, a cast is made only against a mold. Right? A mold is only made by direct contact with the original fossil. Correct?  Therefor my steinkern is an internal mold since it formed against the original shell… right?

    I am a little confused by your use of “positive” and “negative” in reference to impressions. Does “positive” and “negative”  refer to concave and convex? If “negative = cast” then the definition of a “mold” would be a: mold impression of the original formed by contact with the original. That is a confusing definition.

    If “negative” = mold = convex, then some molds break that rule. A steinkern which can be an interior mold is convex in most bivalves. Exterior molds are usually concave but are partly convex when the original bivalve has a concave valve (some pectens and brachiopods.)

    Thanks for you help.




    Bruce MacFadden

    To me a steinkern is the internal sediment filling of a fossil like a gastropod or bivalve. It is a cast. A mold as far as I am concerned is an outer concave thing. For example when you are making a fossil replica you pour a latex or other flexible substance around the real fossil.  When the latex is cured, you cut through it and remove the real fossil, and then you pour plastic (or similar substance) into the void. When the void filling is hardened you remove the fossil replica (the cast) from the surrounding mold.

    There may be other interpretations, but this is mine!


    Benjamin Dattilo

    It really is difficult to use the right words to describe a thing. I meant a different idea.

    Maybe just leave the positive negative off:

    A mold forms from the original, a cast forms against a mold. Steinkerns are definitely molds.

    Jack Kallmeyer

    John (@john-christian),

    I just printed this quote from Amadaeus Grabau in the Dry Dredgers bulletin this past month.  It is in agreement with answers of others but this old time quote is a good one:

    Solution and the Formation of Molds and Casts (p.100) – Amadeus Grabau 1899

    “Carbonated, or otherwise acidulated waters will dissolve calcareous fossils, if the strata in which they are embedded are pervious to the water.  Thus, a mold of the exterior of a dissolved shell, for example, may remain in the rock, while a mold of the interior, formed by the mud or sand which found its way between the valves, will remain within the mold of the exterior.  If the rock is under pressure, it may happen that the two molds are pressed against each other, and the stronger features of one may become superimposed upon the weaker features of the other.  Thus, fossil mussels may show the external striæ impressed on the internal mold, showing at the same time the muscular impressions in relief.  Occasionally the space between the two molds, i.e. that formerly occupied by the shell, may be filled by infiltrations, and a cast of the original shell may thus be produced.”

    I believe the key to remember here is that a mold is made from an original object (or facsimile thereof) while a cast is made from a mold.  A cast reproduces the actual original object in form.  So, your footprint in mud is a mold of your foot.  Plaster poured into that mold makes a cast reproduction of your foot.


    Rebecca Freeman

    The way I always explain it to students is that to get a mold (internal or external impressions) you must have the original shell. To get a cast the shell must disappear first, leaving only its impression(s).

    Love the Grabau quote!

    John Christian

    @rleder, @bdattilo, @bmacfadden, @crobins, @rebecca-freeman, @jkallmeyer

    Thanks for everyone’s help.

    Using suggestions provided by you and information from other sources, I am creating a set of definitions and rules to help everyone determine if a fossil is a mold or a cast. I am interested in making the definitions and rules better, clearer and easier to use. I need the help of everyone.

    Here is what I have:

    1. A mold is an imprint of an original fossil.
    2. Molds can only be made against an original fossil.
    3. A cast is a replica and/or reasonable copy of an original fossil.
    4. A cast can only be made against a mold of an original fossil.
    5. An original fossil is defined as an organism, fossil, fossil to be or a cast thereof.
    6. Trace fossils are various tracks, trails and burrows that give us information about what the original organism did while alive.
    7. The surface of a trace fossil created by the original organism against a material is an original fossil when the surface of a trace fossil created by the original organism against a material cannot create a cast of the original organism that created the trace fossil.
    8. The surface of a trace fossil created by the original organism against a material is a mold when the surface of a trace fossil created by the original organism against a material can create a cast of the original organism that created the trace fossil.

    Here are some related rules or corollaries:

    9. An original fossil must exist before the mold of the original fossil.
    10. The mold of an original fossil must exist before the cast of the original fossil.
    11. A cast of the original fossil cannot be created against the original fossil.

    Let’s apply the definitions and rules to an actual situation. Is a filling contained within the original shells of a closed bivalve a mold or a cast? A mold (the infilling) is an imprint of the original fossil (the closed bivalve.) The mold (infilling) can only be made against the original fossil (closed bivalve.) Remember, a cast (replica and/or reasonable copy of an original fossil) can only be made against a mold. The infilling can create a cast (the interior only) of the original closed bivalve. The infilling is not a cast because the infilling is not a replica and/or reasonable copy of an original fossil.

    It is important that we agree what the original fossil is first. Then we can determine what are the fossil mold(s) and cast(s). We must also remember that a cast of the original fossil must be a reasonable copy of the original fossil.

    I welcome any suggestions, new definitions and rules along with supporting rational arguments and reasoning. Sources for definitions would be helpful.

    I am also willing use these definitions and rules to see if a fossil that you describe is a mold, cast or other type of fossil. The more we use these rules and definitions, the better they become.

    Once we improve these definitions and rules please use them.



    Jack Kallmeyer

    Wow!  I think your definitions are basically correct but my question is whether they improve on those already given by others.

    I would take exception to your definition in #5.  An original fossil can not be a cast.  It may be a body fossil but it is not an original fossil.  Perhaps the problem I am having is the use of the term original fossil.  Where you use this term, you really  mean the original flora or fauna which may never have been preserved as a fossil at all.

    Your statements regarding trace fossils in #7 & 8 do not apply to my senses.  There are trace fossils – evidence of the activity of animals – and there are body fossils – the remains of the actual body whether that be a mold, cast or original remains.  We do not apply the terms mold or cast to trace fossils. There are specific technical terms that apply to these traces as outlined in Osgood, 1970.  You should refer to that document regarding trace fossils.

    I may be wrong here but I don’t believe there is any disagreement about the definitions of molds and casts in the rank of the professional paleontologists.  The only ones whom I have seen confuse this are amateurs.


    John Christian

    Thank you Jack (and everyone else.) @jkallmeyer

    You provided me a very helpful analysis that found the same two weaknesses that I was most concerned about.

    If I substitute “body fossil” for “original fossil” will all the other definitions and rules make sense and create no major contradictions? I hope that a “body fossil” includes a cast because a mold can be made against a cast (second generation mold.)

    I created rules 7 and 8 because I have noticed that many professional paleontologists consider that casts and molds can be made from well executed trace fossils such as dinosaur footprints. I assume that one could reason that a footprint infilling is a cast because a “caste is a replica and/or reasonable copy of a body fossil” (the foot.) Do you think the infilling of a well executed and preserved footprint can ever be considered a cast? Should I take out 7 and 8 and include a note that some paleontologists think that casts and molds can be made from certain trace fossils such as footprints?

    Thanks again for your help Jack and anyone else.


    Jack Kallmeyer


    I would say that body fossils include the molds and casts found as fossils as well as petrified original remains.  I think this works for what you are trying to understand.  Technically the molds may not be body fossils since they represent the negative space left by the now missing original body material.

    As far as trace fossils go, the ones found “in the wild” are preserved in different fashions that are described in Osgood 1970 and by others.  Yes, you can make a mold from these originals and then make a cast from the mold to make a cast replica.  I really think that is all you need to say.  Simpler is better.  No need for long detailed explanations.


    John Christian

    Thanks again Jack. @jkallmeyer

    Since body fossils include molds and casts, using body fossil would not work in the definitions because according to 2 “: Molds can only be made against molds. (Not true.) What is the definition of: “A mold is an imprint of _______________? I could make up and define a new term of art such as “alpha fossil” to put in the “_____________”. For example: “an alpha fossil” is defined as an organism, fossil, fossil to be or a cast thereof. I guess that I am looking for the paleontology equivalent of Black’s Law Dictionary to find definitions of mold and cast.

    Are there any rules when applying the terms mold and cast to trace fossils such as the infilling of a (shrimp) burrow or the infilling of a footprint? The infilling of a burrow is a ____________? I suggest that “infilling of a burrow is a mold of the interior of the burrow.” Also some people claim that: the infilling of a burrow is a cast because the borrow is a mold of the organism that created the burrow. The last sentence makes no logical sense because a “cast is a replica and/or reasonable copy of” an organism. A burrow infilling looks nothing like a shrimp.

    In other words, if there are no coherent definitions for mold and cast referring to trace fossils, I would like to create a set and publish them. My definitions/rules 7 and 8 could be a basis for the new definitions.

    Thanks again.

    Jack Kallmeyer


    I really think you are over complicating this with your attempts to improve these definitions.  The definitions of molds and casts were made very simply at the beginning of this thread and that’s really all you need.  Molds are made from originals, casts are made from molds.  It’s that simple as I see it.

    I suggest that you take trace fossils out of this mold/cast realm. Trace fossils are a whole different bag of worms.  It would be helpful if you would do some research on them before trying to invent new definitions.  I suggest Osgood 1970, Frey 1975 and Seilacher 2007.  Trace fossils fall under a large variety of morphologies.  Trace fossils do not ever include imprints or molds of body fossils although those imprints and sometimes true body fossils are directly associated with the trace that the organism made.  These are wonderful finds as they specifically identify the trace maker, i.e., the horseshoe crab fossil (body fossil) at the end of its own trackway (trace fossil)  in Solnhofen.

    I don’t think I can help you any further.



    @bdattilo, @bmacfadden, @crobins, @john-christian, @rebecca-freeman,

    I agree with Jack’s comment, there is no need to make it more complicated. Grabau’s definition is very good and I guess since he was half German he should have known the meaning of Steinkern also very well 😉

    Btw: with this section: “… Occasionally the space between the two molds, i.e. that formerly occupied by the shell, may be filled by infiltrations, and a cast of the original shell may thus be produced.” he describes the “Ersatzschalenerhaltung” I explained earlier (ersatz shell).



    Jack Kallmeyer

    Ronny, @rleder

    How about providing a phonetic pronunciation key to Ersatzschalenerhaltung ?  I guess I can see why steinkern caught on and this term didn’t. 🙂



    Hey Jack (@jkallmeyer),

    I think ‘ersatz shell’ will do fine, no need for breaking your tongue with ‘Ersatzschalenerhaltung’ 😉




    Hey John (@john-christian),

    I was thinking about your efforts and recommendations to better define the terms of mold, cast, steinkern etc.. I just think that there is some kind of a grey area in all that and that the real meaning will come up with the context. For example do paleontologists still have problems to define a fossil or at least they do it in different ways. Some say a fossil is a remnant of a plant or a animal at least 10000 years old, other think that the structural change within the taphonomic process makes a fossil a fossil. It was also discussed in several forums and the one at researchgate just put it together like this: “fossil” is every bone, imprint, mould, mummified / frozen body, etc. of past living organisms, to the exclusion of those which belonged to still existing populations.”  
    ( [accessed Apr 6, 2016].) That is somehow a bit wobbly because they tried to cover every situation.

    With our case of mold, cast, steinkern etc. we will end up in a similar way. Everything seems to be debatable and we sometimes might have a hard time to follow one definition for every case. Nature is too complex and we have to decide from a contextual basis.

    at the end I really see the benefit for all of us while you are trying to clarify that, because we always should remind ourselves if we use phrases in the correct way.

    Thank you

    all the best


    Dave Carlson

    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?


    Dave Carlson


    Jack Kallmeyer

    @rleder, @bdattilo, @bmacfadden, @crobins, @rebecca-freeman, @dave-carlson

    I stick by my previously submitted quote from Grabau. Your clam on the right in your image is an internal mold, i.e., it was made from the original shell by an infilling of mud.  This is a steinkern. If you coated it with a silicone compound you will have a cast of the original shell interior.

    Let’s say the original shell was buried but did not get infilled at all.  Further the aragonitic shell dissolves through time but the hollow cavity remains.  That hollow cavity has the impression of the outside ornamentation of the shell and would be an external mold.  If you pour silicone compound into that cavity you will reproduce the original shell morphology, i.e., a cast of the original shell’s exterior.  If this cavity were filled naturally, the result would be a natural cast of the shell. Still a steinkern.

    As Grabau says, there are times when an internal mold exists inside of an external mold, and, through pressure, the external mold is pressed against the internal mold producing a steinkern with the features of both the inside of the shell and the outside on the same object.

    As to your question about a source for a definitive definition and why the literature seems to use mold and cast interchangeably, I refer you to the varying opinions we here have all given to you.  Even though my answers are the only correct ones 😉 I believe the others are welcome to their own definitions even if they are wrong….. LOL


    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.


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