Preserved brachs or bivalves in limestone - fleshy parts?

[Pippa]

Found this chunk of limestone at my Lake Michigan's sand depleted "beach". Due to the extremely high water level, storms have washed away pretty much all the sand at this beach, exposing the large underlying rocks.

What do you think of the almond-shaped preservation of the interior parts, while most of the shells themselves have been dissolved away? 

 

 

 

 

 

 

[digit]

The beaches of the Great Lakes are having a bit of a problematic time at the moment with the very high water levels. I'm hoping this might lead to good conditions for hunting for Petoskey Stones in Michigan next April (I may bring my waders).

 

The insides of these brachiopods or bivalve mollusks (I don't know them well enough to differentiate) have been sediment filled and the resulting internal molds are often called steinkerns from the German for "stone seeds". The name may seem odd but I'm certain that the name was attributed to these internal molds on some smaller mollusk where the internal mold likely looked something more like a pistachio seed.

 

We have Eocene sites (Ocala Limestone) which preserves mollusk impressions but not usually the shells which have dissolved away long ago and left the gaps between the outer and inner molds. Often, scientists will fill the negative cavities with materials like silicone to make a positive from the negative outer mold to see what the shell would have looked like. If I remember correctly, some bivalves have a stronger calcium carbonate shell--calcitic rather than aragonitic--which can last quite a bit longer before dissolving. Oysters and scallops, if memory serves, have calcitic shells and preserve better. The tests of many echinoderms are calcitic which causes them to preserve often with beautiful detail on the tests.

 

 

Cheers.

 

-Ken

[Tidgy's Dad]

They seem to show bilateral symmetry and two halves of each valve divided by a septum. 

So they're brachiopods, I think. 

[Pippa]

10 hours ago, digit said:

The beaches of the Great Lakes are having a bit of a problematic time at the moment with the very high water levels. I'm hoping this might lead to good conditions for hunting for Petoskey Stones in Michigan next April (I may bring my waders).

 

The insides of these brachiopods or bivalve mollusks (I don't know them well enough to differentiate) have been sediment filled and the resulting internal molds are often called steinkerns from the German for "stone seeds". The name may seem odd but I'm certain that the name was attributed to these internal molds on some smaller mollusk where the internal mold likely looked something more like a pistachio seed.

 

We have Eocene sites (Ocala Limestone) which preserves mollusk impressions but not usually the shells which have dissolved away long ago and left the gaps between the outer and inner molds. Often, scientists will fill the negative cavities with materials like silicone to make a positive from the negative outer mold to see what the shell would have looked like. If I remember correctly, some bivalves have a stronger calcium carbonate shell--calcitic rather than aragonitic--which can last quite a bit longer before dissolving. Oysters and scallops, if memory serves, have calcitic shells and preserve better. The tests of many echinoderms are calcitic which causes them to preserve often with beautiful detail on the tests.

 

 

Cheers.

 

-Ken

If I remember correctly, I think I read that people find the best Petoskeys in the rivers. You might want to check that out. Don't know if you would still need waders.

 

Haha, I know what "Steinkern" means. I just didn't know it was used for petrified interiors of shells. I grew up in Switzerland and German is my native language. Comes in very handy when trying to understand scientific vocabulary, especially in geology.

So the word "Kern" has many, many meanings:  kernel, pit, seed, core, center, beginning and many more. One being "stone". I kid you not. For example the hard pit of a stone fruit (apricot, peach, cherry or plum) is often just called  "Stein" or Aprikosenstein, Pfirsichstein etc.  So Steinkern could be translated as, petrified pit, stone pit, stone seed, stone core, stone center etc. or even petrified stone. Ha! That creates a tautological error, like "frozen ice" does.

At any rate, now when I google Steinkern brachiopods, bingo! I get many pictures as well as articles on the subject. It's so very helpful  to have the correct terminology when googling.  I really appreciate your help, thank you. 

 

Interesting too, re: creating casts from molds and from those molds casts again? This gets into creating sculptures. I think I will leave that to the professionals though.

[Pippa]

13 hours ago, Tidgy's Dad said:

They seem to show bilateral symmetry and two halves of each valve divided by a septum. 

So they're brachiopods, I think. 

I think you are correct about that. 

Personally, I've been just so thrilled to learn that I've got Steinkerne, I forgot to even wonder what type of shelled animals have been fossilized in that rock.

 

 

 

[FossilNerd]

23 hours ago, Pippa said:

What do you think of the almond-shaped preservation of the interior parts,

8 hours ago, Pippa said:

I just didn't know it was used for petrified interiors of shells.

Since your title says “fleshy parts” I just wanted to clarify that a steinkern is not the preservation of the actual fleshy interior parts of the brachiopod. The fleshy parts have all decomposed and gone away. The interior of the shell then filled with sediment that eventually hardened. Similar to filling an empty bucket up with cement and letting it dry. The resulting chunk of cement would be a steinkern of the bucket. 

[Pippa]

16 minutes ago, FossilNerd said:

Since your title says “fleshy parts” I just wanted to clarify that a steinkern is not the preservation of the actual fleshy interior parts of the brachiopod. The fleshy parts have all decomposed and gone away. The interior of the shell then filled with sediment that eventually hardened. Similar to filling an empty bucket up with cement and letting it dry. The resulting chunk of cement would be a steinkern of the bucket. 

Oh, I do realize that. I guess i should have said "shell interior" vs. fleshy parts, as it's just the interior shapes that are visible.  The word "preserved" was probably not the best choice either, I just didn't know what other term to use.  

[FossilNerd]

40 minutes ago, Pippa said:

Oh, I do realize that. I guess i should have said "shell interior" vs. fleshy parts, as it's just the interior shapes that are visible.  The word "preserved" was probably not the best choice either, I just didn't know what other term to use.  

No worries! Just wanted to clarify.