Brachiopod

[Jim K]

Last week we took a ride to Lake Michigan to do a short search before the weather turned bad. The previous day had strong winds with large waves and I thought we might find some interesting things. We found plenty of Favosites, Halysites, and Rugose corrals as well as some of the other usual finds. The one unusual find was what I believe to be an internal mold of a Pentamerus oblongus brachiopod. I don't find many brachiopods and as such I'm not 100% sure on my ID. Please confirm or correct. Thanks in advance.      Jim

[daves64]

@Tidgy's Dad

[Tidgy's Dad]

Hmmmm.

It's a bit big, but within the size limits for the species, the cross markings at the anterior end are suggestive of the species. 

There's not enough of it left after erosion to be sure. 

It could be a polished bit of quartzite that vaguely resembles a pentamerid internal mould, or it could actually be a very worn Pentamerus. 

I wouldn't like to say either way. 

[Jim K]

2 hours ago, Tidgy's Dad said:

Hmmmm.

It's a bit big, but within the size limits for the species, the cross markings at the anterior end are suggestive of the species. 

There's not enough of it left after erosion to be sure. 

It could be a polished bit of quartzite that vaguely resembles a pentamerid internal mould, or it could actually be a very worn Pentamerus. 

I wouldn't like to say either way. 

Thank you. The size, shape and arrowhead pattern are what led me to my ID. Yes, Lake Michigan has a way of smoothing out the details.

 

[Harry Pristis]

I accept that this is a brachiopod.  It is not, however, a steinkern. 

 

Think carefully about how steinkerns, internal or external CASTS, are formed.  A cast is formed in a MOLD, either the exterior or the interior of the brachiopod, in this case.  When a mold is infilled, no internal structure (the hollow or the pedicle channel, in this case) is preserved in the resultant cast. 

 

The logic dictates that this is NOT a cast; therefor, it's not a steinkern.

 

Same for Jim K's find.

[Al Dente]

5 hours ago, Harry Pristis said:

I accept that this is a brachiopod.  It is not, however, a steinkern. 

 

Think carefully about how steinkerns, internal or external CASTS, are formed.  A cast is formed in a MOLD, either the exterior or the interior of the brachiopod, in this case.  When a mold is infilled, no internal structure (the hollow or the pedicle channel, in this case) is preserved in the resultant cast. 

 

The logic dictates that this is NOT a cast; therefor, it's not a steinkern.

 

Same for Jim K's find.

A steinkern will show the structure of the inside of the brachiopod shell. This steinkern is showing the ridges on the inside of the shell. 

[Al Dente]

Here are some examples pulled from the internet of Pentamerus steinkerns. Most or all of these are from the Midwest Silurian dolomites where calcite shells aren’t preserved, only the steinkerns and external molds.

 

 

[Harry Pristis]

First, we should agree that "steinkern" is a term that includes two different types of cast -- interior cast and exterior cast.  That is, a cast showing the interior of the shell and a cast showing the exterior of the shell.  Neither type of cast preserves interior structure of the animal such as the body lumen or other structure not closely attached to the shell.  Both type of casts produce a massive lump of fossil-lookalike, regardless of how detailed the outside of the cast looks.

I'm not saying there can't be casts of brachiopods.  I'm saying that Ursula's and Jim K's finds are not casts.

[FranzBernhard]

1 hour ago, Harry Pristis said:

I'm saying that Ursula's and Jim K's finds are not casts.

Not being a cast (internal mold) in the strict sense, its the void of a brachiopod mostly filled with sparry calcite (precipitated from solution), leaving a small, crystal-filled hole (at least in the case of Ursula´s specimen).

Franz Bernhard

Edited October 1, 2021 by FranzBernhard

[tombk]

1 hour ago, Harry Pristis said:

Neither type of cast preserves interior structure of the animal such as the body lumen or other structure not closely attached to the shell. 

So the shape of the interior of the shell would be a primary determinant of the shape of an internal cast. If the interior of the shell looked like the following picture, I could see the OP’s find being an internal cast of something like it. 
 

[Harry Pristis]

34 minutes ago, tombk said:

So the shape of the interior of the shell would be a primary determinant of the shape of an internal cast. If the interior of the shell looked like the following picture, I could see the OP’s find being an internal cast of something like it. 
 

 

I do understand your suggestion, tombk.  You are suggesting that this pedicle valve would produce an internal cast which would resemble Ursula's find.  I think you're wrong.  For one thing, the pedicle opening and the lumen are evident on Ursula's find.  A cast of the interior of a brachiopod could not reproduce those features. 

  Your illustration is a representation of the interior surface of a pedicle valve of a brachiopod.   That's about all we can say.