unknown invert

[M Harvey]

This is embedded in hard calcified sandstone layer of the upper Eutaw fm.  I have been collecting from the site for years but have never encountered anything like this.  The paleontologists I have shown it to are also stumped.  The cell structure is similar to a pelecypod, but this is no bivalve.   The shape is reminiscent of an egg but fairly sure it isn't that.  The diameter is 12 cm.   I know that photos cannot capture all the details.  Any guesses?

[Fossildude19]

Some sort of echinoid, perhaps? 

[DPS Ammonite]

If the shell is calcite and fizzes in acid consider a barnacle. They get up to 15 cm across. Note ladder like cross pieces in cross section of shell.

 

Your piece:

 

 

 

https://deepseanews.com/2016/10/tgif-barnacles-put-a-ring-on-it/

 

 

 

https://www.asnailsodyssey.com/index.php?x=420

Edited March 23 by DPS Ammonite

[M Harvey]

16 hours ago, Fossildude19 said:

Some sort of echinoid, perhaps? 

I considered that and there is an echinoid common to the formation but this specimen has a smooth uniform surface.  It also has two attached  individual cells.  It is unfortunate that the tops are missing.

[M Harvey]

17 hours ago, DPS Ammonite said:

If the shell is calcite and fizzes in acid consider a barnacle. They get up to 15 cm across. Note ladder like cross pieces in cross section of shell.

 

Your piece:

 

 

 

https://deepseanews.com/2016/10/tgif-barnacles-put-a-ring-on-it/

 

 

 

https://www.asnailsodyssey.com/index.php?x=420

It is mildly reactive to 10% acetic acid.  I tried researching Cretaceous barnacles and couldn't find a match.  Any idea as to species?

[DPS Ammonite]

3 hours ago, M Harvey said:

It is mildly reactive to 10% acetic acid.  I tried researching Cretaceous barnacles and couldn't find a match.  Any idea as to species?

No idea of barnacle species. Look to see if any Cretaceous echinoids have similar ladder like structures in their tests; I don’t think that they do.

[Al Dente]

This side looks like an oyster.

 

 

[M Harvey]

On 3/23/2024 at 3:22 PM, DPS Ammonite said:

No idea of barnacle species. Look to see if any Cretaceous echinoids have similar ladder like structures in their tests; I don’t think that they do.

I see what you are saying.  At first I was skeptical because my specimen does not have the interlocking plates.  However, the ladder structure does seems diagnostic.  I am going to go with barnacle until proven otherwise.