Need help with a Pennsylvanian gastropod from Texas

[BobWill]

This came from the Late Pennsylvanian, Jacksboro LImestone of the Graham formation near Jacksboro, Texas. At first I thought it was Shansiella but the flat bottom and huge umbilicus doesn't fit. It is 4 cm across and 3 cm high with the tip of the apex probably missing.

 

 

Edited October 20, 2022 by BobWill

[DPS Ammonite]

Does it have a selenizone?

 

Nothing like in McKinzie’s Color Guide and the Dallas Paleo Society’s pictures of Pennsylvanian fossils.

[BobWill]

7 minutes ago, DPS Ammonite said:

Does it have a selenizone?

 

Nothing like in McKinzie’s Color Guide and the Dallas Paleo Society’s pictures of Pennsylvanian fossils.

No selenizone. Josh Handley has suggested Euconospira but the only images I find either have a flat whorl profile on the sides and bottom or rounded on both. Maybe there is enough variation to account for a rounded side and flat bottom. I don't have a picture of the umbilicus on those to compare.

[BobWill]

I just heard from Josh Hanley on the DPS Facebook group who believes it is Shansiella broadheadi Hoare 1961

Any confirmation?

[DPS Ammonite]

Mourlonia?

 

https://fossil.15656.com/2020/05/04/pennsylvanian-mourlonia-from-pennsylvania/

 

Also this is one of the best papers for Desmoinian gastropods from nearby New Mexico. Contact Barry Kues.

 

BARRY S. KUES and  ROGER L. BATTEN"MIDDLE PENNSYLVANIAN GASTROPODS FROM THE FLECHADO FORMATION, NORTH-CENTRAL NEW MEXICO," Journal of Paleontology75(sp54), 1-95, (1 January 2001). https://doi.org/10.1666/0022-

https://www.jstor.org/stable/1315576#metadata_info_tab_contents
 

 

[BobWill]

9 hours ago, DPS Ammonite said:

Mourlonia?

 

https://fossil.15656.com/2020/05/04/pennsylvanian-mourlonia-from-pennsylvania/

 

 

 

This appears to have a smaller umbilicus and the bottom is rounded. 

[cngodles]

So the biggest mystery around Mourlonia is that it is SUPPOSED to be in the Pennsylvanian in North America, but there doesn't appear to be any sort of good documentation to support it. If I remember correctly, it exists before, it exists after, but nobody has seemed to find one in-between.

 

It does look very Shansiella-like to me, I'm assuming this is just a steinkern. A steinkern would give it that umbilicus, which they don't have when the shell remains.

 

I've found compressed Shansiella in the past, which could give it the flat bottom in preservation. This specimen here has quite the flat-bottom.

 

 

Or this comparison of a specimen with normal dimensions and one that has been pancaked down.

 

 

[ClearLake]

Here is a snippet from the above mentioned Hoare (1961) publication defining Shansiella broadheadi.  Note the definition contains "base strongly rounded" and "wide, distinctly marked selenizone".  That would seem to pose a problem for your specimen.  The flatness in yours does not look like it is due to compression to me.  Figure 7 is Shansiella broadheadi and #8 is Shansiella carbonaria.  Also, to my eye, the specimens figured below have a more evenly rounded profile to each whorl then your specimen, but maybe I'm reading too much into that.

 

Just wanted to provide this info, I don't have a better suggestion.  Perhaps the differences can be explained away due to preservation.

 

[BobWill]

On 10/29/2022 at 12:13 PM, cngodles said:

 

 

It does look very Shansiella-like to me, I'm assuming this is just a steinkern. A steinkern would give it that umbilicus, which they don't have when the shell remains.

 

 

I thought it was a replacement fossil like most of what we find there but now you got me wondering. It would have taken a very thick shell to cover that huge gap though.

[cngodles]

11 hours ago, BobWill said:

It would have taken a very thick shell

 

They do have very thick shells. Here is one I cut in half.

 

 

I also think this steinkern is probably one. Note the huge umbilicus-looking thing there.

 

[cngodles]

I'd also like to point out this Shansiella by Batten from a locality nearby, but not the same as above. We've been taking note of shell thickness differences between different locations of the same genus/species. The thickness there is so much thinner than my specimen.

 

 

http://research.amnh.org/paleontology/media/images/6/9/88747_specimen_representations_media_6981_large.jpg

Edited October 31, 2022 by cngodles
to correct/add photo

[BobWill]

19 minutes ago, cngodles said:

 

They do have very thick shells.

 

 

 

 

 

Wow! Would a steinkern show the ornamentation on the outer surface or the growth lines underneath like mine? Most of the gastropod internal molds I find are from Cretaceous deposits and are completely smooth.

[cngodles]

6 minutes ago, BobWill said:

Would a steinkern show the ornamentation on the outer surface

 

In examples I have with the shell removed, the spiral ornament is preserved a little. It's not like full ornament, but you can certainly see waves in the steinkern where the ornament was.

 

This highly eroded shell shows it well. You can see the under sweeping growth lines, and you can see the ornament preserved in sort of a ghostly image. It's not always the case, but if you preserve just a tiny portion of the base shell layers, you can get this effect. You can see it in Batten's specimen above as well, where the shell is two distinct layers, inner and outer. If you preserve the inner and not the outer, you can get that effect.

 

Edited October 31, 2022 by cngodles

[DPS Ammonite]

Sometimes you can have an interior mold, steinkern, that preserves a layer of the shell that is in the middle of the shell layer. The inner layer is usually aragonite, mother of pearl, which usually dissolved before the calcite outer shell. Thus the steinkern preserved features intermediate between the inner and outer surface of a shell: the boundary between the calcite and aragonitic layers in a shell. 

[BobWill]

45 minutes ago, cngodles said:

 

In examples I have with the shell removed, the spiral ornament is preserved a little. It's not like full ornament, but you can certainly see waves in the steinkern where the ornament was.

 

Quote

 

 

 

 

I see. I guess this didn't show up well in my original post but maybe this is why I thought it was a replacement fossil rather than a duplication fossil. There appears to be a layer of shell at the top of this image.

 

Edited October 31, 2022 by BobWill

[BobWill]

8 hours ago, DPS Ammonite said:

Sometimes you can have an interior mold, steinkern, that preserves a layer of the shell that is in the middle of the shell layer. The inner layer is usually aragonite, mother of pearl, which usually dissolved before the calcite outer shell. Thus the steinkern preserved features intermediate between the inner and outer surface of a shell: the boundary between the calcite and aragonitic layers in a shell. 

I used to think of material preserved like that as external casts but I was told by a museum director with a PhD in paleontology that external and internal casts were rare and if you see some shell it's not likely to be a duplication fossil but more likely a replacement fossil, often occurring in conjunction with a mold. His credentials doesn't mean he was right but I wonder how you would confirm which it is.

 

Judging by the one @cngodles cut open above there is a pretty big space to close in at the umbilicus for a single layer, whatever name it goes by. The close-up in my last post shows what looks like a layer of shell and I just, now noticed there is an extremely thin layer on top of that. This shows it in a darker color, on the right.

 

[DPS Ammonite]

2 hours ago, BobWill said:

wonder how you would confirm which it is.

Who’s on first! What are my choices? Does it refer to your fossil?

[BobWill]

14 minutes ago, DPS Ammonite said:

Who’s on first! What are my choices? Does it refer to your fossil?

Sorry, too cryptic...no, the fossil you described, a steinkern having some shell-like material from one layer. I think you described it as part of the steinkern if I understood it correctly. Can it be a piece of shell preserved by replacement rather than part of the steinkern? and how would you tell?

[DPS Ammonite]

Bob, I was trying to describe a steinkern, (hardened sediment inside a shell), as possible being deposited against a shell that had some of its internal layers dissolved, especially the mother of pearl. Such steinkern would show features of both the external and internal part of the shell. Also, original or replaced shell could be attached to a steinkern. I guess that you would have to look at such a steinkern in thin section to see if it is sediment or part of the original shell or replacement of shell.

 

Did I answer your question Bob?