Mississippian mystery ?shell, UK. SOLVED: Isogramma, an unusual brachiopod

[TqB]

I have seven specimens of these, all with the same appearance and strange internal structure. They have perplexed me for years though I'm familiar with most fossils from the area. (I originally had three and have recently collected four more which enables them to be characterised more reliably. Altogether, five individuals are fairly complete).
One small exposure of marine shale, Mississippian, Brigantian Stage, northern England, Co. Durham.

 

The main features are:

1) See photos 3 & 4 -The shell material is strange, being irregularly honeycombed, resulting in a granular apperarance on the surface. I haven't seen any other shells at all like this from the same beds - brachiopods and bivalves being common.

 2)They're roughly semicircular and shell-like. They very possibly have been paired down the straight edge. (Two of my seven specimens each consist of two disarticulated individuals, one on top of the other.) Bilaterally symmetrical, with a central sulcus or ridge.

3)The straight edge is thin and bent over. Where preserved it terminates with distinctive sharply curved ridged ornament.

4)They're all about the same size, 35-45mm across, and seem to be about 1mm thick.

Although superficially brachiopod like, there's no sign of a hinge or umbo, and the texture isn't like any brachiopod I know.

The fine structure closely resembles Jurassic ammonite aptychi of Laevaptychus type so aptychus is one possibility I've been considering though the shape doesn't match any I've found. Also, they're really quite common in the small source exposure whilst cephalopods are scarce (just one small orthocone to date). And Palaeozoic aptychi are generally rare and would be surprising.

I have a few other ideas but will see what people can come up with. I hope it's obvious to someone!

Here are four specimens, two of them being the possibly associated pairs. Thank you for looking!

 

Specimen 1, paired:

 

Specimen 1, top member removed (it broke off when drying!):


Specimen two, worn fragment showing honeycombed internal structure. (All the specimens show this.)

 

Specimen 2, closeup:

Specimen 1, edge detail:

 

 

Specimen 3, paired, partial at bottom overlaying complete one. Note close similarity of edge with previous photo (specimen 1)

 

Specimen 4, with better preserved circular edge:

 

EDIT:
All seven specimens so far collected, in the same orientation to show their morphological consistency.

 

Edited May 27, 2023 by TqB

[westcoast]

This reminds me if something I found in similar aged rocks.This might sound like a strange suggestion but have you considered stromatoporoid?

[Tidgy's Dad]

Interesting. 

It looks like bryozoan zooecia to me, but is it a weird form of massive bryozoan or an encruster on some sort of shell? 

[doushantuo]

Tarq,i take it you have considered Gigantoproductus?

Edited May 24, 2023 by doushantuo

[TqB]

26 minutes ago, westcoast said:

This reminds me if something I found in similar aged rocks.This might sound like a strange suggestion but have you considered stromatoporoid?

I hadn't, but they're all near identical shells with the odd internal structure so I think there's only the one shelly organism involved.

 

20 minutes ago, Tidgy's Dad said:

Interesting. 

It looks like bryozoan zooecia to me, but is it a weird form of massive bryozoan or an encruster on some sort of shell? 

It's definitely the internal structure of all of them, not an encruster.

 

16 minutes ago, doushantuo said:

Tarq, i take it you ahve considered Gigantoproductus?

I'm pretty sure they aren't brachiopods. There are plenty of productids in the beds and they're obvious.

Edited May 24, 2023 by TqB

[doushantuo]

i was about to change my mind,but you were quicker than me.

I think it's SLIGHTLY too large for a Cravenoceras aptychus,but...

[doushantuo]

The polygonal structure reminds me of the shell structure of a hyolith operculum,BTW

edit: I'm convinving myself it IS a hyolith operculum

Edited May 24, 2023 by doushantuo

[Tidgy's Dad]

Not brachiopods. 

I'm sticking with bryozoan colonies for now, 

Really unusual, whatever they are. 

[TqB]

4 minutes ago, doushantuo said:

i was about to change my mind,but you were quicker than me.

I think it's SLIGHTLY too large for a Cravenoceras aptychus,but...

Thank you, that is along possible lines I was considering.  The bilateral symmetry is a problem, though this sort of thing is a bit closer (though Silurian):
(From https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1502-3931.1984.tb00670.x)

 

[doushantuo]

the polygonal "ornamentation" might be trepostome bryozoan(see Adam's remark)or the coral Hyostragulum if that genus ranges up to the Mississippian

Edited May 24, 2023 by doushantuo

[TqB]

55 minutes ago, Tidgy's Dad said:

Not brachiopods. 

I'm sticking with bryozoan colonies for now, 

Really unusual, whatever they are. 

Thanks for confirming not brachiopod, Adam!  I'm sure that bryozoans aren't involved as the cellular structure is internal. This patch shows well preserved shell outer surface with the granulation reflecting the internal porous structure that only shows on worn or broken surfaces.. There are no pores on the surface.

 

 

46 minutes ago, doushantuo said:

the polygonal "ornamentation" might be trepostome bryozoan(see Adam's remark)or the coral Hyostragulum if that genus ranges up to the Mississippian

Thanks for the suggestion, Ben. I don't think any colonial organisms are involved as the structure is internal to the shell sufaces - see above.

 

[TqB]

1 hour ago, doushantuo said:

The polygonal structure reminds me of the shell structure of a hyolith operculum,BTW

edit: I'm convinving myself it IS a hyolith operculum

That's an intriguing idea. I've never found a hyolithid in these beds (or ever, actually). Would they ever get large enough for these to be their opercula?

 

[Tidgy's Dad]

I don't think that hyoliths got much bigger than 4 or 5 cm, though I could be wrong. and I can't see that a hyolithid operculum could possibly be this big, 

They were also pretty rare in the Carboniferous and one of the two orders was already extinct. 

[TqB]

@Tidgy's Dad @doushantuo I think that these Jurassic aptychi show similar structure, b2, c1, c3, though it's on a larger scale.

(From: https://www.researchgate.net/publication/311204509_Late_Jurassic_aptychi_from_the_La_Caja_Formation_of_northeastern_Mexico)

 

 

Edited May 24, 2023 by TqB

[Tidgy's Dad]

Ah, goniatite aptychi, I hadn't thought of that. 

It would also explain why they are sometimes paired, do large goniatites occur in this formation? 

Edited May 24, 2023 by Tidgy's Dad

[doushantuo]

I was aware of that paper,but considered the stratigraphic gap between mississippian and jurassic pretty large,and subsequnetly dismissed the possibility

[doushantuo]

you might like(particularly for fig 3):

size 4.2Mb

a Peltoceras aptychus

 

Mironenko_A_Structure_ASGP_Vol.69_No.4_2021.pdf

 

or:

especially figs 2-4 are extremely relevant

1783257507_Farinaccietal1976_aptychi.pdf

Structural features of some Jurassic and Cretaceous aptychi

This one comes with the HIGHEST possible recommendation from me

for the record: some BDSPI back issues are open access,this being one of them

Edited May 24, 2023 by doushantuo

[TqB]

4 minutes ago, Tidgy's Dad said:

Ah, goniatite aptychi, I hadn't thought of that. 

It would also explain why they are sometimes paired, do large goniatites occur in this formation? 

Not yet. And these mystery things are quite common there really so you'd expect some.  

(Possible orthoconic nautiloid aptychi are known from the Silurian (Aptychopsis, if they're not phyllocarids) though this is going out on a limb as orthocones are of course common in the Carboniferous while any appropriate aptychi are unknown I think? I've found just one small orthocone at this locality, no other cephalopods.)
Basically, I'm still stuck.

 

 

[TqB]

10 minutes ago, doushantuo said:

you might like(particularly for fig 3:

size 4.2Mb

a Peltoceras aptychus

 

Mironenko_A_Structure_ASGP_Vol.69_No.4_2021.pdf 4.19 MB · 0 downloads

Thanks, good paper, similar on the outside.  I know it's a stretch comparing with Mesozoic ammonoids and I'm probably on the wrong track altogether, clutching at straws.  

[doushantuo]

Funnily enough.Ammophila arenaria *leaves concentric circles on beach sand

*Marram Grass root

Edited May 24, 2023 by doushantuo

[doushantuo]

Porpitoids* are pretty durable,are circular and can show concentric AND radial ornament,and are approximately of the right size

*hydrozoans/"chondrophorines

Extant porpitoids are subject to mass strandings,BTW

Edited May 24, 2023 by doushantuo

[TqB]

10 hours ago, doushantuo said:

Porpitoids* are pretty durable,are circular and can show concentric AND radial ornament,and are approximately of the right size

*hydrozoans/"chondrophorines

Extant porpitoids are subject to mass strandings,BTW

Beautiful things, but always full discs I think? I wonder if the quite well known but undescribed "jellyfish" from the Brigantian of Scotland belong there. About the same size, as you say. (30mm+)

 

 

[doushantuo]

Tarq,see if you cant get at:

doi.org/10.1007/BF03031748

(stanley/Yochelson/Pal.zeitschr.1983

It shows a not totally unreasonable visual match to your specimen

 

[TqB]

4 minutes ago, doushantuo said:

Tarq,see if you cant get at:

doi.org/10.1007/BF03031748

(stanley/Yochelson/Pal.zeitschr.1983

It shows a not totally unreasonable visual match to your specimen

 

Thanks, Ben, just a paywalled one I think. I know that some sort of "hard" medusoid is responsible. 

[doushantuo]

Those who know me may suspect that I have a giganormous stack of relevant German literature

Just nod

NEW JELLYFISH TAXA FROM THE UPPER JURASSIC LITHOGRAPHIC LIMESTONES OF CERIN (FRANCE):
TAPHONOMY AND ECOLOGY
CHRISTIAN GAILLARD, JACQUELINE GOY, PAUL BERNIER,
JEAN PAUL BOURSEAU, JEAN CLAUDE GALL§, GEORGES BARALE,
ERIC BUFFETAUT– and SYLVIE WENZ

[Palaeontology, Vol. 49, Part 6, 2006, pp. 1287–1302

1.4 MB

see fig.8

 

 

Palaeontology - 2006 - GAILLARD - NEW JELLYFISH TAXA FROM THE UPPER JURASSIC LITHOGRAPHIC LIMESTONES OF CERIN FRANCE (1).pdf

Edited May 25, 2023 by doushantuo