Hello Achim; How about an internal cast of a belemnite shell?

Best regards, Jim Des Lauriers

Hi Achim,

reminds me of a weathered longitudinal section through the arm of a crinoid in the genus Cupressocrinites.

Cheers

Markus

Can it be a polyplacophoran? Chiton? a guess based on number of plates (8)??

Yes, looks like crinoid segments, as Markus Poschmann pointed out.

Surely resembles a weathered crinoid segment. Attaching a picture of a better preserved specimen from the Jurassic of Jaisalmer, India for comparison.

This is certainly crinoid column or arm

Dear colleagues,

thank you for the helpful hints.

At first I thought that it could actually be a weathered internal cast of a mollusk. In a belemnite, however, the septa would have to be concave at this flat cross-section.

What spoke against a crinoid fossil for me was the fact that the elements are connected in the middle axis (I would rule out a polyplacophoran for that reason.), they seem to have a concentric structure and do not show the typical calcite fracture that can otherwise be observed in fossil echinoderms.

I have a somewhat detailed picture.Can you imagine that diagenetic changes of a crinodite elements actually produce this appearance?

Achim; I agree with your assessment. Cheers, Jim Des Lauriers

Hi Achim,

based on the last picture from you, i think, it could be the internal sipho of a Devonian palcephalopod. You can see the simple sutures of the chambers at the top left. Some forms such as oncoceratids or ascoceratids can have cameral deposits (internal fillings) on their sipho. Would be my guess on it.

I agree with Till Söte, looks like a section through a siphuncle of a cephalopod, more specifically an actinocerid. They generally have large siphuncles with massive siphuncular deposits. They are relatively common in the Devonian (especially early Emsian) of Morocco for example. Most Devonian Actinocerida are assigned to the family Ormoceratidae, but Devonian actinocerids are poorly studied in general and it is probably difficult to identify it further.

Cheers, Alex

Yes, I also agree, these details suggest a cephalopod identity.

Cheers

Markus

Hello, Till,

hello Alexandre

and also all the other colleagues.

The explanation sounds plausible to me. But as a vertebrate paleontologist I still have to ask: The prominent, segmented structure would then be the filling of the sipho. Is the sipho also structured according to the chambers? To me it looks as if it is regularly thickened or constricted. According to your explanation, I would connect the constrictions with the passage through the septa. Is that anatomically correct? The "usual" schematic representation of siphos of cephalopods shows the sipho as a tube-like structure where the diameter does not expand and contract in each chamber.

Thank you very much and stay healthy.

Achim

Hi Achim,

Yes, that is correct. The siphuncle of actinocerids is quite different from the one seen in ammonoids, coleoids or nautilids. It is much larger in relation to the shell and expands strongly within each chamber. I attached a schematic representation of an actinocerid (from Teichert 1933, Palaeontographica, Abt. A), I hope that makes it more clear.

The siphuncle had a much larger diversity in early Palaeozoic cephalopods, there are also other groups with expanded or even concave siphuncles and different types of siphuncle fillings.

Best wishes,

Alex

Yes, I also agree with Achim

I confirm the statements by Till and Alexander!

i agree with Alexander Pohle